Kinetic particle model of matter

The kinetic theory states that all matter is made of tiny particles (atoms or molecules) that are in constant, random motion. The state of matter—solid, liquid, or gas—is determined by the energy of these particles and the strength of the forces between them. In a solid, particles are packed tightly in a fixed, regular pattern. They are held by strong forces of attraction (bonds) and can only vibrate about their fixed positions. This gives solids a fixed shape and volume. In a liquid, particles are still close together but have more kinetic energy than in a solid. The forces between them are weaker, allowing them to move and slide past one another. This is why liquids have a fixed volume but can flow and take the shape of their container. In a gas, particles have the highest kinetic energy. The forces between them are negligible, so they are far apart and move randomly and at high speeds. Gases have no fixed shape or volume and will expand to fill any container.

Brownian motion is the random, jerky movement of microscopic particles (like pollen or dust) suspended in a fluid (a liquid or a gas). It provides direct evidence for the kinetic theory of matter. We can't see the individual air or water molecules, but we can see their effect. The larger, visible suspended particles are constantly being bombarded by the smaller, invisible, fast-moving particles of the fluid. Since the collisions are random and come from all directions, the bombardment is uneven at any instant. This results in a net force that pushes the suspended particle, causing it to move. A moment later, a different set of random collisions causes a net force in another direction, making the particle change its path. This results in the observed haphazard 'zig-zag' motion.

The temperature of an object is a measure of the average kinetic energy of its constituent particles. The faster the particles move or vibrate on average, the higher the temperature. Internal energy is the total energy of all the particles in an object. It is the sum of the total kinetic energy (due to their motion) and the total potential energy (due to the forces between them). A large, cold object can have more internal energy than a small, hot object because it has many more particles, even if the average kinetic energy of each is lower. The Kelvin scale is the absolute temperature scale. Its zero point, 0 K, is called absolute zero. This is the lowest possible temperature, where particles have the minimum possible internal energy (they still have some zero-point energy, but for IGCSE, you can consider them to have stopped moving). To convert from Celsius (°C) to Kelvin (K), you add 273.

Gas pressure is caused by the collisions of gas particles with the surfaces of the container. Each collision exerts a tiny force on the surface. The pressure is the total force of these countless collisions acting on each unit of area. Two key relationships follow from this model:

1. Heating a gas at constant volume: If you heat a gas in a rigid container, the particles gain kinetic energy and move faster. This means they collide with the container walls more frequently and more forcefully (with greater momentum change per collision). Both factors increase the total force on the walls, thus increasing the pressure.
2. Heating a gas at constant pressure: If you heat a gas in a flexible container (like a balloon), the faster-moving particles initially increase the pressure inside. This pressure pushes the walls of the container outwards, increasing its volume. The volume increases until the pressure inside equals the constant external pressure again. This is because in a larger volume, collisions with the wall become less frequent, counteracting the effect of the more forceful collisions.

Thermal expansion is the tendency of matter to increase in volume when it is heated. According to the kinetic theory, heating a substance increases the kinetic energy of its particles, causing them to vibrate (in solids) or move (in liquids and gases) more vigorously. As they move more, they push their neighbours further away, increasing the average separation between particles. This causes the entire substance to expand in size. The degree of expansion varies significantly between the states of matter. Gases expand the most because the forces between particles are negligible, so any increase in kinetic energy easily translates into a larger volume. Liquids expand more than solids, as the forces between liquid particles are weaker than the strong bonds in solids. This principle has many applications, such as in liquid-in-glass thermometers (the liquid expands up the tube) and bimetallic strips used in thermostats. It also causes problems, such as bridges buckling in hot weather, which is why expansion gaps are built into large structures.

Evaporation is the process by which a liquid turns into a gas. It can happen at any temperature, but only occurs at the surface of the liquid. The particles in a liquid have a range of kinetic energies. The fastest-moving particles at the surface may have enough energy to overcome the attractive forces holding them in the liquid and escape into the air as gas particles. Since the particles with the highest energy have left, the average kinetic energy of the remaining particles in the liquid decreases. As temperature is a measure of average kinetic energy, the liquid cools down. This is known as evaporative cooling. The rate of evaporation can be increased by: increasing temperature (more particles have escape energy), increasing surface area (more particles are at the surface), and increasing airflow (wind blows escaped particles away, preventing them from returning). Condensation is the opposite process, where a gas turns into a liquid when it cools down. Gas particles lose kinetic energy, slow down, and the forces of attraction pull them together to form a liquid.