Interaction and motion of molecules. Brownian motion
When a perfume bottle is opened, its smell soon reaches other corners of the room: so perfume particles move through the air. We believe that molecules move in gases and liquids; in solids the particles also vibrate about their places (a heated metal ball expands and cannot pass through a ring). In 1827 the English botanist Robert Brown noticed the motion of particles when he watched pollen grains in water under a microscope: they moved constantly and randomly. The reason is that liquid molecules strike them randomly from all sides. The constant random motion of very small particles in a liquid or gas is called Brownian motion; it becomes stronger when the temperature rises. Although molecules move, a solid or a liquid does not fall apart, because attractive forces act between molecules; at very short distances repulsive forces also act. These forces are felt only at distances close to the size of a molecule: pressing the pieces of a broken stick together does not bring their molecules close enough, but plasticine or dough can be pressed together.
“Random motion”: 6–8 students act as “molecules”; one student is a “pollen particle” (inside a circle or holding a large ball). The “molecules” nudge the particle gently from different sides and it keeps changing direction. Safety: nudge lightly and carefully. At home (with an adult): press two wet smooth-edged glass or plastic plates together and try to pull them apart.