The molecules of liquids are free to move in all directions within the liquid. On heating, the average amplitude of vibration of its molecules increases. The molecules push each other and need more space to occupy. This accounts for the expansion of the liquid when heated. The thermal expansion in liquids is more significant than in solids due to the weak forces between their molecules. Therefore, the coefficient of volume expansion of liquids is greater than solids.

        Liquids have no definite shape of their own. A liquid always attains the shape of the container in which it is poured. Therefore, when a liquid is heated, the liquid and the container change their volume.