CAIE IGCSE Chemistry 6 Chemical Reactions Topic Practice

When calcium carbonate is heated strongly, a gas is given off.
Which word describes this change?

Powdered magnesium carbonate is added to excess dilute hydrochloric acid.
The total volume of gas produced is measured over time.
A graph of the results is shown.

The experiment is repeated but the concentration of the hydrochloric acid is doubled.

All other conditions are kept the same.
Which statements about the second experiment are correct?

1 The final volume of gas is $360 \mathrm{~cm}^{3}$.
2 The reaction finishes before 90 seconds.
3 The activation energy of the reaction is lower.

When the colourless gas $\mathrm{N}_{2} \mathrm{O}_{4}$ is heated, it forms the brown gas $\mathrm{NO}_{2}$.
When the reaction mixture is cooled, the brown colour fades and turns back to colourless.
Which type of reaction is described by these observations?

Chromium forms the compound chromium(III) sulfate.
What does the (III) represent?