An inflatable rescue marker contains solid Q and a solution containing reactant P. Reactant P and solid Q react to produce a gas.
Gas production stops while some solid Q is still present in the liquid. More of the solution containing reactant P, at the same temperature, is added and gas production starts again.
Why does gas production start again?
It replaces reactant P, which had all been used up
It replaces solid Q, which had all been used up
It raises the pressure, so particles collide more often
It acts as a catalyst for the reaction of P and Q
127 exam-style questions on AQA GCSE Chemistry 6.1 Rate of reaction, covering 6.1.1a Calculating rates of reactions, 6.1.1b Gradient of a tangent as rate, 6.1.2 Factors which affect the rates of chemical reactions, 6.1.3 Collision theory and activation energy, and 6.1.4 Catalysts. Each one has a worked solution and a mark scheme showing where the marks go.