Topic 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)
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 understandingA glowing splint is extinguished. Does that identify hydrogen?Think it through, then reveal the answer
No. Hydrogen is tested with a lighted splint and gives a pop. An extinguished glowing splint only shows that the gas did not relight it; it does not uniquely identify the gas.