Lesson 1 of 2
Identify the gas, not just the bubbles
A named gas needs a specific test and observation.
SEC G2 Science K223 / K225 (2027)
I can identify gases.Syllabus
Syllabus K223 / K225, 6(a). Identify gases
Identify the gas, not just the bubbles
A named gas needs a specific test and observation.
| Gas | Test | Positive result | Why it works |
|---|---|---|---|
| Hydrogen, H2 | Hold a lighted splint at the mouth of the test tube | A squeaky pop | Hydrogen burns rapidly with oxygen from the air, forming water |
| Oxygen, O2 | Put a glowing splint into the gas | The splint relights | Oxygen supports burning, so the glowing wood bursts back into flame |
| Carbon dioxide, CO2 | Bubble the gas through limewater | A white precipitate forms: the limewater turns milky | Limewater is aqueous calcium hydroxide; carbon dioxide reacts with it to form insoluble calcium carbonate |
Choose the splint from the gas you expect. A lighted splint, one that is still burning, tests for hydrogen. A glowing splint, one that has been blown out but still glows red, tests for oxygen. Mixing the two up is the most common mistake. A lighted splint going out does not identify a gas: carbon dioxide, nitrogen and many other gases put out a flame.
Confirm carbon dioxide with limewater, not with a splint. Bubble the gas through the limewater using a delivery tube. If a lot of carbon dioxide passes through, the white precipitate can dissolve again, so report the first change you see.
Check your understanding
A glowing splint goes out in a gas. Does that identify hydrogen?
Show the answer
No. Hydrogen is tested with a lighted splint, which gives a pop.
Many gases put out a glowing splint, such as CO2 and N2, so this does not identify the gas. The test for hydrogen is a squeaky pop with a lighted splint.