A student is asked to plan how to identify each of five different gases collected in separate tubes: hydrogen, oxygen, carbon dioxide, ammonia and chlorine....
Question 1 Report
A student is asked to plan how to identify each of five different gases collected in separate tubes: hydrogen, oxygen, carbon dioxide, ammonia and chlorine.
(a) Complete the table by giving a confirming test and the positive result for each of the five gases. [5]
(b) Two of the gases will both affect damp litmus paper. Explain how the student can use damp litmus paper to tell ammonia and chlorine apart. [3]
(c) Describe how you would plan a test to show that a colourless gas is carbon dioxide and not simply air, giving the apparatus and the positive result. [3]
(d) State which two of the five gases are the most hazardous and give one precaution for handling them. [3]
(e) Suggest why a lighted splint should not be used as the first test on an unknown gas. [2]
(f) Name the piece of apparatus used to hold a small volume of gas for testing at the mouth of a tube. [1]
Answer Details
This question tests the standard confirmatory tests for common gases and the safe way to carry them out. Each gas has one characteristic test, and two of the gases both act on damp litmus, so the difference between them must be described precisely.
(a) Confirming tests and positive results [5]:
| gas | test and positive result |
|---|---|
| hydrogen | lighted splint gives a squeaky pop [1] |
| oxygen | relights a glowing splint [1] |
| carbon dioxide | turns limewater milky [1] |
| ammonia | turns damp red litmus paper blue [1] |
| chlorine | bleaches damp litmus paper white [1] |
(b) Both ammonia and chlorine act on damp litmus, but in opposite ways [3]: ammonia is alkaline and turns damp red litmus blue [1]; chlorine turns damp litmus red then bleaches it white [1]; so the permanent loss of colour (bleaching) identifies chlorine, while the blue colour identifies ammonia [1].
(c) To show a colourless gas is carbon dioxide and not just air [3]: bubble the gas through limewater using a delivery tube [1]; if it is carbon dioxide the limewater turns milky [1]; air does not turn limewater milky, so the milkiness is the distinguishing evidence [1].
(d) The two most hazardous gases are chlorine and ammonia, both toxic and harmful to breathe in [1]; a suitable precaution is to work in a fume cupboard [1] and to use only small amounts / avoid breathing them in [1].
(e) A lighted splint should not be the first test because some gases, such as hydrogen or a hydrogen-air mixture, are flammable or explosive [1], so a flame could ignite them dangerously before the identity of the gas is known [1].
(f) The apparatus used to hold a small volume of gas for testing at the mouth of a tube is a test-tube holder / a pair of tongs [1].
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