Most solids and liquids expand when they are heated and contract when they are cooled. Gases also expand when they are heated at a constant pressure. If a gas is heated in a container that prevents expansion, the pressure increases. Heat causes expansion because it increases the vibrations of a material's atoms or molecules. The increased vibrations force the atoms or molecules farther apart and the body becomes larger.
Different materials expand by different amounts when their temperature is raised by one degree.
For example, aluminium expands twice as much as iron under the same temperature increase. Changes in size . As we have seen, when heat flows into a substance, the motion of the atoms or molecules in the substance increases. As a result of their increased motion, the atoms or molecules take up more space and the substance expands. The opposite occurs when heat flows out of a substance.