The four gas tests you must know, what a positive result looks like for each, and the common mistakes that lose marks.
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1.How do you test for hydrogen?
Hold a burning splint at the open end of a test tube of the gas.
2.What is a positive result for hydrogen?
The gas burns rapidly with a squeaky pop.
3.How do you test for oxygen?
Put a glowing splint into a test tube of the gas.
4.What is a positive result for oxygen?
The glowing splint relights.
5.How do you test for carbon dioxide?
Bubble the gas through limewater, which is an aqueous solution of calcium hydroxide.
6.What is a positive result for carbon dioxide?
The limewater turns cloudy or milky white.
7.How do you test for chlorine?
Hold damp litmus paper in the gas.
8.What is a positive result for chlorine?
The litmus paper is bleached and turns white.
9.Why must the litmus paper be damp for the chlorine test?
The chlorine must dissolve in the water on the paper before it can bleach it.
10.Why is a burning splint used for hydrogen but a glowing one for oxygen?
Hydrogen must be ignited to burn, whereas oxygen does not burn itself but relights a splint that is already glowing.
11.A student says oxygen burns with a squeaky pop. What is wrong?
Oxygen does not burn. It supports combustion, so it relights a glowing splint. The squeaky pop is the hydrogen test.
12.Which gas turns damp litmus red then white?
Chlorine. It is acidic in solution as well as being a bleach.
13.Why does limewater go cloudy with carbon dioxide?
An insoluble white precipitate of calcium carbonate is formed.
14.Which reaction would you use to produce carbon dioxide for testing?
A metal carbonate reacting with a dilute acid.
15.Which reaction would you use to produce hydrogen for testing?
A reactive metal such as magnesium reacting with a dilute acid.
16.Why must a test tube of gas be kept upright and stoppered?
Many of these gases are less dense than air or will diffuse away, so the sample would be lost.
17.Which of the four gases is toxic?
Chlorine, which is why the test should be carried out in a fume cupboard or with good ventilation.
18.What safety precaution applies to the hydrogen test?
Only test a small volume, since hydrogen is highly flammable and larger amounts can explode.
19.What would you observe if limewater was bubbled with air?
It would go cloudy only very slowly, because air contains only a small proportion of carbon dioxide.
20.Why are these tests described as qualitative?
They identify which substance is present but say nothing about how much of it there is.
21.If a gas relights a glowing splint, what can you conclude?
It is oxygen, or at least contains a high proportion of oxygen.
22.Which two tests both use a splint?
Hydrogen, with a burning splint, and oxygen, with a glowing splint.
23.Which test involves a precipitate forming?
The carbon dioxide test, where calcium carbonate precipitates and makes the limewater cloudy.
24.Why is it important to describe the result and not just name the gas?
Marks are given for the observation, so the answer must state what is seen, such as the splint relighting.
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