States of Matter
Ice, water, steam - the same stuff in three disguises, and heat is the only difference.
Solid, liquid, gas - it's all about how fast the particles move
Junior level — plain language, no maths
Ice, water, steam - three completely different things, and yet they're all the exact same stuff: \(H_2O\). The only difference is how much energy the tiny particles have and how tightly they hold onto each other. Add heat and you don't change what the particles are, you just make them jiggle faster. That's the whole secret of the three states of matter.
In a solid the particles are packed in a neat, fixed pattern, only vibrating on the spot - so a solid keeps its shape. Add heat and they vibrate harder until they break free of their rows and start sliding past each other: now it's a liquid, which flows and takes the shape of its container but keeps the same volume. Heat it more and the particles tear away from one another completely and fly around freely, bouncing off the walls - a gas, which spreads to fill any space it's given.
The jumps between states have names: melting and freezing (solid ↔ liquid), boiling and condensing (liquid ↔ gas). Here's a surprise: while something is melting or boiling, its temperature doesn't rise at all, even as you keep heating it. All that energy goes into breaking the bonds between particles, not into speeding them up. That's why a pot of boiling water stays stuck at 100 °C no matter how high you crank the hob.
Things worth knowing
- Water is unusual: ice floats on its own liquid, because solid water is less dense than liquid. Almost everything else sinks in its own melt.
- While ice melts or water boils, the temperature holds steady - the heat goes into breaking bonds, not raising temperature.
- There's a fourth state, plasma - superheated gas stripped of electrons. Stars, lightning and neon signs are plasma, the commonest state in the universe.
Particle motion, phase changes, and latent heat
Student level — the core equations
The kinetic theory says matter is made of particles in constant motion, and temperature is simply a measure of their average kinetic energy. The state depends on a tug-of-war between that motion (which scatters particles) and the attractive forces between them (which hold them together). Solid: forces win, particles locked in a lattice, only vibrating. Liquid: a near-draw, particles touching but mobile. Gas: motion wins, particles far apart and independent.
Heating a solid follows a heating curve: temperature rises steadily, flattens at the melting point, rises through the liquid, flattens at the boiling point, then rises through the gas. The flat stretches are the phase changes, and there the added energy - the latent heat - goes entirely into overcoming the intermolecular forces, not into raising temperature. It's why steam burns are so severe: condensing steam dumps a large latent heat straight onto your skin.
Pressure matters too, not just temperature. Squeeze a gas hard enough and it liquefies; drop the pressure and a liquid boils at a lower temperature (which is why water boils below 100 °C up a mountain). A phase diagram maps which state exists at each pressure and temperature, including the triple point (all three coexist) and the critical point (beyond which liquid and gas become one).
Key Formulas
| Temperature | \(T \propto \overline{KE}\) | average kinetic energy |
|---|---|---|
| Latent heat | \(Q = mL\) | energy for a phase change |
| The transitions | \(\text{solid} \xrightarrow{T_m} \text{liquid} \xrightarrow{T_b} \text{gas}\) | |
Things worth knowing
- Steam at 100 °C burns far worse than water at 100 °C: as it condenses it releases its latent heat straight into your skin.
- Water boils below 100 °C up a mountain, where air pressure is lower - which is why cooking takes longer at altitude.
- Dry ice (solid CO₂) skips the liquid state entirely: it sublimes straight from solid to gas at ordinary pressure.
Kinetic theory, phase diagrams, and the states beyond the big three
Scholar level — full mathematical depth
01Kinetic-molecular theory
Particles are in ceaseless motion with a Maxwell-Boltzmann distribution of speeds; temperature sets the average, but there is always a spread. That spread is why evaporation happens well below boiling: the fastest molecules escape the surface, carrying energy away and cooling what remains - exactly how sweating cools you.
02Intermolecular forces set the transitions
Melting and boiling points reflect the strength of the forces between particles - van der Waals < dipole < hydrogen bonding < ionic or metallic. Water's boiling point is strikingly high for such a small molecule because of hydrogen bonding, the same bonding that spaces the molecules out in ice and makes it float.
03The phase diagram
A pressure-temperature map divides solid, liquid and gas with coexistence lines. Two points are special: the triple point, the unique P and T where all three coexist, and the critical point, beyond which the liquid-gas distinction dissolves into a single supercritical fluid. Latent heats are the energies needed to cross each line.
04Beyond the big three
There is plasma - ionized gas, and the most common state in the visible universe - and, near absolute zero, genuinely quantum states: Bose-Einstein condensates, in which atoms merge into a single shared quantum state, and superfluids and superconductors that flow or conduct with zero resistance. "State of matter" is a far richer idea than the three we meet first.
Key Formulas
| Average kinetic energy | \(\overline{KE} = \tfrac{3}{2}k_BT\) | |
|---|---|---|
| Latent heat | \(Q = mL\) | |
| Clausius-Clapeyron | \(\dfrac{dP}{dT} = \dfrac{L}{T\,\Delta V}\) | slope of a phase boundary |
Things worth knowing
- Near absolute zero, atoms can merge into a Bose-Einstein condensate - all sharing one quantum state and behaving as a single "super-atom".
- The triple point of water (0.01 °C, 611 Pa) is so reproducible it was used to help define the kelvin temperature scale.
- Above its critical point (374 °C, 218 atm) water becomes a supercritical fluid - neither liquid nor gas, and a powerful solvent.