Full chapter
Qualitative Analysis
Identify hydrogen, oxygen and carbon dioxide with tests that tell them apart.
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
Predict the gas, then prove it
Link each gas to the reaction that makes it, and record the test as evidence.
| Reaction | Gas produced | Example |
|---|---|---|
| Reactive metal + dilute acid | Hydrogen | Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g) |
| Carbonate + dilute acid | Carbon dioxide | CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) |
| Hydrogen peroxide with a manganese(IV) oxide catalyst, a common laboratory source | Oxygen | 2H2O2(aq) -> 2H2O(l) + O2(g) |
Fizzing, or effervescence, tells you that a gas is being made. It does not tell you which gas. Use what you know about the reaction to predict the gas, then carry out the matching test to prove it. The acid reactions here are the same ones you meet in Acid-Base Chemistry.
Worked example
Identify the gas from an unknown solid
A white solid fizzes when dilute hydrochloric acid is added. How would you show which gas forms?
- Effervescence shows that a gas is produced, but bubbles alone do not identify it.
- A white solid that fizzes with acid could be a carbonate, giving carbon dioxide, or a metal, giving hydrogen.
- Bubble the gas through limewater. A white precipitate confirms carbon dioxide. If there is no precipitate, test a fresh sample of the gas with a lighted splint for hydrogen.
Record the observations (effervescence; limewater turns milky) separately from the conclusion (the gas is carbon dioxide, so the solid is probably a carbonate).
Check your understandingA gas relights a glowing splint. A student writes "the gas is flammable". Correct the conclusion.Think it through, then reveal the answer
Quick revision
Revisit the essentials, then return to an explanation when you need it.
Hydrogen: a lighted splint gives a squeaky pop. Oxygen: a glowing splint relights. Carbon dioxide: limewater turns milky (a white precipitate). Name the test and its result first, then the gas.
Check your understandingWhich single test distinguishes oxygen from carbon dioxide?Think it through, then reveal the answer
Scope
Learning outcomes
6. Qualitative Analysis (K223 / K225). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
6(a) Identify gases
- Carbon dioxide with limewater
- Hydrogen with a lighted splint
- Oxygen with a glowing splint
Identify the gas, not just the bubblesPredict the gas, then prove it