States of Matter: Solid, Liquid and Gas Explained Simply

Matter exists in three familiar states: solid, liquid and gas. The particles are the same in each state; what changes is how tightly they hold together and how energetically they move. In a solid they vibrate in fixed positions, in a liquid they slide past one another, and in a gas they fly freely.

This matters because almost every process you meet, cooking, weather, engines, refrigeration, is particles switching states or moving within them. The particle model behind the three states is the single most useful idea in school chemistry and physics alike.

Key Concepts

• Particle model: all matter is made of tiny particles in constant motion; temperature measures their average kinetic energy.
• Intermolecular forces: attractions between particles that solids and liquids must overcome to change state.
• Solid: fixed shape and volume; particles in a regular lattice, vibrating in place.
• Liquid: fixed volume but no fixed shape; particles close together yet free to flow.
• Gas: neither fixed shape nor volume; particles far apart, filling any container.
• Melting, freezing, vaporisation, condensation, sublimation: the names of the six transitions between states.
• Latent heat: energy absorbed or released during a change of state with no temperature change.
• Plasma: a fourth state, an ionised gas found in lightning, neon signs and stars.

Formulas

• Charles's law: V₁/T₁ = V₂/T₂, where V is gas volume in litres (L) and T is absolute temperature in kelvin (K).

Charles's law assumes the pressure and the amount of gas stay constant, and the gas behaves ideally. Crucially, T must be in kelvin; using Celsius gives nonsense because the scale does not start at absolute zero.

Worked Example

A balloon holds 3.0 L of air at 27°C. It warms in the sun to 47°C at the same pressure. What is its new volume?

1. Convert to kelvin: T₁ = 27 + 273 = 300 K; T₂ = 47 + 273 = 320 K.
2. V₂ = V₁ × T₂/T₁ = 3.0 × 320/300.
3. V₂ = 3.0 × 1.0667 = 3.2 L.

The gas expands by 0.2 L because faster particles push harder against the balloon skin. Forgetting the kelvin conversion is the classic exam trap.

Common Misconceptions

• "Particles get bigger when heated." They do not. Heating makes particles move faster and spread apart; each particle stays the same size.
• "The white cloud above a kettle is water vapour." Water vapour is invisible; the white cloud is millions of tiny liquid droplets where the vapour has re-condensed in cool air.
• "Only three states exist." Plasma is a genuine fourth state, and physicists have found more exotic ones like Bose-Einstein condensates near absolute zero.

Real-World Applications

• Refrigeration: fridges cycle a fluid between liquid and gas, absorbing heat as it evaporates inside and dumping it as it condenses outside.
• Hot-air balloons: heating the air lowers its density so the balloon rises, applied Charles's law.
• Metal casting: foundries melt and freeze tonnes of metal, controlling cooling to set crystal structure.
• Freeze-drying: food and vaccines are preserved by sublimating ice directly to vapour.
• Weather: clouds, rain and fog are water cycling between vapour, liquid and ice in the sky.

Solid, liquid and gas are not three kinds of stuff but three moods of the same particles, set by the battle between their motion and their mutual attraction.