Microscopic pollen grains suspended in still water are observed under a microscope to move in a random, jerky path. This phenomenon is known as Brownian motion.
Which statement correctly explains this observation?
Convection currents in the water push the pollen grains continuously in a circular path.
Extremely small, fast-moving water molecules, which are invisible under a standard microscope, randomly collide with the larger pollen grains.
The pollen grains are living organisms that swim actively to find nutrients in the water.
The pollen grains repeatedly collide with one another due to mutual gravitational attraction.
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.