# Explain why, in a closed system, a reversible reaction eventually reaches a state where the concentrations of reactants and products remain constant, even though the reaction has not stopped.

> Cambridge International IGCSE Chemistry (0620) — 6.3 Reversible reactions and equilibrium · Explain · 4 marks

> Hydrogen iodide can be formed from hydrogen and iodine gases in a sealed container: H₂(g) + I₂(g) ⇌ 2HI(g). When hydrogen and iodine are first mixed, no hydrogen iodide is present.

## Mark scheme (4 marks)

1. At the start, only the forward reaction occurs / the forward reaction is faster than the reverse reaction initially
2. As the reaction proceeds, the rate of the forward reaction decreases and the rate of the reverse reaction increases
3. Equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction
4. Both forward and reverse reactions continue at equal rates, so concentrations do not change / the system is in dynamic equilibrium

## Key terms

- [reversible reaction](https://www.gradenine.co.uk/glossary/reversible-reaction)
- [closed system](https://www.gradenine.co.uk/glossary/closed-system)

## Related

- [Revision notes for Cambridge International IGCSE Chemistry (0620)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-in-a-closed-system-8f42925c) · Published by Druglandscape Ltd.