Heat energy is measured in terms of temperature, or the hotness or coldness of matter. Matter with very active molecules has high temperature and is said to be hot while matter with molecules that move around less has low temperature and is referred to as cold.

Changes in Phases of Matter

Heat energy has such an impact on matter that it can change matter from one phase to another. Changes in temperature can cause changes in the phases of some matter, for example, food.

A bar of butter will melt when you heat it in a pan placed over a fire. Ice cubes will turn to liquid water when you take them out of the cold confines of a freezer and into room temperature. Both butter and ice cubes change from solid to liquid as they absorb heat from their surroundings. The change in phase from solid to liquid is called melting.

When you boil water in a pot, the amount of water in the pot will decrease after some time because part of it has turned into water vapor, or water that is in the gaseous phase. The change in phase from liquid into gas is called evaporation.

When water vapor cools, it releases heat and turns back into liquid. The change in phase from water vapor to liquid is called condensation.