AQA · GCSE · Combined Science · Higher

Steam expansion, particles and density

Q04.5 3 marks

If the pressure inside the pressure cooker becomes greater than 200 kPa then some of the steam is released through the safety valve.

The released steam expands as it moves into the atmosphere.

Explain how a change in density of the steam is caused by a change in the arrangement of particles in the steam as it is released.

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Marking points

1 State that the particles in the steam spread out.
2 Explain that the steam or gas takes up a greater volume.
3 Link the greater volume to density decreasing using $\text{density}=\frac{\text{mass}}{\text{volume}}$ .

Full-mark answer

As the steam is released, its particles spread out. The steam therefore takes up a greater volume. Since $\text{density}=\frac{\text{mass}}{\text{volume}}$ , increasing the volume means the density decreases.

Why this answer loses marks

The steam particles expand so the density gets lower.

The particles themselves do not expand; they spread out. The answer also needs the link to greater volume and density = mass / volume.

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