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Water is unusual in that you can easily see all three of its everyday states in one kitchen: ice in the freezer, liquid in the tap, steam from the kettle. Every substance can exist in these states under the right conditions. What changes between them isn’t the molecules themselves, but how they’re arranged and how much they move.
The particle model
All matter is made of particles — atoms, molecules or ions — that are always moving, and that attract each other through intermolecular forces or bonds. The state of a substance depends on the balance between those attractions and the particles’ kinetic energy (which rises with temperature).
Solids
- Particles are tightly packed, usually in a regular pattern (a crystal lattice).
- They vibrate in fixed positions but don’t move past each other.
- Fixed shape and fixed volume.
- Very hard to compress.
Not all solids are crystalline. Glass and many plastics are amorphous solids, with particles locked in place but in a random arrangement.
Liquids
- Particles are close together but not in fixed positions.
- They move past each other, so liquids flow.
- Fixed volume, but take the shape of their container.
- Hard to compress.
Gases
- Particles are far apart — in air at room conditions, the space between molecules is roughly ten times their own size.
- They move fast and randomly in straight lines until they collide.
- No fixed shape or volume — a gas fills any container.
- Easy to compress, which is why gas behaviour follows the gas laws.
Plasma: the fourth state
Heat a gas enough, or pass a strong electric current through it, and the atoms lose electrons. The result is plasma — a glowing mixture of positive ions and free electrons. It conducts electricity and responds to magnetic fields.
Plasma is actually the most common state of ordinary matter in the universe, because stars are made of it. On Earth you see it in lightning, neon signs, auroras and some flat-screen TVs of the past.
(There are more exotic states too, such as Bose–Einstein condensates, formed near absolute zero — but they’re rarely part of introductory chemistry.)
Changes of state
| From → to | Name | Energy |
|---|---|---|
| Solid → liquid | Melting (fusion) | absorbed |
| Liquid → solid | Freezing (solidification) | released |
| Liquid → gas | Evaporation / boiling (vaporization) | absorbed |
| Gas → liquid | Condensation | released |
| Solid → gas | Sublimation | absorbed |
| Gas → solid | Deposition | released |
Sublimation skips the liquid state. Dry ice (solid CO₂) sublimes at −78.5 °C under normal pressure, which is why it makes fog without leaving a puddle. Iodine crystals sublime into a purple vapour when warmed. Frost forming on a window is deposition.
Evaporation vs. boiling: evaporation happens only at the surface, at any temperature. Boiling happens throughout the liquid, at a specific temperature where the liquid’s vapour pressure equals the surrounding pressure. At lower pressure — high on a mountain — water boils at a lower temperature, which is why pasta takes longer to cook there.
Why temperature stops rising during melting and boiling
Heat a block of ice steadily and measure its temperature. It rises to 0 °C, then stays at 0 °C until all the ice has melted, then rises again to 100 °C, then stays at 100 °C until all the water has boiled away.
During those flat stretches, the energy you’re adding isn’t speeding up the particles. It’s going into breaking the attractions between them. That energy is called latent heat. It’s released again when the substance condenses or freezes — which is why steam burns are so severe.
Which elements are solid, liquid or gas?
At standard room conditions (25 °C and 1 atm):
- Gases (11): hydrogen, nitrogen, oxygen, fluorine, chlorine, and the six natural noble gases (helium, neon, argon, krypton, xenon, radon).
- Liquids (2): bromine and mercury — see liquid elements at room temperature.
- Solids: everything else, although cesium, gallium, rubidium and francium melt only just above room temperature.
The home page table’s temperature slider lets you watch elements change state as you drag from absolute zero upwards — try it on the periodic table.
Quick answers
Is fire a state of matter? A flame is mostly hot glowing gas; the hottest parts of some flames are partly ionized, so they’re weakly plasma-like.
Is glass a slow-moving liquid? No — that’s a myth. Glass is an amorphous solid. Old windows that are thicker at the bottom were made that way.
Why does the same substance have different states? Because temperature and pressure change the balance between particle motion and the attractions holding particles together.
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