The arrangement of the particles
2 marks · HardExplain, using the particle model, why solids have a fixed shape and gases do not.
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Also accept: Solids have particles that are closely packed in a fixed, regular arrangement. These particles can only vibrate, so the solid keeps its shape.; In gases the particles are far apart and move freely in all directions, so gases do not keep a fixed shape.
Common mistakes: Not mentioning the fixed arrangement for solids.; Omitting the random movement in gases.
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Marking points
B1 · 1 mark · Explains that solids have closely packed particles in a fixed, regular arrangement and that particles can only vibrate.
B2 · 1 mark · Explains that gases have particles far apart that move freely, so gases do not keep a fixed shape.
Worked solution
Solids have a fixed shape because their particles are closely packed in a fixed, regular pattern and can only vibrate in place. Gases do not have a fixed shape because their particles are far apart and move freely in all directions, so they fill the container.