The graph below shows the volume of gas produced over time when marble chips react with hydrochloric acid. Why is the graph steepest at the very beginning of the reaction?

What does the horizontal (flat) part of the volume–time graph indicate about the reaction?

A student says: 'The graph is steep at first because heat is released, making the reaction faster.' Identify the error in this statement and give the correct scientific explanation.

Look at the rate–time graph for the same reaction. How does the rate of reaction change as the reaction progresses? Explain using collision theory.

Two reactions produce the same final volume of gas. Reaction X reaches this volume in 40 s. Reaction Y reaches it in 80 s. Which reaction has the greater average rate? Show your working.