All substances are made up of molecules. These molecules are generally in motion and possess kinetic energy; each molecule also exerts a force of attraction on other molecules, so they possess potential energy. The sum of the kinetic and potential energy is called the internal energy of the molecules. This internal energy, when it flows out, is called heat energy. It is more in hot substances and less in cold substances, and it flows from hot substances to cold substances.
1. Effects of Heat
When a substance is heated, the following things can happen.
| Effect | What happens |
|---|---|
| Expansion | Molecules gain energy, vibrate and force other molecules apart. Space is left in railway tracks because more heat in summer causes expansion. Expansion is greater in liquids than solids and maximum in gases. |
| Change in state | Ice becomes water and water on further heating changes into vapour – solid becomes liquid and liquid becomes gas when heat is added. The reverse takes place when heat is removed. |
| Change in temperature | The kinetic energy of the particles increases, they move at higher speed and the temperature rises. When heat is removed, the temperature falls. |
| Chemical changes | Some chemical reactions need heat to begin, and heat determines the speed at which reactions occur (e.g. cooking food). |