Testing for Gases and Identifying Ions

This lesson covers the chemical tests for gases and methods for identifying ions that you need to know for the AQA GCSE Combined Science Trilogy (8464) specification. Being able to carry out and describe these tests is essential for both practical work and written exams.

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Tests for Gases

You must know the tests for four gases: hydrogen, oxygen, carbon dioxide, and chlorine.

Gas	Test	Positive Result
Hydrogen (H₂)	Hold a burning splint near the gas	Burns with a squeaky pop
Oxygen (O₂)	Insert a glowing splint into the gas	Splint relights
Carbon dioxide (CO₂)	Bubble through limewater (calcium hydroxide solution)	Limewater turns milky (cloudy white)
Chlorine (Cl₂)	Hold damp litmus paper in the gas	Litmus paper is bleached white

Explaining the Limewater Test

$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$Ca(OH)2​(aq)+CO2​(g)→CaCO3​(s)+H2​O(l)

The milky appearance is caused by tiny particles of insoluble calcium carbonate (CaCO₃) suspended in the solution.