05 Purif
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PURIFYING COPPER (II) SULPHATE AND POTASSIUM NITRATE Introduction A substance is said to be soluble in a solvent (a liquid in most cases) when it forms a solution (“disappears” in it). But the solubilities of different substances (that is, how much of them will dissolve in 100 g of the solvent) are very different. Moreover, solubility also depends on temperature. Look carefully at the chart below. If you read along a row, say, the one that starts with “20 degrees” you will find that the solubility in water for the four substances at that temperature is very different. On the other hand, if you read down a column, say, the potassium nitrate column, you will notice how dramatically solubility changes with temperature for this substance. For sodium chloride instead, solubility changes just slightly.
Examples of
SOLUBILITY DATA salt / 100g water
g
Salt name and potassium formula → potassium sulphate nitrate KNO3 K2SO4 Temp. deg C ↓
sodium NaCl
0
13.9
7.4
35.7
14.3
10
21.2
9.3
35.8
17.4
20
31.6
11.1
36.0
20.7
30
45.3
13.0
36.2
24.2
40
61.4
14.8
36.5
28.7
50
83.5
16.5
36.8
33.8
60
106.0
18.2
37.3
40.0
19.8
37.6
47.0
21.4
38.1
56.0
22.9
38.6
67.5
24.1
39.2
80.0
70 80
169
90 100
246
chloride
copper(II)sulphate CuSO4 • 5 H2O
answer Try to answer these questions finding the relevant information in the table: 1- Which is the less soluble substance at 0°C? 2- Which is the less soluble substance at 100°C? 3- Which is the substance that has the highest solubility at 20°C? 4- Suppose you add 100g of water to 60 g of copper (II) sulphate and heat to 80°C: will it dissolve completely? 5- How many grams will remain undissolved? 6- What would happen if you cool down the solution in (5) to 10°C? These differences in solubility are used by chemists to purify substances: you can dissolve them in a little hot water, then filter hot to get rid of the insoluble dirt and then cool down and wait for crystals to appear. Impurities that are soluble will remain dissolved and if the crystals are filtered, washed and dried, the sample will be purer than before. This procedure can be repeated over and over again and is called recrystallisation. Crystals look very nice both with the naked eye or looking through a microscope. That is because the particles that form the crystals are aligned (as gymnasts and soldiers do) in regular though different patterns. Aims To obtain pure substances starting with dirty samples. Apparatus Funnel, iron ring, retort stand, beaker, stirring rod, Bunsen burner, tripod, wire gauze, measuring cylinder, filter paper, watch glass, and a laboratory scale or balance. The Purification of Copper (II) Sulphate Procedure 1- Fix the apparatus as shown in the figure on the right side of the page 2- Weigh 15 g of impure copper (II) sulphate in a small beaker and add 30 ml of distilled water. Heat close to the boil while stirring with the glass rod. The solid should be completely dissolved. 3- The insoluble residue will be filtered in an apparatus as shown on the next page. Pour the solution carefully using a stirring rod to avoid spillages (the teacher will show you). 4- The filtrate is received on a second beaker or directly in an evaporating dish. Rinse the first beaker with some 10 ml of hot distilled water twice so that the filtrate looks nearly colourless. 5- As too much water may have been used, evaporate the filtrate on a boiling water bath
(already used in experiment Nr 2) moving the liquid with the rod frequently. As soon as you notice that crystals appear put out the burner. 6- Allow to cool slowly to room temperature. 7- If crystals do not appear leave for the next class. Filter the crystals with the apparatus previously described. Wash them with a few ml of iced water and put them on a watch glass or a piece of filter paper. 8- Let them dry. You can keep the crystals in a polythene bag. Copper (II) sulphate is an algaecide used for maintenance of swimming pools. Have you got some at home? Be careful with it because it is poisonous.
The Purification of Potassium Nitrate Procedure 1- Fix once again the apparatus as shown for copper (II) sulphate. 2- Weigh 15 g of impure potassium nitrate in a boiling tube and add 15 ml of distilled water. Heat in a boiling water bath while stirring with the glass rod. The solid should be completely dissolved; otherwise add small portions of distilled water until it does. 3- As previously done, filter hot to eliminate insoluble grit. Use the same apparatus as you used for the copper (II) sulphate experiment. 4- The filtrate is received on a second beaker or directly in an evaporating dish. Rinse the first beaker with some 5 ml of hot distilled water twice so that the filtrate looks nearly colourless. 5- As too much water may have been used, evaporate partially on a boiling water bath moving the liquid with the rod frequently. 6- As soon as you notice that crystals appear transfer the liquid to a small test tube and cool it in an ice – water bath. 7- When the solution is chilled, filter the crystals. 8- Let the crystals dry on a piece of filter paper.
Examples of
SOLUBILITY DATA salt / 100g water
g
Salt name and potassium formula → potassium sulphate nitrate KNO3 K2SO4 Temp. deg C ↓
sodium NaCl
0
13.9
7.4
35.7
14.3
10
21.2
9.3
35.8
17.4
20
31.6
11.1
36.0
20.7
30
45.3
13.0
36.2
24.2
40
61.4
14.8
36.5
28.7
50
83.5
16.5
36.8
33.8
60
106.0
18.2
37.3
40.0
19.8
37.6
47.0
21.4
38.1
56.0
22.9
38.6
67.5
24.1
39.2
80.0
70 80
169
90 100
246
chloride
copper(II)sulphate CuSO4 • 5 H2O
answer Try to answer these questions finding the relevant information in the table: 1- Which is the less soluble substance at 0°C? 2- Which is the less soluble substance at 100°C? 3- Which is the substance that has the highest solubility at 20°C? 4- Suppose you add 100g of water to 60 g of copper (II) sulphate and heat to 80°C: will it dissolve completely? 5- How many grams will remain undissolved? 6- What would happen if you cool down the solution in (5) to 10°C? These differences in solubility are used by chemists to purify substances: you can dissolve them in a little hot water, then filter hot to get rid of the insoluble dirt and then cool down and wait for crystals to appear. Impurities that are soluble will remain dissolved and if the crystals are filtered, washed and dried, the sample will be purer than before. This procedure can be repeated over and over again and is called recrystallisation. Crystals look very nice both with the naked eye or looking through a microscope. That is because the particles that form the crystals are aligned (as gymnasts and soldiers do) in regular though different patterns. Aims To obtain pure substances starting with dirty samples. Apparatus Funnel, iron ring, retort stand, beaker, stirring rod, Bunsen burner, tripod, wire gauze, measuring cylinder, filter paper, watch glass, and a laboratory scale or balance. The Purification of Copper (II) Sulphate Procedure 1- Fix the apparatus as shown in the figure on the right side of the page 2- Weigh 15 g of impure copper (II) sulphate in a small beaker and add 30 ml of distilled water. Heat close to the boil while stirring with the glass rod. The solid should be completely dissolved. 3- The insoluble residue will be filtered in an apparatus as shown on the next page. Pour the solution carefully using a stirring rod to avoid spillages (the teacher will show you). 4- The filtrate is received on a second beaker or directly in an evaporating dish. Rinse the first beaker with some 10 ml of hot distilled water twice so that the filtrate looks nearly colourless. 5- As too much water may have been used, evaporate the filtrate on a boiling water bath
(already used in experiment Nr 2) moving the liquid with the rod frequently. As soon as you notice that crystals appear put out the burner. 6- Allow to cool slowly to room temperature. 7- If crystals do not appear leave for the next class. Filter the crystals with the apparatus previously described. Wash them with a few ml of iced water and put them on a watch glass or a piece of filter paper. 8- Let them dry. You can keep the crystals in a polythene bag. Copper (II) sulphate is an algaecide used for maintenance of swimming pools. Have you got some at home? Be careful with it because it is poisonous.
The Purification of Potassium Nitrate Procedure 1- Fix once again the apparatus as shown for copper (II) sulphate. 2- Weigh 15 g of impure potassium nitrate in a boiling tube and add 15 ml of distilled water. Heat in a boiling water bath while stirring with the glass rod. The solid should be completely dissolved; otherwise add small portions of distilled water until it does. 3- As previously done, filter hot to eliminate insoluble grit. Use the same apparatus as you used for the copper (II) sulphate experiment. 4- The filtrate is received on a second beaker or directly in an evaporating dish. Rinse the first beaker with some 5 ml of hot distilled water twice so that the filtrate looks nearly colourless. 5- As too much water may have been used, evaporate partially on a boiling water bath moving the liquid with the rod frequently. 6- As soon as you notice that crystals appear transfer the liquid to a small test tube and cool it in an ice – water bath. 7- When the solution is chilled, filter the crystals. 8- Let the crystals dry on a piece of filter paper.
