Tests for gases – GCSE Combined Science Chemistry

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Most gases are colourless, so you cannot tell one from another by looking. Chemists use a quick test for each gas. Each test gives a result you can see or hear.

These tests matter because many reactions give off a gas. If you can name the gas, you can work out what happened. A fizz with a metal and an acid is hydrogen. A fizz with a carbonate and an acid is carbon dioxide.

By the end you will know four tests and what a positive result looks like. You will also be able to say which gas is which from the results in a table, and how to do the tests safely.

Why we test gases

A positive test means the gas gives the result in the table. Gases give different results, so you can tell them apart. Always give both parts of an answer: the test and the result.

Gases come from reactions you already know. A metal plus an acid makes hydrogen. A carbonate plus an acid makes carbon dioxide. Hydrogen peroxide breaks down to make oxygen. The electrolysis of salt solution makes hydrogen and chlorine.

Hydrogen: the squeaky pop

Collect the gas in a test tube. Hold a lit splint at the open end of the tube. If the gas is hydrogen, you hear a squeaky pop.

The pop is a small explosion. The hydrogen burns very quickly with the oxygen in the air to make water. The equation is hydrogen + oxygen → water.

Hydrogen is made when a metal such as magnesium or zinc reacts with an acid. Hydrogen is flammable, so keep naked flames away until you do the test and use only a small amount.

Oxygen: the glowing splint

Light a wooden splint, then blow it out so that it is only glowing. Put the glowing splint into the test tube. If the gas is oxygen, the splint relights.

Oxygen is needed for burning. In pure oxygen a glowing splint has enough oxygen to burst back into flame. In ordinary air there is only about one fifth oxygen, so a glowing splint does not relight.

You can make oxygen by breaking down hydrogen peroxide with manganese dioxide. The manganese dioxide is a catalyst. It speeds up the reaction and is not used up.

Carbon dioxide: limewater

Bubble the gas through limewater. Limewater is a clear, colourless solution of calcium hydroxide. If the gas is carbon dioxide, the limewater turns milky (cloudy).

The milky colour comes from a white solid called calcium carbonate. It forms when carbon dioxide reacts with the calcium hydroxide: calcium hydroxide + carbon dioxide → calcium carbonate + water.

You get carbon dioxide when an acid reacts with a carbonate, for example marble chips (calcium carbonate) with hydrochloric acid. You also breathe it out. Limewater is the standard test. A burning splint going out does not prove it is carbon dioxide, because other gases put out a flame too.

Chlorine: damp litmus paper

Hold a piece of damp blue litmus paper in the gas. If the gas is chlorine, the litmus paper is bleached white. Dry paper is not bleached, so the paper must be damp.

Chlorine is a pale green, poisonous gas with a sharp smell. It irritates the eyes and lungs. Always test for it in a fume cupboard, and use only a small amount.

Chlorine is made at the positive electrode when sodium chloride solution is electrolysed. Hydrogen is made at the negative electrode in the same experiment, so you can test both gases from one cell.

Which gas is it?

To name a gas, match the result to the table. Squeaky pop means hydrogen. Relights a glowing splint means oxygen. Limewater milky means carbon dioxide. Damp litmus bleached means chlorine.

Use the reaction as a clue too. A metal with an acid gives hydrogen. A carbonate with an acid gives carbon dioxide. A test result is more reliable than a guess, so always carry out the test.

Stay safe. Wear eye protection. Use small amounts of gas. Work in a fume cupboard for chlorine. Do not hold a flame near hydrogen in a large amount, because it can explode. Limewater is mildly irritating, so wash your hands if you spill it.

Recap

Hydrogen gives a squeaky pop with a lit splint. Oxygen relights a glowing splint.

Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper white.

Write the test and the result in your answer. Use the reaction as a clue, then prove it with the test.