When microscopic smoke particles suspended in air are observed using a microscope, they are seen to move in a continuous, rapid, and random jerky motion.
What causes a smoke particle to move in this way?
convection currents in the air due to heating
continuous collisions with a single air molecule at any given instant
unbalanced bombardment of the smoke particle on different sides by fast-moving air molecules
electrostatic repulsion and attraction between neighboring smoke particles
321 exam-style questions on CIE IGCSE Physics 2.1 Kinetic particle model of matter, covering 2.1.1 States of matter and changes of state, 2.1.2a Particle structure and motion in solids, liquids and gases, 2.1.2b Gas pressure and particle collisions, 2.1.2c Brownian motion, 2.1.3a The absolute scale of temperature, and 2.1.3b Pressure, volume and temperature of a gas. Each one has a worked solution and a mark scheme showing where the marks go.