The three most common forms, or phases, that substances can take are solids, liquids, and gases. Changes in phase, or the physical state of matter, are prompted by physical forces, such as temperature. Water is an easy substance to study because it goes through different phases – ice, liquid, and steam. No matter what its form, water is always water with the same chemical makeup, two hydrogen atoms and one atom of oxygen.

A phase change, or a physical change, occurs when a substance changes from one form to another. Matter can change from one phase to another by adding or subtracting energy. To make ice, take energy away from liquid water. The cooled molecules slow down, move close together and become solid ice.

To melt the ice cube, add heat energy. The molecules move faster, spread apart, and solid ice changes to water. Add more heat energy to the water and the molecules move even faster and farther apart, changing liquid water to water vapor, a gas.

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