Thermal properties and temperature

Temperature is a measure of the average kinetic energy of the particles within a substance. The faster the particles are moving or vibrating, the higher the temperature. We use fixed points, like the melting point of ice (0 °C) and the boiling point of water (100 °C), to define temperature scales. If you cool a substance down, its particles slow down. The theoretical point at which particles stop moving as much as possible is called absolute zero. This is the starting point for the Kelvin scale, which is used in scientific work. A change of one Kelvin (1 K) is the same size as a change of one degree Celsius (1 °C).

When most substances are heated, their particles gain kinetic energy, vibrate more, and push each other further apart. This causes the substance as a whole to expand (increase in size). This is called thermal expansion. For the same temperature rise, gases expand the most, followed by liquids, and then solids. This effect has important consequences. For example, gaps must be left in bridges and railway lines to allow for expansion in hot weather, preventing them from buckling. Thermal expansion is also useful, for instance in bimetallic strips used in thermostats, which bend when heated to operate a switch.

Gas pressure is the result of countless tiny collisions. The particles in a gas are in constant, random motion at high speeds. They collide with each other and with the walls of their container. Each collision with a wall exerts a tiny force. The total effect of all these collisions over the area of the walls creates the pressure of the gas. If you heat a gas in a fixed container (constant volume), its particles gain kinetic energy and move faster. This means they hit the walls harder and more often, increasing the pressure. If you heat a gas but allow it to expand (constant pressure), its volume will increase.

Different materials require different amounts of energy to heat up. The 'specific heat capacity' (symbol 'c') of a substance is the amount of thermal energy needed to raise the temperature of 1 kg of that substance by 1 °C (or 1 K). A substance with a high specific heat capacity, like water (c = 4200 J/kg°C), needs a lot of energy to get hot and cools down slowly. A substance with a low specific heat capacity, like copper (c = 400 J/kg°C), heats up and cools down quickly. We can calculate the energy transferred using the equation: Energy transferred (E) = mass(m) × specific heat capacity(c) × temperature change (ΔT).

Evaporation is the process where a liquid turns into a gas from its surface, without boiling. Inside a liquid, particles have a range of kinetic energies. The 'fastest' (most energetic) particles at the surface can have enough energy to overcome the forces holding them in the liquid and escape as a gas. Because the most energetic particles leave, the average kinetic energy of the remaining particles decreases. This means the temperature of the remaining liquid drops, which is why evaporation causes cooling. This process can be sped up by increasing temperature, increasing surface area, or increasing airflow over the surface. Condensation is the opposite: a gas turns into a liquid when its particles cool, slow down, and are captured by intermolecular forces.