Tiny smoke particles are suspended in air inside a sealed container. A bright light is shone from the side, and the container is viewed from above through a microscope. Many tiny, bright specks of light are seen moving in a random, jerky, zig-zag path. What are these bright specks of light and what causes their motion?
They are air molecules being struck by other air molecules.
They are smoke particles being struck by fast-moving, invisible air molecules.
They are air molecules being struck by smoke particles.
They are smoke particles being struck by other smoke particles.
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.