Everything around us — the air, water, rocks, your pen — is matter. This matter can take three main forms called physical states: solid, liquid, and gas. What distinguishes these three states is how the particles (atoms or molecules) are arranged and bound together.
The three states
In a solid, particles are tightly packed and ordered. They vibrate in place but do not move around freely. As a result, a solid has its own shape and volume. An ice cube in a glass keeps its shape until it melts.
In a liquid, particles still touch each other but can slide past one another. A liquid has its own volume but takes the shape of its container. Pour water into a bottle then into a bowl: the volume stays the same, the shape changes.
In a gas, particles are far apart and move in every direction. A gas has neither shape nor fixed volume: it fills any available space. That is why you can smell a perfume across a room.
Phase transitions
Heating or cooling matter triggers transitions between states:
- Melting: solid → liquid (ice melts at 0 °C).
- Freezing: liquid → solid (water freezes at 0 °C).
- Vaporization: liquid → gas (water boils at 100 °C).
- Condensation: gas → liquid (fog on a cold window).
- Sublimation: solid → gas directly (dry ice).
Each substance has its own transition temperatures. This is a characteristic property: you can identify a pure substance by measuring its melting or boiling point.
Why it matters
Understanding states of matter means understanding that the same substance — water, H₂O, for instance — can show up in radically different forms depending on temperature, while the molecules themselves do not change. This idea prepares you directly for high school, where you will discover that these states depend on a balance between thermal agitation and the forces that bind particles together.