0-4 General Laboratory Technique

IV. General Laboratory Technique

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Topic

IV.

General Laboratory Technique

Reference Reading

0.4 Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 5 – 6, 216 – 226

Objectives Notes

A. Common Apparatus / Equipment

1.

test tube

7.

round-bottomed flask

2. 3.

test tube rack test tube holder

8. 9.

flat-bottomed flask spatula

4. 5. 6.

beaker wash bottle reagent bottle

10. wire gauze 11. tripod 12. Bunsen burner

13. heat resistant mat / fireproof mat 14. evaporating dish 15. crucible with lid 16. stand and clamp 17. measuring cylinder 18. dropper

19. filter funnel 20. mortar and pestle 21. triple beam balance 22. electronic balance 23. safety glasses

B. Common Laboratory Technique 1.

Operation of Bunsen burner

1.

Check the connection of the town gas supply. Insert the hose fully into the gas outlet. it is easier to insert the hose into the outlet by wetting the hose with a little water first. Place the Bunsen burner on a heat resistant mat to protect the table. Close the air hole to make sure that the gas will not burn in the barrel of the burner. Strike a match and place it 5 cm above the tip of the burner. Switch on the gas supply fully. Open the air hole to obtain a non-luminous flame. After use, close the air hole and then the gas supply.

2. 3. 4. 5. 6.

N.B.

1. 2.

Only a non-luminous flame should be used to heat a glassware. A yellow flame contains unburnt carbon particles which will blacken glasswares. The used match should be place on the heat resistant mat until it is completely extinguished.

a) Handling of striking back If the air hole of a Bunsen burner is opened and the gas supply is weak, the flame will burn inside the barrel. This is called striking back and a roaring sound will be heard. The flame will become green because the metal barrel is being burnt. When striking back happens, switch off the gas supply immediately. Do not touch the Bunsen burner because it is will be very hot. Allow it to cool down and may be used again later.

IV. General Laboratory Technique 2.

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Transferring chemical

Transferring of solid

Transferring of liquid

Spatula is used to transfer solid chemical. Unless a very large amount of solid is to be transferred, only the smaller end of the spatula should be used to avoid spillage.

When transferring liquid chemical from a reagent bottle, we follow the convention that the chemical would be poured from the side with no label. Thus, the label will not be stained by the chemical seeping down the edge of the bottle and your hand will not become the cloth to wipe the chemical away.

spatula

It is important to wait and allow the last drop of chemical to run into the test tube before taking the bottle away to avoid spillage. This can be done by tapping the bottle mouth against the mouth of the test tube. N.B.

3.

More importantly, NO excess chemical should be returned into the reagent bottle otherwise the chemical may be contaminated (get dirty). Put the extra chemical at the proper place of disposal as directed by the teacher.

Mixing solution

In order to mix the solution in a test tube well, different methods may be used depending on the volume of the solution. Shaking

Less than 2 cm3 of solution

Swirling

More than 2 cm3 but less than

Pouring fro and back

2 of the test tube 3

More than

2 of the test tube 3

IV. General Laboratory Technique 4.

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Heating technique

a) Heating hydrated solid When a solid containing water is heated in a test tube, the tube should be pointed a little downward as shown in the diagram. This is because the mouth of the tube is cooler than the bottom of the tube. The steam will condensed into water at the mouth. If the test tube is held vertically, the water condensed may fall back to the bottom and crack the test tube because of local contraction of glass.

b) Heating solution (1) Heating solution in a test tube First of all, test tube is not designed for heating solution strongly, the water will spurt out from the narrow mouth. Therefore, solution can only be heated up to 70 ºC. Furthermore, in order to avoid spurting of solution, the tube should be swirled continuously and taken away from the flame occasionally while being heated. Antibumping granules (boiling stones) may also be added to make the boiling smoother by breaking the large bubble into small ones.

(2) Heating flammable liquid Flammable liquid should never be heated with a naked flame. The vapour evaporated may catch fire suddenly. Flammable liquid should be heated indirectly by using a steam bath, a water bath and an oil bath depending on the temperature required.

steam bath

water bath

oil bath

 → increasing temperature

A steam bath is usually used instead of a water bath if the liquid have to be evaporated to extract the solid from the solution.

IV. General Laboratory Technique 5

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Collection of gases

There are different methods of collection of gas and each method has its limitation.

Displacement of water It cannot be used for soluble gas. e.g. ammonia and sulphur dioxide.. The gas collected will also be wet.

Upward delivery of gas / Downward displacement of air Only gases lighter than air could be collected. e.g. hydrogen and helium. The gas would be contaminated by air.

Gas syringe – it can be used to collect all kind of gas but it is expensive and only a limited volume of gas can be collected.

a) Prevention of sucking back If a gas evolving from a heated source is to be collected, special precaution have to be taken when displacement of water is used. The delivery tube must be removed from the trough before the Bunsen burner is switched off. If the flame is removed first, the gas pressure inside the tube will drop and the atmospheric pressure will force the cool water up the delivery tube and crack the hot test tube. This is called sucking back.

Downward delivery of gas / Upward displacement of air Only gases heavier than air could be collected. e.g. carbon dioxide. The gas would be contaminated by air.

IV. General Laboratory Technique 6.

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Drying substances

Desiccator It is the dry box used in laboratory. Common solid drying agents like silica gel, anhydrous calcium chloride, anhydrous calcium oxide, are put at the bottom to absorb moisture.

Wash bottle It is used to dry a gas by allowing it to pass through a liquid drying agent, concentrated sulphuric acid. The incoming delivery tube should be immersed in the acid and the outgoing tube must be above the acid surface. If the connection is reversed mistakenly, the acid would be pushed out of the bottle by the pressure of the incoming gas. U-tube The operation of an U-tube is similar to that of a wash bottle. In an U-tube, solid drying agent is used instead.

Glossary

Past Paper Questions

apparatus equipment reagent crucible spatula teat pipette mortar non-luminous flame striking back contamination swirling fro and back local contraction anti-bumping granules naked flame steam bath water bath displacement of water upward delivery of gas / downward displacement of air downward delivery of gas / upward displacement of air gas syringe sucking back desiccator wash bottle U-tube silica gel anhydrous calcium chloride anhydrous calcium oxide concentrated sulphuric acid

pestle oil bath

93 I 5 b i 94 I 2 a b

93 I 5 b i 5b The diagram below shows an apparatus, which, when it contains water, is part of the set-up used in the preparation of chlorine gas.

i

Copy the diagram into your answer book. Label clearly (1) the level of water that should be used in the apparatus. (2) the direction of the chlorine gas passing through the apparatus.

2

IV. General Laboratory Technique

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2 marks 94 I 2 a b 2 For each of the following experiments, decide and explain which of the experimental set-ups, X or Y, should be used. Experiment Set-up X Set-up Y 2a (a) To dehydrate hydrated iron(II) sulphate crystals

2b

C

NOTE : Award 0 mark for wrong choice of set-up. X The test tube is slanted / tilted to prevent the (condensed) water from running back and crack / break the (hot) test tube. (b) To dry chlorine gas

2 1 mark 1 mark

NOTE : Award 0 mark for wrong choice of set-up. X With set-up X, most / all of the gas can get into contact with / flow (pass / go) through the drying agent, 1 mark hence the gas can be dried more effectively. 1 mark In part (b), some candidates chose set-up Y because they thought that chlorine, being a dense gas, would stay at the bottom of the U-tube and could not pass out after drying.

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