A solution of potassium chromate is yellow. A student adds dilute acid and the solution turns orange. The student then adds an alkali and the solution turns yellow again.
What does this show?
The acid acts as a catalyst for the colour change.
The reaction can be reversed by changing the conditions.
The reaction can only go in the forward direction.
A new substance forms that cannot be changed back.
47 exam-style questions on OCR GCSE Chemistry 5.3 Equilibria, covering 5.3.1 Reversible reactions and dynamic equilibrium and 5.3.2 Changing the position of equilibrium. Each one has a worked solution and a mark scheme showing where the marks go.