# A scientist seals a small piece of solid iodine inside a glass flask containing air at room temperature. After several hours, the entire flask appears filled with a purple-brown colour. Explain why the purple-brown colour spreads to fill the entire flask.

> Cambridge International IGCSE Chemistry (0620) — 1.2 Diffusion · Explain · 4 marks

## Mark scheme (4 marks)

1. Iodine sublimes / turns directly from solid to vapour (gas), producing iodine molecules in the gas phase
2. Iodine molecules (particles) move randomly / are in constant random motion
3. Iodine molecules spread out / mix throughout the flask by diffusion, moving from a region of higher concentration to lower concentration
4. Gas molecules have large spaces between them / are far apart, allowing iodine molecules to move freely and fill the available space

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