In an experiment to demonstrate Brownian motion, tiny pollen grains suspended in a beaker of water are observed through a microscope. Some pollen grains are seen to move in a continuous, random, zig-zag path.
Which of the following statements best explains this observation?
The pollen grains are alive and move on their own.
Heat from the microscope's light source causes convection currents that drive the pollen grains in random directions.
Extremely small, rapidly moving water molecules collide continuously and unevenly with the much larger pollen grains.
Water molecules are larger than the pollen grains and push them aside as they flow.
151 exam-style questions on CIE IGCSE Physics 2.1.2a Particle structure and motion in solids, liquids and gases. Each one has a worked solution and a mark scheme showing where the marks go.