Solids, Liquids and Gases
4 marks · NormalA laboratory demonstration shows the random motion of gas particles.
Explain why gases exert pressure on the walls of their container, using the idea of particles.
State and describe one property of gases which is evidence for the random motion of their particles.
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(a) — · must show explanation mentions collisions, force on walls causes pressure
(b) — · must show Brownian motion, motion of smoke/dust particles
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Explains that gas particles collide with the walls of the container.
B2 (a) · 1 mark · Mentions that many collisions create a net force, which is observed as pressure.
B3 (b) · 1 mark · States Brownian motion or similar evidence (e.g. motion of smoke).
B4 (b) · 1 mark · Describes movement as random or unpredictable.
Worked solution
a) Gas molecules move in all directions, colliding with the walls of the container. Each collision exerts a tiny force; the sum of these forces over the area creates pressure.
b) Evidence: Brownian motion—the observed random movement of smoke or dust particles in air, which results from collisions with randomly moving gas molecules.