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 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.
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 exampleIdentify the gas from an unknown solid
A white solid fizzes when dilute hydrochloric acid is added. How would you show which gas forms?
Try it first. Then show the worked stepsHide the worked steps
- 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).
Watch out for this
A student says: "Observation: carbon dioxide is produced when the acid is added to the carbonate." What is wrong with this?
Show the answer
"Bubbles form" and "the limewater turns milky" are observations. "Carbon dioxide is produced" is a conclusion drawn from them. Questions often give a mark for each, so write both.
Check your understanding
A gas relights a glowing splint. A student writes, "The gas is flammable." Which correction is right?
Show the answer
The gas is oxygen, which helps things burn but does not burn itself.
Relighting a glowing splint is the test for oxygen. Oxygen supports burning. A flammable gas such as hydrogen gives a squeaky pop with a lighted splint instead.
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 understanding
Which single test distinguishes oxygen from carbon dioxide?
Show the answer
A glowing splint, which relights in oxygen but goes out in CO2
Oxygen relights a glowing splint; carbon dioxide puts it out. Confirm CO2 with limewater, which turns milky.
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