What is Matter?

Matter is anything that has mass and occupies space (has volume). Everything around us — air, water, rocks, living beings — is made of matter. Matter is made up of extremely small particles called atoms and molecules.

States of Matter

Solid:
In a solid, particles are tightly packed in a fixed, orderly arrangement. They vibrate in place but cannot move freely. Solids have a definite shape and definite volume. Example: ice, rock, wood.

Liquid:
In a liquid, particles are close together but can slide past each other. Liquids have a definite volume but no definite shape — they take the shape of their container. Example: water, mercury, oil.

Gas:
In a gas, particles are far apart and move very fast in random directions. Gases have no definite shape and no definite volume — they expand to fill whatever container they are in. Example: oxygen, nitrogen, water vapor.

Plasma:
Plasma is the fourth state of matter, found in stars and lightning. It is a superheated gas where electrons are separated from atoms. It is the most common state of matter in the universe.

Changes of State
Solid to liquid = melting (requires heat)
Liquid to gas = evaporation / boiling (requires heat)
Gas to liquid = condensation (releases heat)
Liquid to solid = freezing (releases heat)
Solid to gas = sublimation (e.g., dry ice)