Topic 2 of 2
Predict the gas, then prove it
Link each gas to the reaction that makes it, and keep observations separate from conclusions.
SEC G2 Science K223 / K225 (2027)
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).