Tests for gases (AQA GCSE Chemistry): Revision Notes
Tests for gases
Gas testing is a key practical skill in chemistry. You need to know how to identify four common gases using simple laboratory tests. These tests are based on the unique properties each gas shows when it reacts with certain substances.
Test for hydrogen
Hydrogen gas can be identified using a burning splint test. This test works because hydrogen is highly flammable and reacts rapidly with oxygen in the air.
Method:
- Light a wooden splint so it burns with a flame
- Put the burning splint near the mouth of the test tube containing the gas
- Listen carefully for the sound
Result: Hydrogen burns very quickly and makes a distinctive squeaky 'pop' sound. This happens because hydrogen reacts rapidly with oxygen in the air to form water.
Remember: Hydrogen goes 'pop'!
Test for oxygen
Oxygen gas is tested using a glowing splint test. This works because oxygen is essential for combustion and will rekindle any glowing material.
Method:
- Light a wooden splint and then blow out the flame
- Leave the splint glowing (with a red hot end)
- Put the glowing splint into the gas
Result: Oxygen will make the glowing splint burst back into flames. This happens because oxygen supports combustion (burning).
Remember: Oxygen relights a glowing splint.
Test for carbon dioxide
Carbon dioxide is identified using limewater. This test works because carbon dioxide forms an insoluble precipitate when it reacts with calcium hydroxide.
Method:
- Put some clear limewater into a test tube
- Bubble the gas through the limewater
- Watch what happens to the limewater
Result: Carbon dioxide makes the clear limewater turn milky or cloudy white. This happens because carbon dioxide reacts with calcium hydroxide (limewater) to make calcium carbonate, which is insoluble.
Chemical equation:
Remember: Carbon dioxide turns limewater milky.
Test for chlorine
Chlorine gas is tested using litmus paper. This test works because chlorine is both acidic and a powerful bleaching agent.
Method:
- Dampen a piece of blue litmus paper with water
- Hold the damp litmus paper in the gas
- Watch the colour changes
Result: Chlorine first turns the blue litmus paper red, then bleaches it completely white. This shows chlorine is acidic (turns blue litmus red) and is also a powerful bleaching agent.
Remember: Chlorine bleaches litmus paper white.
Safety considerations
Testing gases requires careful attention to safety procedures, especially when working with toxic or reactive substances.
Safety Warning: Always work in a well-ventilated area when testing gases. Some gases like chlorine are toxic and should only be tested in small amounts. Never inhale the gases directly.
Key takeaways
Understanding these four gas tests is essential for practical chemistry work and examinations.
Key Points to Remember:
- Hydrogen makes a squeaky pop with a burning splint
- Oxygen relights a glowing splint
- Carbon dioxide turns limewater milky
- Chlorine bleaches damp litmus paper from blue to red to white
- These are the four main gas tests you need to know for your exam