Why Does Matter Change State?

Science of Why a Substance Changes State

Close up Photograph of Ice Sculpture
by Simon Gakhar / Getty Images

You have observed matter changing state, like when an ice cube melts from a solid into liquid water or the water boils into vapor, but do you know why a substance changes from? The reason is because matter is affected by energy. If a substance absorbs enough energy, atoms, and molecules move around more. The increased kinetic energy can push particles far enough apart that they change form. Also, increased energy affects the electrons surrounding atoms, sometimes allowing them to break chemical bonds or even escape the nucleus of their atoms.

Usually, this energy is heat or thermal energy. Increased temperature is a measure of increased thermal energy, which can lead solids to change to liquids to gasses to plasma and additional states. Decreased temperature reverses the progression, so a gas may become a liquid which may freeze into a solid.

Pressure plays a role, too. The particles of a substance seek the most stable configuration. Sometimes the combination of temperature and pressure permits a substance to "skip" phase transition, so a solid can go directly to the gas phase or a gas may become a solid, with no liquid intermediate state.

Other forms of energy besides thermal energy can change the state of matter. For example, adding electrical energy can ionize atoms and change a gas into plasma. Energy from light can break chemical bonds to change a solid into a liquid. Often, types of energy are absorbed by a material and change into thermal energy.