Tests for gases

Each common gas has its own simple test. Hydrogen burns with a squeaky pop, oxygen relights a glowing splint, carbon dioxide turns limewater milky, chlorine bleaches damp litmus paper and ammonia turns damp red litmus paper blue. A good test gives a result that only that gas gives.

Key facts

  • Gases: hydrogen burns with a squeaky pop; oxygen relights a glowing splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper.
  • Ammonia turns damp red litmus paper blue.
  • A test must be specific: many gases put out a lit splint, but only carbon dioxide turns limewater milky.

Worked examples

Example 1

A gas is collected and tested: a lighted splint burns with a squeaky pop. Which gas is it?

  1. The test for hydrogen: a lighted splint burns with a squeaky pop.
  2. So the gas is hydrogen.

Answer: hydrogen

Example 2

A gas is collected and tested: it relights a glowing splint. Which gas is it?

  1. The test for oxygen: it relights a glowing splint.
  2. So the gas is oxygen.

Answer: oxygen

Example 3

Describe the tests for hydrogen, oxygen and carbon dioxide, and give the result of each.

  1. Hydrogen: hold a lit splint at the mouth of the tube. It burns with a squeaky pop.
  2. Oxygen: put a glowing splint into the gas. It relights.
  3. Carbon dioxide: bubble it through limewater. The limewater turns milky (cloudy).

Answer: hydrogen: a lit splint gives a squeaky pop; oxygen: it relights a glowing splint; carbon dioxide: it turns limewater milky

Example 4

How do you test for chlorine, and what is the result?

  1. Chlorine dissolves in the water on damp litmus paper, making an acidic solution, so blue litmus turns red.
  2. Chlorine is also a bleach, so the paper then turns white. Do the test in a fume cupboard: chlorine is toxic.

Answer: hold damp blue litmus paper in the gas: it turns red and then white (it is bleached)

Example 5

A gas from a reaction relights a glowing splint. Which gas is it? Name a reaction that could have made it.

  1. Relighting a glowing splint is the test for oxygen.
  2. Hydrogen peroxide breaks down into water and oxygen, quickly when a catalyst such as manganese(IV) oxide is added.

Answer: oxygen; for example, hydrogen peroxide breaking down with manganese(IV) oxide as a catalyst

Example 6

Why is limewater used to test for carbon dioxide, rather than seeing whether the gas puts out a lit splint?

  1. A good test gives its result with one substance only.
  2. A lit splint goes out in carbon dioxide, but also in nitrogen and other gases that do not support burning.
  3. Carbon dioxide reacts with limewater (calcium hydroxide solution) to form insoluble calcium carbonate, which makes it milky.

Answer: many gases, such as nitrogen, put out a lit splint, so that does not identify carbon dioxide; only carbon dioxide turns limewater milky

Example 7

A gas is collected and tested: it turns damp red litmus paper blue. Which gas is it?

  1. The test for ammonia: it turns damp red litmus paper blue.
  2. So the gas is ammonia.

Answer: ammonia