Matter is anything that has mass and takes up space. Solid, liquid and gas are the three states of matter found most often on Earth. All three are made of tiny particles. Water can be ice, which is a solid. It can be liquid water. It can be steam, which is a gas. Plasma is a fourth state of matter. The Sun is a ball of plasma. A solid, like an ice cube, keeps its shape.

In a solid, the tiny particles are packed tightly together. They vibrate in place but do not move past one another. In a liquid, the particles are close together. They can slide past one another. In a gas, the particles are far apart. They move around on their own. A gas is easy to squeeze into a smaller space. Liquids and solids hardly change their size when squeezed. A liquid flows and takes the shape of its container.

Water freezes at 0 degrees Celsius. At normal air pressure, it boils at 100 degrees Celsius. Melting is a solid changing into a liquid. Freezing is a liquid changing into a solid. While ice is melting in water, the mixture stays at 0 degrees Celsius. When liquid water is heated enough, it turns into a gas called steam. Gases can be turned into liquids by cooling them. Liquids can be turned into solids by cooling them more.

Air always holds some water as water vapor, an invisible gas. Evaporation is liquid water changing into water vapor, a gas. It happens when heat energy makes water molecules break free of their liquid. Evaporation takes heat away. When water on your skin evaporates, you feel cold. Condensation is water vapor turning back into liquid water. Drops of water form on the outside of a glass of ice water. This is because water vapor in the air gets cold. In the water cycle, water evaporates into vapor. It condenses into clouds. Then it falls back to Earth as precipitation.

Ice floats because it is less dense than liquid water. Water expands when it freezes. So ice takes up more room than the liquid water it came from. Sublimation is a solid turning straight into a gas with no liquid stage. Snow and ice can sublime, slowly turning into water vapor without melting first. Dry ice is frozen carbon dioxide. It turns straight into a gas instead of melting. It does this at about -78.5 degrees Celsius.

Words to know

• particles: tiny bits
• vibrate: shake in place
• sublimation: solid to gas
• condensation: gas to liquid
• plasma: fourth state