States of Matter and Changes of State

What is Matter?

Matter is anything that has mass and occupies space (volume). Everything around us — air, water, wood, metal — is made of matter, which is itself composed of tiny particles (atoms and molecules).

The Three Common States of Matter

• Solid: has a definite shape and definite volume. Particles are tightly packed and vibrate in fixed positions. Example: ice, wood, iron.
• Liquid: has a definite volume but takes the shape of its container. Particles are close together but can move past one another. Example: water, oil, milk.
• Gas: has no definite shape or volume; it expands to fill its container. Particles are far apart and move freely and rapidly. Example: oxygen, carbon dioxide, water vapour.

A fourth state, plasma, exists at very high temperatures (e.g. inside stars, lightning) and consists of charged particles, but it is less commonly encountered in daily life.

Particle Arrangement and Energy

The state of matter depends on how much energy the particles have and how strongly they attract one another. Solids have the strongest particle attraction and lowest particle energy; gases have the weakest attraction and highest particle energy.

Changes of State

• Melting: solid → liquid (by heating), e.g. ice melting to water.
• Freezing: liquid → solid (by cooling), e.g. water freezing to ice.
• Evaporation/Boiling: liquid → gas (by heating), e.g. water boiling to steam.
• Condensation: gas → liquid (by cooling), e.g. steam condensing on a cold surface.
• Sublimation: solid → gas directly, without becoming liquid, e.g. dry ice (solid carbon dioxide).
• Deposition: gas → solid directly, e.g. frost forming from water vapour.

Why This Matters

Understanding states of matter and how they change helps explain everyday phenomena — why ice cubes melt in a drink, why clothes dry in the sun, and why steam forms on a cold mirror after a hot shower — and forms the foundation for understanding chemical reactions and physical processes.