Solids, Liquids and Gases Notes

Their properties result from how the particles are arranged and how much energy they have. Justify a state with at least two properties: “it is a gas because it has no fixed shape and no fixed volume”.

In a solid, particles are tightly packed in a regular arrangement with strong forces, and vibrate about fixed positions. In a liquid they are close but random, with weaker forces, and slide past each other. In a gas they are far apart, forces are negligible and they move rapidly and randomly; most of a gas is empty space.

Melting, freezing and boiling happen at a fixed temperature, because the energy goes into breaking or forming forces of attraction rather than raising temperature. All changes of state are reversible.

Evaporation is different: the most energetic surface particles escape at any temperature below boiling point.

On a heating curve the temperature rises while particles gain kinetic energy, then stays level at melting and boiling points because the energy overcomes the forces of attraction. A longer plateau needs more energy. A cooling curve is the reverse, with plateaus at condensing and freezing.

At constant pressure, volume is directly proportional to absolute temperature ($V \propto T$, Charles’s law): hotter particles hit the walls harder and more often. At constant temperature, volume is inversely proportional to pressure ($P \propto \dfrac{1}{V}$, Boyle’s law): squeezing the particles together makes more collisions. Always use kelvin.