Chemical Tests for Gases in the ESAT
Updated July 2026
Identification of gases is a fundamental laboratory skill in Chemistry. For the ESAT, you must recognise the characteristic qualitative tests for hydrogen, oxygen, carbon dioxide, and chlorine. These tests involve distinctive observations, such as specific acoustic signatures, changes in combustion, or chemical bleaching of indicator papers.
Chemical identification tests use characteristic reactions to confirm the identity of unknown gases: hydrogen is identified by a squeaky pop, oxygen by relighting a glowing splint, carbon dioxide by clouding limewater, and chlorine by bleaching damp litmus paper.
Qualitative Analysis of Gases
Qualitative analysis is used to identify the presence of specific substances based on their unique chemical properties. In laboratory chemistry, four gases are frequently encountered as products of reactions: hydrogen, oxygen, carbon dioxide, and chlorine. To succeed in the ESAT, you must know the procedure and the expected observation for each of these tests.
The Test for Hydrogen Gas
Hydrogen () is a colourless, odourless, and highly flammable gas. When hydrogen is present in a test tube, it can be identified using a burning splint. The procedure involves holding the burning splint at the open end of the test tube. If the gas is hydrogen, it will react with the oxygen in the surrounding air to produce an explosion that manifests as a characteristic 'squeaky pop' sound. This occurs because the hydrogen gas burns rapidly in a small, contained space. The chemical reaction occurring is:
The Test for Oxygen Gas
Oxygen () is essential for combustion but does not burn itself. Instead, it supports the combustion of other materials. The test for oxygen involves using a glowing splint. A glowing splint is a wooden stick that has been lit and then blown out so that only the tip remains smouldering and red. When this glowing splint is inserted into a container of oxygen, the splint will relight and burst back into flame. The increased concentration of oxygen molecules at the surface of the wood accelerates the rate of oxidation, causing the flame to reform.
The Test for Carbon Dioxide Gas
Carbon dioxide () is a colourless gas that is often produced in combustion or acid-carbonate reactions. It is identified using an aqueous solution of calcium hydroxide, commonly referred to as limewater. To perform the test, the gas is shaken with the limewater or bubbled through it. If the gas is carbon dioxide, the limewater turns cloudy or milky. This change is caused by the formation of an insoluble white precipitate of calcium carbonate (). The equation for the reaction is:
The Test for Chlorine Gas
Chlorine () is a pale green gas that is both toxic and highly reactive. It acts as an oxidising agent and a bleaching agent. To test for chlorine, damp blue litmus paper is used. When the paper is exposed to the gas, it first turns red and then is bleached white. It is essential that the litmus paper is damp because the chlorine gas must dissolve in water to produce the chemical species responsible for the changes. Chlorine reacts with the water on the paper to form hydrochloric acid (), which turns the litmus red, and hypochlorous acid (), which acts as the bleach and turns the paper white.
Key takeaways
- Hydrogen creates a squeaky pop when a burning splint is held at the test tube mouth.
- Oxygen relights a glowing splint due to supported combustion.
- Carbon dioxide forms a white precipitate of calcium carbonate when shaken with limewater.
- Chlorine gas bleaches damp blue litmus paper after an initial turn to red.
In the ESAT exam, look for descriptions of 'squeaky pops' or 'relighting' to identify gases in the context of longer multi-step reaction questions. Always confirm if the litmus paper mentioned is 'damp' in the chlorine test description.
Do not confuse the hydrogen and oxygen splint tests. Using a glowing splint for hydrogen will often fail to produce the pop because it lacks the activation energy to start the reaction, while using a burning splint for oxygen will simply make the flame bigger rather than demonstrating the 'relighting' of an extinguished splint.
The chlorine test is an example of a redox process where the chlorine gas is acting as a strong oxidising agent. The bleaching effect occurs because the hypochlorous acid () oxidises the organic dyes in the litmus paper, breaking the chemical bonds that allow them to absorb specific wavelengths of light.
Frequently asked questions
Why must the litmus paper be damp when testing for chlorine?
The litmus paper must be damp because chlorine gas reacts with water to form the acids ( and ) required for the chemical change. Without water, the gas cannot interact effectively with the indicator dye in the paper.
What is the difference between the splint used for oxygen and the one used for hydrogen?
The oxygen test requires a glowing splint (smouldering but no flame), which then relights. The hydrogen test requires a burning splint (an active flame), which causes the squeaky pop explosion.
Will carbon dioxide relight a glowing splint?
No, carbon dioxide does not support combustion. In fact, it will extinguish a burning or glowing splint, though this is not a definitive test because other gases like nitrogen also extinguish flames. The limewater test is the specific identification test.