Take an ice cube, warm it and you get water; keep going and you get steam. Same H₂O molecules every time. A state of matter is not a different substance — it is a different way for the same particles to behave.
This article describes the states of matter at the particle level, the phase changes between them, why temperature pauses while ice melts, how a phase diagram works and how gases follow the ideal gas law.
The particle picture
All matter is made of particles that are always moving and that attract each other. The state depends on the contest between those two: thermal energy pulling particles apart and intermolecular attractions holding them together.
Solids, liquids and gases
In a solid the particles are locked in position. In crystalline solids like salt, ice or metals they form a regular repeating lattice; in amorphous solids like glass they are frozen in disorder.
In a liquid the particles are almost as close as in a solid but can move past one another, so a liquid flows. Attractions still matter: they cause surface tension, which lets a water strider stand on a pond.
In a gas the particles are far apart — in air at room conditions, about ten molecular diameters on average — and move at hundreds of metres per second. Gases are mostly empty space, which is why they compress easily.
Plasma, the fourth state
Heat a gas enough, or hit it with a strong electric field, and electrons are torn from the atoms. The result is plasma: a mix of positive ions and free electrons that conducts electricity and glows. Stars, lightning, auroras and neon signs are plasma — by mass it is the most common state of ordinary matter in the universe.
Changes of state
While ice melts, the thermometer stays at 0 °C even though you keep heating. The energy, called latent heat, goes into breaking the attractions between molecules rather than speeding them up. Water needs 334 J per gram to melt and 2 260 J per gram to boil — which is why steam burns are so severe.
Boiling point depends on pressure. On a mountain top, with less air pressing down, water boils below 100 °C, so pasta takes longer to cook; a pressure cooker raises the pressure and the boiling point to about 120 °C.
Phase diagrams
A phase diagram maps which state is stable at each temperature and pressure. Lines mark the conditions where two states coexist. Two special points:
- Triple point: the single temperature and pressure where solid, liquid and gas coexist — for water, 0.01 °C at 611.657 Pa.
- Critical point: above it, liquid and gas can no longer be told apart and the substance becomes a supercritical fluid — used, for example, to decaffeinate coffee with CO₂.
Gases and the ideal gas law
Because gas particles are far apart, most gases behave alike and follow one equation, the ideal gas law:
Heat a sealed gas and its pressure rises (why aerosol cans warn against heat); squeeze it and the pressure rises too. The ideal gas calculator solves for any one of P, V, n or T. Temperature must always be in kelvin.
Common misconceptions
“The bubbles in boiling water are air.”
They are water vapour. Dissolved air leaves early, as the small bubbles you see before the boil.
“Particles in a solid don’t move.”
They vibrate constantly. Only at absolute zero (0 K) would that motion reach its minimum.
Frequently asked questions
What are the main states of matter?
The three everyday states are solid, liquid and gas. Plasma, an ionized gas found in stars and lightning, is often counted as the fourth.
What causes a change of state?
Adding or removing energy (usually heat) or changing the pressure. Energy lets particles overcome the attractions holding them in place.
What is sublimation?
Sublimation is a solid turning directly into a gas without becoming liquid, like dry ice (solid CO₂) at room pressure.
Why does the temperature stay constant while ice melts?
The heat goes into breaking the attractions between molecules (latent heat) instead of making them move faster, so the temperature stays at 0 °C until all the ice has melted.
References and further reading
- IUPAC — Gold Book: phase, phase transition, triple point, critical point.
- IAPWS — Revised release on the triple point and thermodynamic properties of water.
- Atkins, P.; de Paula, J. — Physical Chemistry, chapters on gases and phase diagrams.