Tests for gases (AQA GCSE Chemistry Combined Science): Revision Notes
Tests for gases
Gas identification is an important skill in chemistry. You can identify four common gases - hydrogen, oxygen, carbon dioxide, and chlorine - using simple laboratory tests. Each gas has its own unique test that gives a clear, observable result.
Test for hydrogen
Hydrogen gas can be identified using the burning splint test. This is one of the most dramatic gas tests you'll see!
Procedure for the Burning Splint Test:
- Light a wooden splint so it has a flame
- Hold the burning splint near the mouth of the test tube containing the gas
- Listen carefully for the result
What happens: Hydrogen burns very quickly when it meets the flame. This creates a distinctive squeaky pop sound. The pop happens because hydrogen burns rapidly and produces a small explosion.
Why this works: Hydrogen is highly flammable and reacts instantly with oxygen in the air when a flame is present.
Worked Example: Identifying Hydrogen Gas
- Setup: You have a test tube containing an unknown gas that you suspect is hydrogen
- Test: Light a wooden splint and carefully bring the flame to the mouth of the test tube
- Observation: You hear a distinctive "pop" sound as the gas ignites
- Conclusion: The squeaky pop sound confirms the gas is hydrogen
Test for oxygen
Oxygen gas is tested using the glowing splint test. This test shows oxygen's ability to support combustion.
Procedure for the Glowing Splint Test:
- Light a wooden splint and let it burn for a moment
- Blow out the flame but leave the end glowing red-hot
- Quickly place the glowing splint into the gas
What happens: The glowing splint will burst back into flames (relight). This happens because oxygen supports burning very well.
Why this works: Oxygen helps things burn. When there's extra oxygen present, even a barely glowing piece of wood can catch fire again.
Test for carbon dioxide
Carbon dioxide is identified using limewater (calcium hydroxide solution). This test produces a very clear visual change.
Procedure for the Limewater Test:
- Put some clear limewater into a test tube
- Bubble the suspected carbon dioxide gas through the limewater
- Watch for changes in the limewater's appearance
What happens: The clear limewater turns milky or cloudy white. This milky substance is calcium carbonate, which forms when carbon dioxide reacts with calcium hydroxide.
The chemical reaction:
Why this works: Carbon dioxide reacts with calcium hydroxide to make tiny particles of calcium carbonate that make the solution look milky.
Worked Example: Testing for Carbon dioxide
- Setup: Prepare clear limewater in a test tube
- Test: Bubble the unknown gas through the limewater using a delivery tube
- Observation: The limewater changes from clear to milky white
- Conclusion: The formation of the milky precipitate confirms carbon dioxide is present
Test for chlorine
Chlorine gas is identified using litmus paper. This test shows chlorine's powerful bleaching properties.
Procedure for the Litmus Paper Test:
- Take a piece of damp litmus paper (red or blue both work)
- Place the litmus paper into the gas or hold it near the gas source
- Observe any colour changes
What happens:
- Red litmus paper: Changes from red to white (bleached)
- Blue litmus paper: First changes from blue to red, then becomes white (bleached)
Why this works: Chlorine is a strong bleaching agent. It removes colour from dyes and indicators by breaking down the molecules that create the colour.
Critical Safety Information
Always work in a well-ventilated area when testing gases. Chlorine gas is particularly harmful, so only use small amounts and avoid breathing it in. Never test large quantities of chlorine gas without proper safety equipment.
Key Points to Remember:
- Hydrogen: Makes a squeaky pop with a burning splint
- Oxygen: Relights a glowing splint
- Carbon dioxide: Turns limewater milky white
- Chlorine: Bleaches litmus paper to white
- Each gas has a unique test that gives a clear, unmistakable result
- Always follow proper safety procedures when working with gases