Exptans Bk3(e)
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Chemistry A Modern View Experiment Workbook 3 Suggested answers Contents PART VIII FOSSIL FUELS AND CARBON COMPOUNDS Chapter 30
Fossil fuels
30.1 To investigate the colour, viscosity, volatility and burning characteristics of crude oil fractions (T/S)
Chapter 31
3
Homologous series, structural formulae and naming of carbon compounds
31.1 To build models of alkanes, alkenes, alkanols and alkanoic acids
Chapter 32
5
Alkanes and alkenes
32.1 To investigate chemical properties of alkanes (S/T)
7
32.2 To crack medicinal paraffin and test for flammability of product (S/T)
8
32.3 To carry out chemical tests on unsaturated hydrocarbons
9
Chapter 33
Consequences of using fossil fuels (Part One): Burning of fuels
Chapter 34
Consequences of using fossil fuels (Part Two): Environmental problems associated with the use of fossil fuels
34.1 To test car exhaust gases (S/T)
10
34.2 To test sulphur dioxide and nitrogen dioxide for acidity
11
Chapter 35
Alcohols
35.1 Oxidation of ethanol to ethanoic acid by acidified potassium dichromate solution (T)
12
35.2 To prepare an ester
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PART IX
PLASTICS AND DETERGENTS
Chapter 36
Plastics
36.1 To test the strength of some plastic articles and ease of softening of plastics (S/T)
14
36.2 To prepare polystyrene
15
36.3 To prepare Perspex
16
36.4 To prepare nylon 6.6 and urea-methanal
17
Chapter 37
Detergents
37.1 To test properties of a detergent
19
37.2 To prepare a soap and test its properties (Extension)
20
37.3 To compare action of soaps and soapless detergents in soft water and hard water (Sample laboratory report) (Extension)
21
PART X DETECTION AND ANALYSIS Chapter 38
Separation of Mixtures
38.1 Extraction
25
38.2 Fractional distillation (T)
26
38.3 Chromatography
27
38.4 To separate a mixture of known substances (Sample laboratory report)
28
Chapter 39
Detection of Substances
39.1 To test for carbonates, chlorides and sulphates
32
39.2 To deduce the chemical nature of a sample (Sample laboratory report) (Extension)
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Experiment 30.1
To investigate the colour, viscosity, volatility and burning characteristics of crude oil fractions (T/S)
4.
(c)
Fraction 1 is quite volatile and has to be condensed out by cooling with water.
8.
Summary of results
Oil fraction
1
2
3
4
room temperature
100°C 150°C
150°C 200°C
200°C 250°C
Property Boiling point
−100°C
range Volatility (judge
(
low
)
from b.p. range)
colourless (
Ease of ignition Colour and sootiness of flame
(
high
non-viscous )
very pale yellow
brown
viscosity ( increases )
very easy yellow with blue edges; non-sooty
yellow
( fairly viscous )
difficult, requires wick yellow/orange; slightly sooty
orange; sooty
orange; very sooty
9.
decreases; colourless; brown; increases; decreases; yellow; orange; increases; less
10.
Hydrocarbons (or alkanes).
11.
a. b.
)
(more difficult)
Colour Viscosity
evaporation becomes
No. Each fraction is still a mixture. It boils over a range of temperatures, not at a fixed temperature. No. (It is only separated into simpler mixtures.)
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12.
The number of carbon atoms becomes larger.
13.
Yes. Higher-boiling fractions contain larger molecules with greater intermolecular forces. Hence the molecules move past each other with greater difficulty.
14. 15.
A fraction consisting of small molecules. Crude oil itself is too complex a mixture to be of any use. Refining of crude oil gives simpler mixtures (fractions), each being suited for some particular uses.
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Experiment 31.1
To build models of alkanes, alkenes, alkanols and alkanoic acids
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1.
(i)
(ii)
(iii)
2 3.
(i)
(ii)
3
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5.
(i)
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(ii)
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(iii)
2 4 7.
(i)
(ii)
(iii)
2
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Experiment 32.1
To investigate chemical properties of alkanes (S/T)
1.
(d)
2.
(b)
Yes. It burns with a yellow flame which is slightly sooty. 2C6H14(l) + 19O2(g) → 12CO2(g) + 14H2O(l)
Condition
Observations
Bromine solution kept in
The red-orange colour of bromine remains
darkness
unchanged.
Bromine solution exposed to
The red-orange colour of bromine is discharged (or becomes paler).
sunlight (or lamp-light)
n-hexane can react with bromine, but some form of energy (e.g. light) has to be supplied to start the reaction. C6H14(l) + Br2(g) → C6H13Br(l) + HBr(g) (or other answers) 3.
burns
4.
light
5.
substitution
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Experiment 32.2
To crack medicinal paraffin and test for flammability of product (S/T)
1.
(c)
(i) No. (ii) Yes. The paraffin burns with a yellow orange flame which is very sooty.
2.
(c)
When heated, it provides a hot catalytic surface on which cracking of alkane molecules occurs.
3.
(a)
(ii)
(b) (c)
If so, the medicinal paraffin absorbed by the rocksil would vaporize too fast. Most of the vapour would pass over the hot pumice stone without reaction. (ii) This is mainly air expelled from inside the apparatus, not cracking products. To prevent sucking back of water which may crack the hot reaction tube.
4.
(c)
A yellow flame is observed, which is a little sooty.
5.
(a) (b)
smaller lower
6.
less
7.
heat; smaller; more
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Experiment 32.3
To carry out chemical tests on unsaturated hydrocarbons
1.
(b)
The red-orange colour of bromine is discharged immediately.
2.
(b)
The purple colour of the solution is discharged immediately.
3.
Addition reaction.
4.
addition; red-orange; colourless; purple; colourless
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Experiment 34.1
To test car exhaust gases (S/T)
9.
It turns milky. Car exhaust gases contain carbon dioxide.
10.
Colour changes from green to red. 4 5. Carbon dioxide, nitrogen oxides and sulphur dioxide.
11.
Sodium citrate prevents coagulation (clotting) of blood. The blood changes from red to cherry red. Carbon monoxide. Carbon monoxide is produced by the incomplete combustion of petrol in the car engine. Carbon monoxide combines with red haemoglobin in blood to form carboxyhaemoglobin, which is cherry red in colour.
12.
(a) (b) (c)
milky; carbon dioxide red; acidic red; cherry red; carbon monoxide
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Experiment 34.2
To test sulphur dioxide and nitrogen dioxide for acidity
1.
(a) (b)
Colourless gas (or slightly misty due to the presence of impurity). Reddish brown gas.
2.
(c)
Water rises up inside the test tube. Sulphur dioxide gas is soluble in water. The blue litmus paper turns red. Sulphur dioxide gas dissolves in water to form an acidic solution.
(d)
3.
Water rises up inside the test tube. Nitrogen dioxide gas is soluble in water. The blue litmus paper turns red. Nitrogen dioxide gas dissolves in water to form an acidic solution.
4.
Acid rain is formed, with pH less than 5.7.
5.
(a) (b)
acidic; blue; red acidic; blue; red
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Experiment 35.1
Oxidation of ethanol to ethanoic acid by acidified potassium dichromate solution (T)
2.
(b)
Orange. Dark green. The mixture is orange due to the colour of the Cr2O72−(aq) ions. Redox reactions occur on heating. Cr2O72−(aq) ions are reduced to Cr3+(aq) ions, which are dark green.
3.
No. As soon as ethanal is formed, it escapes as vapour.
7.
(b) Properties
Distillate from Part B
Ethanol
Colour
colourless
colourless
Smell
smell of vinegar
smell of alcohol
turns red
no change
no change
no change
Effect on blue litmus paper Effect on red litmus paper
8.
An aqueous solution of ethanoic acid.
9.
oxidized; acidified potassium dichromate; oxidizing; oxidized; ethanal; oxidized; ethanoic acid; under reflux; ethanoic acid; distillation
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Experiment 35.2
To prepare an ester
5.
Mixture ‘y’. Sweet fruity smell, like an all-purpose adhesive (e.g. UHU).
6.
(a) (b)
7.
ethanoic acid; vinegar; ester; ethanoic acid; ester; ester
CH3COOH(l) + CH3CH2OH(l) CH3COOCH2CH3(l) + H2O(l) It acts as a catalyst to speed up the reaction. (It also absorbs the water formed.)
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Experiment 36.1
To test the strength of some plastic articles and ease of softening of plastics (S/T)
3.
(d) Plastic
4.
(c)
Does it soften (or melt) on heating?
Polythene
Yes
Polyvinyl chloride
Yes
Polystyrene
Yes
Perspex
Yes
Nylon
Yes
Urea-methanal
No
Yes
5. Plastic
6.
(a) (b)
Does it soften (or melt) again on heating?
Polythene
Yes
Polyvinyl chloride
Yes
Polystyrene
Yes
Perspex
Yes
Nylon
Yes
Urea-methanal
No
heating; cooling; any number of; polythene, polyvinyl chloride, polystyrene, perspex and nylon heating; urea-methanal
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Experiment 36.2 3.
No.
4.
(c)
5. ...
6.
(a) (b)
To prepare polystyrene
Colourless, transparent solid.
H
H
H
C
C C
C C
C
H
H H
H H
H
...
Addition polymer. It is formed by the repeated addition of many styrene molecules, without the elimination of small molecules during the process.
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Experiment 36.3
To prepare perspex
3.
(b)
Colourless liquid.
5.
(a) (b)
(iii) It becomes thicker and thicker. (ii) Usually about 1 hour. (Answer varies, depending on the amount of initiator used, temperature etc.)
6.
(d)
Colourless, transparent solid (probably with a few gas bubbles trapped inside).
(a) (b)
Addition polymer. It is formed by the repeated addition of many monomer molecules, without the elimination of small molecules during the process.
7.
8.
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Experiment 36.4
To prepare nylon 6.6 and urea-methanal
1.
(b)
A thin white film is formed.
4.
A white lump of solid is formed.
5.
nylon; nylon; nylon; junction; thread; Nylon
6.
7.
(a) (b)
Condensation polymer. During polymerization, small molecules (hydrogen chloride in this case) are eliminated.
8.
The first number 6 indicates the number of carbon atoms in the diamine monomer; the second number 6 indicates the number of carbon atoms in the dioyl dichloride monomer.
9.
(a) (b)
Thermoplastic. There are only weak intermolecular forces between polymer chains. No cross links can be formed. Hence nylon can be softened easily by heat repeatedly.
10.
(e)
The colourless solution first turns cloudy. It then becomes more and more viscous (sticky). It eventually turns into a white mass of solid.
11.
It is very hot. The reaction is strongly exothermic.
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13.
(a possible answer) 14.
(a) (b)
Condensation polymer. During polymerization, small molecules (water in this case) are eliminated.
15.
(a) (b)
Thermosetting plastic. When urea-methanal is set hard, extensive cross-links (strong covalent bonds) are formed between polymer chains. The giant covalent network produced cannot be softened by heat again.
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Experiment 37.1
To test properties of a detergent
2.
(b)
The water drop forms a ‘ball’ on the surface of the cloth. No. No.
3.
The detergent solution soaks into the cloth and spreads out. Yes. Yes.
6.
Yes; Yes
7.
(c) Tube ‘x’
x
oil layer clear water
Tube ‘y’
y
foam layer cloudy yellow oily layer milky mixture (emulsion)
A thin layer of oil (still about 2 mm thick) floats on colourless liquid (water) the mixture takes up about the same appearance as before shaking.
A cloudy yellow oily layer (about 1 mm thick) floats on milky mixture (emulsion). On top of the oily layer is a layer of foam (about 1 cm thick). (Thickness for reference only)
(d) No. Quite stable. It is. 8.
(a) (b)
wetting agent; reduce; wet emulsifying agent; emulsion
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Experiment 37.2
To prepare a soap and test its properties (Extension)
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3.
(b)
White creamy solid. To ‘salt out’ (precipitate) the soap from solution.
4.
(b)
To wash away soluble impurities (unreacted alkali, glycerol, sodium chloride).
6.
(e)
Yes.
7.
Yes. Yes.
8.
(a) (b)
castor oil; sodium hydroxide solution; saturated sodium chloride solution (or brine) wetting; emulsifying
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Experiment 37.3 sample laboratory report (Extension)
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Title: To compare action of soaps and soapless detergents in soft water, hard water and tap water Purpose (A) To investigate the cleaning ability of soaps in soft water and in hard water. (B) To investigate the cleaning ability of soapless detergents in soft water and hard water.
Apparatus and chemicals used • Test tubes in rack • Stoppers that fit 150 × 18 mm test tubes • Small self-adhesive labels (or ‘Wytebord’ markers) • Measuring cylinder (10 cm3)
• Liquid soap solution in water (1% w/w) • Liquid detergent solution in water (1% w/w) • Distilled water (as soft water) • Hard water (0.05 M MgSO4 or limewater) • Tap water
Chemical reactions involved COO−(aq) + Ca2+(aq) → (
2 soap anion
COO−)2Ca2+(s) scum
COO−(aq) + H+(aq) → soap anion
COOH(s)
insoluble long chained alkanoic acid
Procedure Notes: (a) In this experiment, the cleaning power of one soap solution and one soapless detergent solution are to be investigated. (b) For simplicity, foam formation was taken as an indication of cleaning power in this experiment. (c) Hard water is the water which contains appreciable concentrations of dissolved calcium and/or magnesium ions. Soft water contains no or very low concentrations of such ions.
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(A)
Action of soaps on soft water, hard water and tap water
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1.
Three test tubes were labelled ‘1’, ‘2’ and ‘3’ respectively.
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2.
(a)
The test tubes were filled as follows:
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Tube ‘1’: 10 cm3 distilled water (as soft water) + 1 cm3 soap solution
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Tube ‘2’: 10 cm3 hard water + 1 cm3 soap solution
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Tube ‘3’: 10 cm3 tap water + 1 cm3 soap solution.
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See Figure 1. add 1 cm3 of soap solution to each
1
measuring cylinder
2
3
test tube
soap solution
distilled water
hard water
tap water
Figure 1 (b) (c) (d)
The tubes were stoppered. The tubes were shaken in each case. The tubes were allowed to stand for 5 minutes (Figure 2). The formation (if any) of lather (Figure 3) and other changes (such as the appearance of scum) were observed. (After recording the observations, the test tubes and stoppers had to be washed thoroughly with tap water.) stopper stopper
test tube rack 1
2
3
test tube rack lather
Figure2
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Figure 3
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(B)
Action of soapless detergent on soft water, hard water and tap water
3.
Step 2 was repeated, but soapless detergent solution was used instead.
Observation 1.
2.
(Reference to Step 2): Tube ‘1’: Some lather (foam) formed on top of the solution which remained quite clear. Tube ‘2’: White solid (scum) formed — some floated and some suspended in the solution. Tube ‘3’: Some lather (foam) formed on top of the solution which was slightly milky. (Reference to Step 3): Lather (foam) formed in all test tubes. No white solid (scum) formed in all test tubes.
Interpretation 1. Tube ‘1’: Soaps worked well in soft water but not in hard water. No scum was formed. Tube ‘2’: The presence of magnesium and/or calcium ions in ‘hard’ water formed insoluble salt (scum) with soap solution. 2 COO−(aq) + Ca2+(aq) → ( COO−)2 Ca2+(s) soap anion
2.
scum
Tube ‘3’ : Tap water is soft water with very low concentrations of metal ions and/or hydrogen ions that formed insoluble precipitates with soaps. Soapless detergents worked very well in all cases. The presence of magnesium and/or calcium ions in ‘hard’ water did not cause the formation of insoluble salt (scum) with soapless detergent and hence no scum was formed.
Discussion 1. 2.
3.
Since formation of lather (foam) is used here as an indication of detergent (cleaning) properties, foamless soapless detergents are not suitable for our purpose in this experiment. Distilled water or deionized water should be used for preparing the soap solution and the soapless detergent solution. If tap water is used, the results of subsequent tests may be slightly affected. The quantities of hard water and soap used here are such that all the soap is ‘used up’ by the Ca2+(aq) / Mg2+(aq) ions. Thus no lather is formed. If a large excess of soap is added to hard water, lather will also be produced.
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Conclusion 1. 2. 3.
Soaps work well in soft water; they do not work well in hard water, in which a precipitate called scum is formed. Soapless detergents work well in both soft water and hard water. Tap water in Hong Kong is soft water.
Answers to questions for further thought 1.
No, sea water is a hard water containing appreciable amounts of magnesium and calcium ions. Scum will be formed. 2. It removes the hardness of water by precipitating calcium and magnesium ions as insoluble carbonates. Ca2+(aq) + CO32−(aq) → CaCO3(s) Mg2+(aq) + CO32−(aq) → MgCO3(s) Washing soda is alkaline. It also helps to remove acidity of water at the same time. 3. In strongly acidic solutions, the soap anions form long chained alkanoic acid which is insoluble in water. 4. We may add tube ‘4’ in this experiment in steps 2 and 3, in which 10 cm3 of 0.1 M HCl and 1 cm3 of either soap solution or soapless detergent solution are mixed. White suspension is formed in soap solution (milky solution) but not in the soapless detergent solution. Thus, it can be concluded that soaps do not work well in strongly acidic solutions but soapless detergents do. COO−(aq) + H+(aq) → COOH(s) soap anion
©Aristo Educational Press Ltd. 2004
insoluble long chained alkanoic acid
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Experiment 38.1
Extraction
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1.
(d) Tube
Solvent
A propanone
B ethanol
added
C petroleum
D heptane
spirit
E aqueous
F water
potassium iodide solution
yes
yes
yes
yes
yes
almost insoluble
Colour of
reddish
reddish
violet
violet
reddish
colourless
the solution formed (if any)
brown
brown
brown
(or very pale yellow)
Does iodine dissolve?
2.
No. Methylbenzene, 1,1,1-trichloroethane (ether, ‘thinner’ or others).
3.
(b)
It is immiscible with water and lighter than water. Light brown. Almost colourless.
4.
(c)
Yellow (colour paler than before). Violet. more soluble; iodine; aqueous potassium iodide; heptane
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Experiment 38.2
Fractional distillation (T)
1.
(c) (e)
Yes. No.
2.
(d)
Vapour rises up; some of it is condensed in the column. This vaporization/condensation process is repeated many times as the vapour rises up the column. The Liebig condenser condenses any vapour escaping from the fractionating column into a liquid. The running water serves as a cooling agent in the process. To get a better cooling effect. To record the temperature of the vapour actually being collected. This is the boiling point of the distillate.
3
(f)
Yes. An ethanol-water mixture with a high proportion of ethanol.
4.
(d)
No. Almost pure water.
5.
miscible liquids; boiling points; water; ethanol
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Experiment 38.3
Chromatography filter paper
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2.
(f)
original position of ink spot
blue orange green
Blue, orange, green. 6. 9.
The column becomes warm as there is a reaction between nitric acid and aluminium oxide and some heat is given out. As an effluent. glass dropper
orange red cotton wool
purple aluminium oxide
C. Summary Paper chromatography; column chromatography; dyes (colours)
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Experiment 38.4 sample laboratory report
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Title:
To separate a mixture of known substances
Purpose To separate a mixture of known substances.
Apparatus and chemicals used • • • • • • • • • • •
Bunsen burner and matches Heat-resistant mat Evaporating basin Stand, boss and clamp Test tubes Beakers Measuring cylinders Glass rod Filter paper Funnel Wash bottle with distilled water
• • • • •
Hydrochloric acid (0.1 M) Sodium hydroxide solution (0.1 M) Copper powder Copper(II) chloride solid Copper(II) hydroxide solid
Chemical reactions involved Cu(OH)2(s) + 2HCl(aq) → CuCl2(aq) + 2H2O(l) CuCl2(aq) + 2NaOH(aq) → 2NaCl(aq) + Cu(OH)2(s) blue gelatinous precipitate
Procedure (A)
Isolation of copper(II) chloride
1. 2.
50 cm3 of water was added to 5 g of the mixture. A glass rod was used to stir the solution. The mixture was filtered (Figure 1a) and the clear solution was collected.
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glass rod glass rod mixture folded filter paper
filtrate
residue filter funnel
evaporating basin
heat crystal
filtrate
(a)
(b)
(c)
Figure 1 3. 4.
6.
Wash the residue with a little cold water. Residue obtained was left for use in part B. The filtrate was boiled in an evaporating basin (Figure 1b). In every 10 seconds, a glass rod was dipped into the boiling solution and then taken out. If the immersed end became cloudy within 5 or 6 seconds, heating was stopped at once. The solution was left to cool and evaporate (Figure 1c). Eventually crystals of copper(II) chloride appeared. The crystals were filtered out from the remaining solution.
(B)
Isolation of copper powder
7. 8. 9. 10. 11. 12.
The residue from A was washed with a little cold distilled water from a washbottle. The residue was transferred into a 100 cm3 beaker. Small portion of dilute hydrochloric acid (0.1 M) was added into the beaker. The solution was stirred with a glass rod. More hydrochloric acid was added until no more solid dissolved. The mixture was filtered. Filtrate obtained was left for experiment in part C. The residue was washed with a little cold water and dried by leaving in air.
(C)
Isolation of copper(II) hydroxide
13. 14. 15. 16.
The clear solution obtained in step 11 was collected in a beaker. Dilute sodium hydroxide was added to the clear solution until no more precipitate formed. The solids were filtered from the remaining solution. The sample was dried by leaving in air.
5.
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Observation (A)
Isolation of copper(II) chloride:
1. 2.
(Reference to Step 1): Some of the solids dissolved and a blue solution appeared. (Reference to Step 6): Small blue crystals of copper(II) chloride formed.
(B)
Isolation of copper powder
3. 4.
(Reference to Step 9): Some of the solids dissolved, forming a clear blue solution. Some reddish brown residue was observed. (Reference to Step 12): Dry reddish brown solids were obtained.
(C)
Isolation of copper(II) hydroxide
5.
(Reference to Step 14): Blue precipitate was formed upon the addition of sodium hydroxide.
Interpretation 1. 2. 3.
The solids dissolved is copper(II) chloride which is soluble in water. Water soluble copper(II) chloride can be obtained by crystallization. When dilute hydrochloric acid is added, the copper(II) hydroxide precipitate dissolves as a result of neutralization. Cu(OH)2(s) + 2HCl(aq) → CuCl2(aq) + 2H2O(l) 4. Copper metal has no reaction with dilute hydrochloric acid. Therefore it appears as solids. 5. Insoluble copper(II) hydroxide precipitate can be obtained when sodium hydroxide solution is added to the copper(II) chloride solution. CuCl2(aq) + 2NaOH(aq) → 2NaCl(aq) + Cu(OH)2(s) blue precipitate
Discussion 1. 2.
Too strong heating may cause the solution to spurt out, especially when it is very concentrated. Thus safety spectacles must be worn. It is not easy to determine when to stop heating during the concentrating process. If the hot solution is not concentrated enough, it will not deposit crystals on subsequent cooling even overnight.
Conclusion The substances in a mixture can be separated according to the differences in their physical property and chemical property.
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Answers to questions for further thought 1. 2.
Acid will react with both magnesium metal and magnesium oxide. Add excess sodium hydroxide solution to the mixture, a clear solution [Zn(OH)4]2− containing blue precipitate Cu(OH)2 was formed. (a) Filter the solution to obtain precipitate of copper(II) hydroxide. Generate copper(II) chloride by adding dilute hydrochloric acid. (b) Add dilute hydrochloric acid to the clear filtrate to generate zinc chloride.
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Experiment 39.1
To test for carbonates, chlorides and sulphates
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6. Effect of adding dilute
Effect of adding silver
Effect of adding barium
hydrochloric acid and
nitrate solution + dilute
chloride solution + dilute
testing with limewater
nitric acid
hydrochloric acid
Effervescence.
White precipitate formed. Precipitate redissolved and a gas was evolved when nitric acid was added.
White precipitate formed. Precipitate redissolved and a gas was evolved when hydrochloric acid was added.
Effect of adding dilute
Effect of adding silver
Effect of adding barium
hydrochloric acid and
nitrate solution + dilute
chloride solution + dilute
testing with limewater
nitric acid
hydrochloric acid
No observation
White precipitate formed. Precipitate did not redissolve when nitric acid was added.
No observation
Effect of adding dilute
Effect of adding silver
Effect of adding barium
hydrochloric acid and
nitrate solution + dilute
chloride solution + dilute
testing with limewater
nitric acid
hydrochloric acid
No observation
No observation
White precipitate formed. Precipitate did not redissolve when hydrochloric acid was added.
The gas evolved turns limewater milky. CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l)
8.
10.
11.
dilute hydrochloric acid; carbon dioxide acidified silver nitrate solution; acidified barium chloride solution
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Experiment 39.2 sample laboratory report (Extension) Title: To deduce the chemical nature of a sample Purpose To deduce the chemical nature of unknown substances.
Apparatus and chemicals used • Boiling tube (fitted with a rubber stopper carrying a bent delivery tube) • Test tubes (fitted with rubber stoppers) • Test tube rack • Bunsen burner and matches • Heat-resistant mat • Stand, boss and clamp • Platinum / nichrome wire
• Unknown solutions labelled A, B and C • Concentrated hydrochloric acid in a test tube (11 M, for flame test) • Dilute hydrochloric acid (1 M) • Dilute sodium hydroxide (1 M) • Barium chloride solution (1 M) • Dilute nitric acid (1 M) • Silver nitrate solution (0.05 M) • Limewater
Chemical reactions involved For solution A
Test for Cu2+ions Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) Test for SO42−ions Ba2+(aq) + SO42−(aq) → BaSO4(s) For solution B
Test for CO32−ions CO32−(aq) + 2H+(aq) → H2O(l) + CO2(g) Limewater test CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) For solution C
Test for Cl−ions Ag+(aq) + Cl−(aq) → AgCl(s)
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Procedure 1. 2.
3.
The appearance of the samples were recorded. Flame tests were carried out to prove the identity of cations. (a) A nichrome wire was dipped in concentrated hydrochloric acid and then heated in a Bunsen flame, with air hole fully opened, until it no longer gave any characteristic coloured flame. (b) The sample was mixed with concentrated hydrochloric acid on a watch glass and the nichrome wire was moistened with this solution. The wire was then heated strongly in a non-luminous flame. The flame colour was observed and recorded. Each sample was divided into four equal portions in four separate test tubes. (a) To the first test tube of solution, a few drops of sodium hydroxide solution was added. The precipitate that was formed (if any) was tested by the addition of excess sodium hydroxide solution to see whether it re-dissolved or not. (b) To the second test tube of solution, a few drops of dilute hydrochloric acid was added. The gas evolved (if any) was then tested by bubbling into another test tube containing limewater. (c) To the third test tube of solution, a few drops of barium chloride solution was added. The precipitate formed (if any) was tested by adding some dilute hydrochloric acid to see whether it re-dissolved or not. Any gas evolved was tested with limewater. (d) To the fourth test tube of solution, a few drops of silver nitrate solution was added. The precipitate formed (if any) was tested by adding some dilute nitric acid to see whether it re-dissolved or not.
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Observation A
B
C
Appearance of solution
Blue
Colourless
Colourless
Flame test
Bluish green flame
Brilliant golden yellow flame
Brick red flame
Addition of sodium hydroxide
Blue precipitate formed
No observable change
White precipitate formed
Addition of hydrochloric acid and testing with limewater
The ppt. did not
The ppt. did not
re-dissolve in excess NaOH
re-dissolve in excess NaOH
No observable change
Effervescence
No observable change
Limewater turned milky Addition of barium chloride solution + dilute HCl and testing with limewater
White precipitate formed
White precipitate formed
No observable change
Effervescence Limewater turned milky Addition of silver nitrate solution + dilute HNO3
No observable change
White precipitate formed
After HNO3 was added, effervescence occurred and ppt.
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White precipitate formed
After HNO3 was added, ppt. remained undissolved
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dissolved
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Interpretation
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1.
For solution A
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Test for Cu2+ions
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The flame colour observed was bluish green.
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Blue precipitate formed when a few drops of sodium hydroxide solution was added.
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Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
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The blue precipitate remained undissolved in excess sodium hydroxide solution.
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Test for SO42−ions
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White precipitate formed when a few drops of barium chloride solution was added.
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Ba2+(aq) + SO42−(aq) → BaSO4(s)
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The white precipitate formed did not re-dissolve in dilute hydrochloric acid.
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So, A is copper(II) sulphate solution.
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2.
For solution B
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Test for Na+ions
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The flame colour observed was brilliant golden yellow.
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Test for CO32−ions
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Carbon dioxide was formed when dilute hydrochloric acid was added.
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CO32−(aq) + 2H+(aq) → H2O(l) + CO2(g)
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Limewater test
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Carbon dioxide turned limewater milky.
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CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
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So, B is sodium carbonate solution.
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3.
For solution C
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Test for Ca2+ions
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The flame colour observed was brick red. White precipitate (calcium hydroxide) was formed when sodium hydroxide solution was added. Ca2+(aq) + 2OH−(aq) → Ca(OH)2(s) The white precipitate formed did not re-dissolve in excess sodium hydroxide solution.
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Test for Cl−ions White precipitate (silver chloride) was formed when a few drops of silver nitrate solution was added. Ag+(aq) + Cl−(aq) → AgCl(s) The white precipitate formed did not re-dissolve in dilute nitric acid. So, C is calcium chloride solution.
Discussion 1.
Test for halides To test for halides, we often use silver nitrate (AgNO3) solution because it forms insoluble silver halides. Ag+(aq) + Cl−(aq) → AgCl(s) white precipitate
However, carbonate and hydroxide of silver are also insoluble in water. Thus, we have to get rid of carbonate and hydroxide by reacting them with dilute nitric acid. (precipitate remains insoluble) AgCl(s) + HNO3(aq) → no reaction (precipitate dissolved) AgOH(s) + HNO3(aq) → AgNO3(aq) + H2O(l) (precipitate dissolved) Ag2CO3(s) + 2HNO3 (aq) → 2AgNO3(aq) + CO2(g) + H2O(l) 2.
Test for sulphates Barium ions form insoluble salts with sulphate ions. Thus we can use barium ion-containing reagents such as barium chloride or barium nitrate to test for the presence of sulphate ions. Ba2+(aq) + SO42−(aq) → BaSO4(s) white precipitate
Carbonates and sulphites can also react with barium ions to form insoluble salts, which react with acids to give colourless carbon dioxide gas and colourless sulphur dioxide gas respectively. 2+ Ba (aq) + CO32−(aq) → BaCO3(s) white precipitate
BaCO3(s) + 2H (aq) → Ba2+(aq) + CO2(g) + H2O(l) Ba2+(aq) + SO32−(aq) → BaSO3(s) +
white precipitate
BaSO3(s) + 2H+(aq) → Ba2+(aq) + SO2(g) + H2O(l)
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3.
Test for copper(II) ions Copper(II) ions form precipitate with sodium hydroxide solution. Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) blue precipitate
The precipitate remained undissolved in excess sodium hydroxide. 4. Test for carbonates Addition of dilute acids to carbonates produce colourless carbon dioxide gas. CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) colourless gas
Carbonates and hydrogencarbonates show very similar behaviour in the above test. We can use magnesium sulphate solution to distinguish them. Mg2+(aq) + CO32−(aq) → MgCO3(s) white precipitate − 2+ (colourless solution formed) Mg (aq) + 2HCO3 (aq) → Mg (HCO3)2(aq)
Conclusion 1. 2. 3. 4. 5.
Metals and metal-containing compounds give characteristic flame colour when heated in Bunsen flame. The colour can help to identify the metal ions present in a compound. Some metal ions form precipitates with sodium hydroxide solution. Halides can be tested by silver nitrate solution. Carbonates can be tested by dilute acids. Carbon dioxide is formed in the reaction which turns limewater milky. Barium ion-containing reagents such as barium chloride, can be used to test for the presence of sulphate.
Answers to questions for further thought 1.
(a)
(b) 2.
(a)
(b)
Physical properties do not involve the change of a substance into other substances. Examples: Appearance, colour, taste, hardness, density, solubility behaviour, melting point, boiling point, malleability, ductility, (or other reasonable answers) Chemical properties involve the chemical reactions and the conditions of the reactions. Examples: Sulphur burns in air to form sulphur dioxide, (or other reasonable answers) Much of the attention is paid on the qualitative analysis but not quantitative ones. For example, we can only tell whether a sample of water contains chloride ions, but not the concentration of the ions in water. A large quantity of sample is required to perform the tests.
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Fossil fuels
30.1 To investigate the colour, viscosity, volatility and burning characteristics of crude oil fractions (T/S)
Chapter 31
3
Homologous series, structural formulae and naming of carbon compounds
31.1 To build models of alkanes, alkenes, alkanols and alkanoic acids
Chapter 32
5
Alkanes and alkenes
32.1 To investigate chemical properties of alkanes (S/T)
7
32.2 To crack medicinal paraffin and test for flammability of product (S/T)
8
32.3 To carry out chemical tests on unsaturated hydrocarbons
9
Chapter 33
Consequences of using fossil fuels (Part One): Burning of fuels
Chapter 34
Consequences of using fossil fuels (Part Two): Environmental problems associated with the use of fossil fuels
34.1 To test car exhaust gases (S/T)
10
34.2 To test sulphur dioxide and nitrogen dioxide for acidity
11
Chapter 35
Alcohols
35.1 Oxidation of ethanol to ethanoic acid by acidified potassium dichromate solution (T)
12
35.2 To prepare an ester
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PART IX
PLASTICS AND DETERGENTS
Chapter 36
Plastics
36.1 To test the strength of some plastic articles and ease of softening of plastics (S/T)
14
36.2 To prepare polystyrene
15
36.3 To prepare Perspex
16
36.4 To prepare nylon 6.6 and urea-methanal
17
Chapter 37
Detergents
37.1 To test properties of a detergent
19
37.2 To prepare a soap and test its properties (Extension)
20
37.3 To compare action of soaps and soapless detergents in soft water and hard water (Sample laboratory report) (Extension)
21
PART X DETECTION AND ANALYSIS Chapter 38
Separation of Mixtures
38.1 Extraction
25
38.2 Fractional distillation (T)
26
38.3 Chromatography
27
38.4 To separate a mixture of known substances (Sample laboratory report)
28
Chapter 39
Detection of Substances
39.1 To test for carbonates, chlorides and sulphates
32
39.2 To deduce the chemical nature of a sample (Sample laboratory report) (Extension)
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Experiment 30.1
To investigate the colour, viscosity, volatility and burning characteristics of crude oil fractions (T/S)
4.
(c)
Fraction 1 is quite volatile and has to be condensed out by cooling with water.
8.
Summary of results
Oil fraction
1
2
3
4
room temperature
100°C 150°C
150°C 200°C
200°C 250°C
Property Boiling point
−100°C
range Volatility (judge
(
low
)
from b.p. range)
colourless (
Ease of ignition Colour and sootiness of flame
(
high
non-viscous )
very pale yellow
brown
viscosity ( increases )
very easy yellow with blue edges; non-sooty
yellow
( fairly viscous )
difficult, requires wick yellow/orange; slightly sooty
orange; sooty
orange; very sooty
9.
decreases; colourless; brown; increases; decreases; yellow; orange; increases; less
10.
Hydrocarbons (or alkanes).
11.
a. b.
)
(more difficult)
Colour Viscosity
evaporation becomes
No. Each fraction is still a mixture. It boils over a range of temperatures, not at a fixed temperature. No. (It is only separated into simpler mixtures.)
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12.
The number of carbon atoms becomes larger.
13.
Yes. Higher-boiling fractions contain larger molecules with greater intermolecular forces. Hence the molecules move past each other with greater difficulty.
14. 15.
A fraction consisting of small molecules. Crude oil itself is too complex a mixture to be of any use. Refining of crude oil gives simpler mixtures (fractions), each being suited for some particular uses.
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Experiment 31.1
To build models of alkanes, alkenes, alkanols and alkanoic acids
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1.
(i)
(ii)
(iii)
2 3.
(i)
(ii)
3
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5.
(i)
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(ii)
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(iii)
2 4 7.
(i)
(ii)
(iii)
2
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Experiment 32.1
To investigate chemical properties of alkanes (S/T)
1.
(d)
2.
(b)
Yes. It burns with a yellow flame which is slightly sooty. 2C6H14(l) + 19O2(g) → 12CO2(g) + 14H2O(l)
Condition
Observations
Bromine solution kept in
The red-orange colour of bromine remains
darkness
unchanged.
Bromine solution exposed to
The red-orange colour of bromine is discharged (or becomes paler).
sunlight (or lamp-light)
n-hexane can react with bromine, but some form of energy (e.g. light) has to be supplied to start the reaction. C6H14(l) + Br2(g) → C6H13Br(l) + HBr(g) (or other answers) 3.
burns
4.
light
5.
substitution
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Experiment 32.2
To crack medicinal paraffin and test for flammability of product (S/T)
1.
(c)
(i) No. (ii) Yes. The paraffin burns with a yellow orange flame which is very sooty.
2.
(c)
When heated, it provides a hot catalytic surface on which cracking of alkane molecules occurs.
3.
(a)
(ii)
(b) (c)
If so, the medicinal paraffin absorbed by the rocksil would vaporize too fast. Most of the vapour would pass over the hot pumice stone without reaction. (ii) This is mainly air expelled from inside the apparatus, not cracking products. To prevent sucking back of water which may crack the hot reaction tube.
4.
(c)
A yellow flame is observed, which is a little sooty.
5.
(a) (b)
smaller lower
6.
less
7.
heat; smaller; more
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Experiment 32.3
To carry out chemical tests on unsaturated hydrocarbons
1.
(b)
The red-orange colour of bromine is discharged immediately.
2.
(b)
The purple colour of the solution is discharged immediately.
3.
Addition reaction.
4.
addition; red-orange; colourless; purple; colourless
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Experiment 34.1
To test car exhaust gases (S/T)
9.
It turns milky. Car exhaust gases contain carbon dioxide.
10.
Colour changes from green to red. 4 5. Carbon dioxide, nitrogen oxides and sulphur dioxide.
11.
Sodium citrate prevents coagulation (clotting) of blood. The blood changes from red to cherry red. Carbon monoxide. Carbon monoxide is produced by the incomplete combustion of petrol in the car engine. Carbon monoxide combines with red haemoglobin in blood to form carboxyhaemoglobin, which is cherry red in colour.
12.
(a) (b) (c)
milky; carbon dioxide red; acidic red; cherry red; carbon monoxide
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Experiment 34.2
To test sulphur dioxide and nitrogen dioxide for acidity
1.
(a) (b)
Colourless gas (or slightly misty due to the presence of impurity). Reddish brown gas.
2.
(c)
Water rises up inside the test tube. Sulphur dioxide gas is soluble in water. The blue litmus paper turns red. Sulphur dioxide gas dissolves in water to form an acidic solution.
(d)
3.
Water rises up inside the test tube. Nitrogen dioxide gas is soluble in water. The blue litmus paper turns red. Nitrogen dioxide gas dissolves in water to form an acidic solution.
4.
Acid rain is formed, with pH less than 5.7.
5.
(a) (b)
acidic; blue; red acidic; blue; red
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Experiment 35.1
Oxidation of ethanol to ethanoic acid by acidified potassium dichromate solution (T)
2.
(b)
Orange. Dark green. The mixture is orange due to the colour of the Cr2O72−(aq) ions. Redox reactions occur on heating. Cr2O72−(aq) ions are reduced to Cr3+(aq) ions, which are dark green.
3.
No. As soon as ethanal is formed, it escapes as vapour.
7.
(b) Properties
Distillate from Part B
Ethanol
Colour
colourless
colourless
Smell
smell of vinegar
smell of alcohol
turns red
no change
no change
no change
Effect on blue litmus paper Effect on red litmus paper
8.
An aqueous solution of ethanoic acid.
9.
oxidized; acidified potassium dichromate; oxidizing; oxidized; ethanal; oxidized; ethanoic acid; under reflux; ethanoic acid; distillation
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Experiment 35.2
To prepare an ester
5.
Mixture ‘y’. Sweet fruity smell, like an all-purpose adhesive (e.g. UHU).
6.
(a) (b)
7.
ethanoic acid; vinegar; ester; ethanoic acid; ester; ester
CH3COOH(l) + CH3CH2OH(l) CH3COOCH2CH3(l) + H2O(l) It acts as a catalyst to speed up the reaction. (It also absorbs the water formed.)
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Experiment 36.1
To test the strength of some plastic articles and ease of softening of plastics (S/T)
3.
(d) Plastic
4.
(c)
Does it soften (or melt) on heating?
Polythene
Yes
Polyvinyl chloride
Yes
Polystyrene
Yes
Perspex
Yes
Nylon
Yes
Urea-methanal
No
Yes
5. Plastic
6.
(a) (b)
Does it soften (or melt) again on heating?
Polythene
Yes
Polyvinyl chloride
Yes
Polystyrene
Yes
Perspex
Yes
Nylon
Yes
Urea-methanal
No
heating; cooling; any number of; polythene, polyvinyl chloride, polystyrene, perspex and nylon heating; urea-methanal
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Experiment 36.2 3.
No.
4.
(c)
5. ...
6.
(a) (b)
To prepare polystyrene
Colourless, transparent solid.
H
H
H
C
C C
C C
C
H
H H
H H
H
...
Addition polymer. It is formed by the repeated addition of many styrene molecules, without the elimination of small molecules during the process.
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Experiment 36.3
To prepare perspex
3.
(b)
Colourless liquid.
5.
(a) (b)
(iii) It becomes thicker and thicker. (ii) Usually about 1 hour. (Answer varies, depending on the amount of initiator used, temperature etc.)
6.
(d)
Colourless, transparent solid (probably with a few gas bubbles trapped inside).
(a) (b)
Addition polymer. It is formed by the repeated addition of many monomer molecules, without the elimination of small molecules during the process.
7.
8.
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Experiment 36.4
To prepare nylon 6.6 and urea-methanal
1.
(b)
A thin white film is formed.
4.
A white lump of solid is formed.
5.
nylon; nylon; nylon; junction; thread; Nylon
6.
7.
(a) (b)
Condensation polymer. During polymerization, small molecules (hydrogen chloride in this case) are eliminated.
8.
The first number 6 indicates the number of carbon atoms in the diamine monomer; the second number 6 indicates the number of carbon atoms in the dioyl dichloride monomer.
9.
(a) (b)
Thermoplastic. There are only weak intermolecular forces between polymer chains. No cross links can be formed. Hence nylon can be softened easily by heat repeatedly.
10.
(e)
The colourless solution first turns cloudy. It then becomes more and more viscous (sticky). It eventually turns into a white mass of solid.
11.
It is very hot. The reaction is strongly exothermic.
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13.
(a possible answer) 14.
(a) (b)
Condensation polymer. During polymerization, small molecules (water in this case) are eliminated.
15.
(a) (b)
Thermosetting plastic. When urea-methanal is set hard, extensive cross-links (strong covalent bonds) are formed between polymer chains. The giant covalent network produced cannot be softened by heat again.
©Aristo Educational Press Ltd. 2004
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Experiment 37.1
To test properties of a detergent
2.
(b)
The water drop forms a ‘ball’ on the surface of the cloth. No. No.
3.
The detergent solution soaks into the cloth and spreads out. Yes. Yes.
6.
Yes; Yes
7.
(c) Tube ‘x’
x
oil layer clear water
Tube ‘y’
y
foam layer cloudy yellow oily layer milky mixture (emulsion)
A thin layer of oil (still about 2 mm thick) floats on colourless liquid (water) the mixture takes up about the same appearance as before shaking.
A cloudy yellow oily layer (about 1 mm thick) floats on milky mixture (emulsion). On top of the oily layer is a layer of foam (about 1 cm thick). (Thickness for reference only)
(d) No. Quite stable. It is. 8.
(a) (b)
wetting agent; reduce; wet emulsifying agent; emulsion
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Experiment 37.2
To prepare a soap and test its properties (Extension)
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3.
(b)
White creamy solid. To ‘salt out’ (precipitate) the soap from solution.
4.
(b)
To wash away soluble impurities (unreacted alkali, glycerol, sodium chloride).
6.
(e)
Yes.
7.
Yes. Yes.
8.
(a) (b)
castor oil; sodium hydroxide solution; saturated sodium chloride solution (or brine) wetting; emulsifying
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Experiment 37.3 sample laboratory report (Extension)
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Title: To compare action of soaps and soapless detergents in soft water, hard water and tap water Purpose (A) To investigate the cleaning ability of soaps in soft water and in hard water. (B) To investigate the cleaning ability of soapless detergents in soft water and hard water.
Apparatus and chemicals used • Test tubes in rack • Stoppers that fit 150 × 18 mm test tubes • Small self-adhesive labels (or ‘Wytebord’ markers) • Measuring cylinder (10 cm3)
• Liquid soap solution in water (1% w/w) • Liquid detergent solution in water (1% w/w) • Distilled water (as soft water) • Hard water (0.05 M MgSO4 or limewater) • Tap water
Chemical reactions involved COO−(aq) + Ca2+(aq) → (
2 soap anion
COO−)2Ca2+(s) scum
COO−(aq) + H+(aq) → soap anion
COOH(s)
insoluble long chained alkanoic acid
Procedure Notes: (a) In this experiment, the cleaning power of one soap solution and one soapless detergent solution are to be investigated. (b) For simplicity, foam formation was taken as an indication of cleaning power in this experiment. (c) Hard water is the water which contains appreciable concentrations of dissolved calcium and/or magnesium ions. Soft water contains no or very low concentrations of such ions.
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(A)
Action of soaps on soft water, hard water and tap water
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1.
Three test tubes were labelled ‘1’, ‘2’ and ‘3’ respectively.
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2.
(a)
The test tubes were filled as follows:
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Tube ‘1’: 10 cm3 distilled water (as soft water) + 1 cm3 soap solution
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Tube ‘2’: 10 cm3 hard water + 1 cm3 soap solution
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Tube ‘3’: 10 cm3 tap water + 1 cm3 soap solution.
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See Figure 1. add 1 cm3 of soap solution to each
1
measuring cylinder
2
3
test tube
soap solution
distilled water
hard water
tap water
Figure 1 (b) (c) (d)
The tubes were stoppered. The tubes were shaken in each case. The tubes were allowed to stand for 5 minutes (Figure 2). The formation (if any) of lather (Figure 3) and other changes (such as the appearance of scum) were observed. (After recording the observations, the test tubes and stoppers had to be washed thoroughly with tap water.) stopper stopper
test tube rack 1
2
3
test tube rack lather
Figure2
©Aristo Educational Press Ltd. 2004
Figure 3
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(B)
Action of soapless detergent on soft water, hard water and tap water
3.
Step 2 was repeated, but soapless detergent solution was used instead.
Observation 1.
2.
(Reference to Step 2): Tube ‘1’: Some lather (foam) formed on top of the solution which remained quite clear. Tube ‘2’: White solid (scum) formed — some floated and some suspended in the solution. Tube ‘3’: Some lather (foam) formed on top of the solution which was slightly milky. (Reference to Step 3): Lather (foam) formed in all test tubes. No white solid (scum) formed in all test tubes.
Interpretation 1. Tube ‘1’: Soaps worked well in soft water but not in hard water. No scum was formed. Tube ‘2’: The presence of magnesium and/or calcium ions in ‘hard’ water formed insoluble salt (scum) with soap solution. 2 COO−(aq) + Ca2+(aq) → ( COO−)2 Ca2+(s) soap anion
2.
scum
Tube ‘3’ : Tap water is soft water with very low concentrations of metal ions and/or hydrogen ions that formed insoluble precipitates with soaps. Soapless detergents worked very well in all cases. The presence of magnesium and/or calcium ions in ‘hard’ water did not cause the formation of insoluble salt (scum) with soapless detergent and hence no scum was formed.
Discussion 1. 2.
3.
Since formation of lather (foam) is used here as an indication of detergent (cleaning) properties, foamless soapless detergents are not suitable for our purpose in this experiment. Distilled water or deionized water should be used for preparing the soap solution and the soapless detergent solution. If tap water is used, the results of subsequent tests may be slightly affected. The quantities of hard water and soap used here are such that all the soap is ‘used up’ by the Ca2+(aq) / Mg2+(aq) ions. Thus no lather is formed. If a large excess of soap is added to hard water, lather will also be produced.
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Conclusion 1. 2. 3.
Soaps work well in soft water; they do not work well in hard water, in which a precipitate called scum is formed. Soapless detergents work well in both soft water and hard water. Tap water in Hong Kong is soft water.
Answers to questions for further thought 1.
No, sea water is a hard water containing appreciable amounts of magnesium and calcium ions. Scum will be formed. 2. It removes the hardness of water by precipitating calcium and magnesium ions as insoluble carbonates. Ca2+(aq) + CO32−(aq) → CaCO3(s) Mg2+(aq) + CO32−(aq) → MgCO3(s) Washing soda is alkaline. It also helps to remove acidity of water at the same time. 3. In strongly acidic solutions, the soap anions form long chained alkanoic acid which is insoluble in water. 4. We may add tube ‘4’ in this experiment in steps 2 and 3, in which 10 cm3 of 0.1 M HCl and 1 cm3 of either soap solution or soapless detergent solution are mixed. White suspension is formed in soap solution (milky solution) but not in the soapless detergent solution. Thus, it can be concluded that soaps do not work well in strongly acidic solutions but soapless detergents do. COO−(aq) + H+(aq) → COOH(s) soap anion
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insoluble long chained alkanoic acid
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Experiment 38.1
Extraction
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1.
(d) Tube
Solvent
A propanone
B ethanol
added
C petroleum
D heptane
spirit
E aqueous
F water
potassium iodide solution
yes
yes
yes
yes
yes
almost insoluble
Colour of
reddish
reddish
violet
violet
reddish
colourless
the solution formed (if any)
brown
brown
brown
(or very pale yellow)
Does iodine dissolve?
2.
No. Methylbenzene, 1,1,1-trichloroethane (ether, ‘thinner’ or others).
3.
(b)
It is immiscible with water and lighter than water. Light brown. Almost colourless.
4.
(c)
Yellow (colour paler than before). Violet. more soluble; iodine; aqueous potassium iodide; heptane
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Experiment 38.2
Fractional distillation (T)
1.
(c) (e)
Yes. No.
2.
(d)
Vapour rises up; some of it is condensed in the column. This vaporization/condensation process is repeated many times as the vapour rises up the column. The Liebig condenser condenses any vapour escaping from the fractionating column into a liquid. The running water serves as a cooling agent in the process. To get a better cooling effect. To record the temperature of the vapour actually being collected. This is the boiling point of the distillate.
3
(f)
Yes. An ethanol-water mixture with a high proportion of ethanol.
4.
(d)
No. Almost pure water.
5.
miscible liquids; boiling points; water; ethanol
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Experiment 38.3
Chromatography filter paper
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2.
(f)
original position of ink spot
blue orange green
Blue, orange, green. 6. 9.
The column becomes warm as there is a reaction between nitric acid and aluminium oxide and some heat is given out. As an effluent. glass dropper
orange red cotton wool
purple aluminium oxide
C. Summary Paper chromatography; column chromatography; dyes (colours)
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Experiment 38.4 sample laboratory report
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Title:
To separate a mixture of known substances
Purpose To separate a mixture of known substances.
Apparatus and chemicals used • • • • • • • • • • •
Bunsen burner and matches Heat-resistant mat Evaporating basin Stand, boss and clamp Test tubes Beakers Measuring cylinders Glass rod Filter paper Funnel Wash bottle with distilled water
• • • • •
Hydrochloric acid (0.1 M) Sodium hydroxide solution (0.1 M) Copper powder Copper(II) chloride solid Copper(II) hydroxide solid
Chemical reactions involved Cu(OH)2(s) + 2HCl(aq) → CuCl2(aq) + 2H2O(l) CuCl2(aq) + 2NaOH(aq) → 2NaCl(aq) + Cu(OH)2(s) blue gelatinous precipitate
Procedure (A)
Isolation of copper(II) chloride
1. 2.
50 cm3 of water was added to 5 g of the mixture. A glass rod was used to stir the solution. The mixture was filtered (Figure 1a) and the clear solution was collected.
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glass rod glass rod mixture folded filter paper
filtrate
residue filter funnel
evaporating basin
heat crystal
filtrate
(a)
(b)
(c)
Figure 1 3. 4.
6.
Wash the residue with a little cold water. Residue obtained was left for use in part B. The filtrate was boiled in an evaporating basin (Figure 1b). In every 10 seconds, a glass rod was dipped into the boiling solution and then taken out. If the immersed end became cloudy within 5 or 6 seconds, heating was stopped at once. The solution was left to cool and evaporate (Figure 1c). Eventually crystals of copper(II) chloride appeared. The crystals were filtered out from the remaining solution.
(B)
Isolation of copper powder
7. 8. 9. 10. 11. 12.
The residue from A was washed with a little cold distilled water from a washbottle. The residue was transferred into a 100 cm3 beaker. Small portion of dilute hydrochloric acid (0.1 M) was added into the beaker. The solution was stirred with a glass rod. More hydrochloric acid was added until no more solid dissolved. The mixture was filtered. Filtrate obtained was left for experiment in part C. The residue was washed with a little cold water and dried by leaving in air.
(C)
Isolation of copper(II) hydroxide
13. 14. 15. 16.
The clear solution obtained in step 11 was collected in a beaker. Dilute sodium hydroxide was added to the clear solution until no more precipitate formed. The solids were filtered from the remaining solution. The sample was dried by leaving in air.
5.
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Observation (A)
Isolation of copper(II) chloride:
1. 2.
(Reference to Step 1): Some of the solids dissolved and a blue solution appeared. (Reference to Step 6): Small blue crystals of copper(II) chloride formed.
(B)
Isolation of copper powder
3. 4.
(Reference to Step 9): Some of the solids dissolved, forming a clear blue solution. Some reddish brown residue was observed. (Reference to Step 12): Dry reddish brown solids were obtained.
(C)
Isolation of copper(II) hydroxide
5.
(Reference to Step 14): Blue precipitate was formed upon the addition of sodium hydroxide.
Interpretation 1. 2. 3.
The solids dissolved is copper(II) chloride which is soluble in water. Water soluble copper(II) chloride can be obtained by crystallization. When dilute hydrochloric acid is added, the copper(II) hydroxide precipitate dissolves as a result of neutralization. Cu(OH)2(s) + 2HCl(aq) → CuCl2(aq) + 2H2O(l) 4. Copper metal has no reaction with dilute hydrochloric acid. Therefore it appears as solids. 5. Insoluble copper(II) hydroxide precipitate can be obtained when sodium hydroxide solution is added to the copper(II) chloride solution. CuCl2(aq) + 2NaOH(aq) → 2NaCl(aq) + Cu(OH)2(s) blue precipitate
Discussion 1. 2.
Too strong heating may cause the solution to spurt out, especially when it is very concentrated. Thus safety spectacles must be worn. It is not easy to determine when to stop heating during the concentrating process. If the hot solution is not concentrated enough, it will not deposit crystals on subsequent cooling even overnight.
Conclusion The substances in a mixture can be separated according to the differences in their physical property and chemical property.
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Answers to questions for further thought 1. 2.
Acid will react with both magnesium metal and magnesium oxide. Add excess sodium hydroxide solution to the mixture, a clear solution [Zn(OH)4]2− containing blue precipitate Cu(OH)2 was formed. (a) Filter the solution to obtain precipitate of copper(II) hydroxide. Generate copper(II) chloride by adding dilute hydrochloric acid. (b) Add dilute hydrochloric acid to the clear filtrate to generate zinc chloride.
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Experiment 39.1
To test for carbonates, chlorides and sulphates
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6. Effect of adding dilute
Effect of adding silver
Effect of adding barium
hydrochloric acid and
nitrate solution + dilute
chloride solution + dilute
testing with limewater
nitric acid
hydrochloric acid
Effervescence.
White precipitate formed. Precipitate redissolved and a gas was evolved when nitric acid was added.
White precipitate formed. Precipitate redissolved and a gas was evolved when hydrochloric acid was added.
Effect of adding dilute
Effect of adding silver
Effect of adding barium
hydrochloric acid and
nitrate solution + dilute
chloride solution + dilute
testing with limewater
nitric acid
hydrochloric acid
No observation
White precipitate formed. Precipitate did not redissolve when nitric acid was added.
No observation
Effect of adding dilute
Effect of adding silver
Effect of adding barium
hydrochloric acid and
nitrate solution + dilute
chloride solution + dilute
testing with limewater
nitric acid
hydrochloric acid
No observation
No observation
White precipitate formed. Precipitate did not redissolve when hydrochloric acid was added.
The gas evolved turns limewater milky. CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l)
8.
10.
11.
dilute hydrochloric acid; carbon dioxide acidified silver nitrate solution; acidified barium chloride solution
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Experiment 39.2 sample laboratory report (Extension) Title: To deduce the chemical nature of a sample Purpose To deduce the chemical nature of unknown substances.
Apparatus and chemicals used • Boiling tube (fitted with a rubber stopper carrying a bent delivery tube) • Test tubes (fitted with rubber stoppers) • Test tube rack • Bunsen burner and matches • Heat-resistant mat • Stand, boss and clamp • Platinum / nichrome wire
• Unknown solutions labelled A, B and C • Concentrated hydrochloric acid in a test tube (11 M, for flame test) • Dilute hydrochloric acid (1 M) • Dilute sodium hydroxide (1 M) • Barium chloride solution (1 M) • Dilute nitric acid (1 M) • Silver nitrate solution (0.05 M) • Limewater
Chemical reactions involved For solution A
Test for Cu2+ions Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) Test for SO42−ions Ba2+(aq) + SO42−(aq) → BaSO4(s) For solution B
Test for CO32−ions CO32−(aq) + 2H+(aq) → H2O(l) + CO2(g) Limewater test CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) For solution C
Test for Cl−ions Ag+(aq) + Cl−(aq) → AgCl(s)
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Procedure 1. 2.
3.
The appearance of the samples were recorded. Flame tests were carried out to prove the identity of cations. (a) A nichrome wire was dipped in concentrated hydrochloric acid and then heated in a Bunsen flame, with air hole fully opened, until it no longer gave any characteristic coloured flame. (b) The sample was mixed with concentrated hydrochloric acid on a watch glass and the nichrome wire was moistened with this solution. The wire was then heated strongly in a non-luminous flame. The flame colour was observed and recorded. Each sample was divided into four equal portions in four separate test tubes. (a) To the first test tube of solution, a few drops of sodium hydroxide solution was added. The precipitate that was formed (if any) was tested by the addition of excess sodium hydroxide solution to see whether it re-dissolved or not. (b) To the second test tube of solution, a few drops of dilute hydrochloric acid was added. The gas evolved (if any) was then tested by bubbling into another test tube containing limewater. (c) To the third test tube of solution, a few drops of barium chloride solution was added. The precipitate formed (if any) was tested by adding some dilute hydrochloric acid to see whether it re-dissolved or not. Any gas evolved was tested with limewater. (d) To the fourth test tube of solution, a few drops of silver nitrate solution was added. The precipitate formed (if any) was tested by adding some dilute nitric acid to see whether it re-dissolved or not.
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Observation A
B
C
Appearance of solution
Blue
Colourless
Colourless
Flame test
Bluish green flame
Brilliant golden yellow flame
Brick red flame
Addition of sodium hydroxide
Blue precipitate formed
No observable change
White precipitate formed
Addition of hydrochloric acid and testing with limewater
The ppt. did not
The ppt. did not
re-dissolve in excess NaOH
re-dissolve in excess NaOH
No observable change
Effervescence
No observable change
Limewater turned milky Addition of barium chloride solution + dilute HCl and testing with limewater
White precipitate formed
White precipitate formed
No observable change
Effervescence Limewater turned milky Addition of silver nitrate solution + dilute HNO3
No observable change
White precipitate formed
After HNO3 was added, effervescence occurred and ppt.
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White precipitate formed
After HNO3 was added, ppt. remained undissolved
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dissolved
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Interpretation
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1.
For solution A
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Test for Cu2+ions
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The flame colour observed was bluish green.
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Blue precipitate formed when a few drops of sodium hydroxide solution was added.
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Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
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The blue precipitate remained undissolved in excess sodium hydroxide solution.
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Test for SO42−ions
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White precipitate formed when a few drops of barium chloride solution was added.
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Ba2+(aq) + SO42−(aq) → BaSO4(s)
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The white precipitate formed did not re-dissolve in dilute hydrochloric acid.
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So, A is copper(II) sulphate solution.
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2.
For solution B
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Test for Na+ions
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The flame colour observed was brilliant golden yellow.
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Test for CO32−ions
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Carbon dioxide was formed when dilute hydrochloric acid was added.
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CO32−(aq) + 2H+(aq) → H2O(l) + CO2(g)
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Limewater test
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Carbon dioxide turned limewater milky.
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CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
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So, B is sodium carbonate solution.
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3.
For solution C
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Test for Ca2+ions
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The flame colour observed was brick red. White precipitate (calcium hydroxide) was formed when sodium hydroxide solution was added. Ca2+(aq) + 2OH−(aq) → Ca(OH)2(s) The white precipitate formed did not re-dissolve in excess sodium hydroxide solution.
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Test for Cl−ions White precipitate (silver chloride) was formed when a few drops of silver nitrate solution was added. Ag+(aq) + Cl−(aq) → AgCl(s) The white precipitate formed did not re-dissolve in dilute nitric acid. So, C is calcium chloride solution.
Discussion 1.
Test for halides To test for halides, we often use silver nitrate (AgNO3) solution because it forms insoluble silver halides. Ag+(aq) + Cl−(aq) → AgCl(s) white precipitate
However, carbonate and hydroxide of silver are also insoluble in water. Thus, we have to get rid of carbonate and hydroxide by reacting them with dilute nitric acid. (precipitate remains insoluble) AgCl(s) + HNO3(aq) → no reaction (precipitate dissolved) AgOH(s) + HNO3(aq) → AgNO3(aq) + H2O(l) (precipitate dissolved) Ag2CO3(s) + 2HNO3 (aq) → 2AgNO3(aq) + CO2(g) + H2O(l) 2.
Test for sulphates Barium ions form insoluble salts with sulphate ions. Thus we can use barium ion-containing reagents such as barium chloride or barium nitrate to test for the presence of sulphate ions. Ba2+(aq) + SO42−(aq) → BaSO4(s) white precipitate
Carbonates and sulphites can also react with barium ions to form insoluble salts, which react with acids to give colourless carbon dioxide gas and colourless sulphur dioxide gas respectively. 2+ Ba (aq) + CO32−(aq) → BaCO3(s) white precipitate
BaCO3(s) + 2H (aq) → Ba2+(aq) + CO2(g) + H2O(l) Ba2+(aq) + SO32−(aq) → BaSO3(s) +
white precipitate
BaSO3(s) + 2H+(aq) → Ba2+(aq) + SO2(g) + H2O(l)
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3.
Test for copper(II) ions Copper(II) ions form precipitate with sodium hydroxide solution. Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) blue precipitate
The precipitate remained undissolved in excess sodium hydroxide. 4. Test for carbonates Addition of dilute acids to carbonates produce colourless carbon dioxide gas. CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) colourless gas
Carbonates and hydrogencarbonates show very similar behaviour in the above test. We can use magnesium sulphate solution to distinguish them. Mg2+(aq) + CO32−(aq) → MgCO3(s) white precipitate − 2+ (colourless solution formed) Mg (aq) + 2HCO3 (aq) → Mg (HCO3)2(aq)
Conclusion 1. 2. 3. 4. 5.
Metals and metal-containing compounds give characteristic flame colour when heated in Bunsen flame. The colour can help to identify the metal ions present in a compound. Some metal ions form precipitates with sodium hydroxide solution. Halides can be tested by silver nitrate solution. Carbonates can be tested by dilute acids. Carbon dioxide is formed in the reaction which turns limewater milky. Barium ion-containing reagents such as barium chloride, can be used to test for the presence of sulphate.
Answers to questions for further thought 1.
(a)
(b) 2.
(a)
(b)
Physical properties do not involve the change of a substance into other substances. Examples: Appearance, colour, taste, hardness, density, solubility behaviour, melting point, boiling point, malleability, ductility, (or other reasonable answers) Chemical properties involve the chemical reactions and the conditions of the reactions. Examples: Sulphur burns in air to form sulphur dioxide, (or other reasonable answers) Much of the attention is paid on the qualitative analysis but not quantitative ones. For example, we can only tell whether a sample of water contains chloride ions, but not the concentration of the ions in water. A large quantity of sample is required to perform the tests.
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