# A student dissolves a dark brown solid, dinitrogen tetroxide (N₂O₄), in a sealed flask. The following reversible reaction occurs: N₂O₄(g) ⇌ 2NO₂(g). N₂O₄ is colourless and NO₂ is brown. When the flask is placed in ice-cold water, the brown colour fades but does not disappear completely. Explain these observations in terms of equilibrium.

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

> N₂O₄(g) ⇌ 2NO₂(g)   The forward reaction is endothermic. N₂O₄ is colourless; NO₂ is brown.

## Mark scheme (4 marks)

1. The reaction is reversible, so both forward and reverse reactions occur simultaneously / at the same time
2. At equilibrium, the rates of the forward and reverse reactions are equal, so concentrations of N₂O₄ and NO₂ remain constant (explaining why some brown colour persists)
3. Lowering the temperature favours the exothermic (reverse) reaction, shifting the equilibrium position to the left / towards N₂O₄
4. More N₂O₄ is produced / less NO₂ remains, so the concentration of brown NO₂ decreases, causing the colour to fade (but not disappear, as equilibrium is still established)

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-dissolves-a-dark-brown-8d201e70) · Published by Druglandscape Ltd.