The temperature at which a liquid becomes a solid is called the freezing point. The solid becomes a liquid at its melting point. Water also has a high latent heat of fusion. Latent heat of fusion refers to the amount of heat gained or lost when a substance changes from a solid to a liquid, or a liquid to a solid. When ice is formed, large quantities of heat are given off.
Most liquids become more dense as they cool.
If cooled until they become solid, the solid phase is more dense than the liquid phase. However, this is not true of water. Pure water becomes more dense as it cools until it reaches 4 degrees C and further cooling decreases the density. The fact that water ice is lighter than liquid water has key implications for Earth. Have them think about the implications this has for ocean life and life in our lakes and rivers.