Lesson 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)
I can identify gases.Syllabus
Syllabus K223 / K225, 6(a). Identify gases
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.