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Chemistry · No. 74 of the first 100

States of matter

Ice, water, and steam are the same molecule in three moods — and the mood is set by one number.

Plate XXVI — One slider, three worlds solid → liquid → gas
Sweep the heat slowly and watch the particles change personality.
solid — locked in place, vibratingsame particles all the way through — only the jiggling changes
FIG. XXVI — ONE SLIDER, THREE WORLDS
Heat 20%
Sweep slowly through 34% and 67% — watch the personality change.
State
solid
Particle jiggle
20%
Cold: every particle vibrates in its parking spot — that's a solid holding its shape. Warmer: they break parking and tumble over each other, still touching — a liquid that pours. Hotter still: they fly apart and fill the whole box — a gas. Ice, water, steam: same molecules, different dance.
The short answer

Everything is made of particles that jiggle, and heat is the jiggling. A little jiggle: particles hold formation — a solid. More: they tumble over each other but stay touching — a liquid. Lots: they fly apart and fill the room — a gas.

What's actually happening

Hold the thought that heat is motion. Not a fluid, not a glow — literal jiggling of particles too small to see. The temperature of your coffee is the average speed of its molecules; "absolute zero" is simply the bottom of the scale where the jiggling effectively stops. With that one idea, the three states of matter stop being three kinds of stuff and become three intensities of dance.

In a cold solid, every particle is held in a lattice by its neighbours' attractions, vibrating on the spot like a person shivering in a queue — which is why solids hold their shape. Warm it and the vibrations grow until, at the melting point, they're violent enough to break the lattice grip: now particles tumble past each other while still touching. That's a liquid — it flows, but it doesn't expand. Keep heating and at the boiling point the fastest particles tear free of the liquid's surface attraction entirely, flying off to fill whatever space exists. Gas.

The subtle part is what happens at the transitions. While ice melts, you can pour in heat and the thermometer refuses to move — every joule is being spent breaking bonds, none on speeding particles up. This "latent heat" is why ice cubes hold a drink at 0 °C for ages, why steam burns far worse than boiling water (it dumps its enormous bond-breaking energy back into your skin as it condenses), and why sweating cools you: the escaping fastest molecules carry away more than their share of heat.

Try it at home Catch the flat spot
  1. 1Fill a glass with ice and a splash of water, stir, and take its temperature every minute while it sits.
  2. 2It pins to 0 °C and stays — for as long as ice remains, all incoming room heat is spent melting, not warming.
  3. 3The moment the last sliver vanishes, the temperature starts to climb. You've seen latent heat on a kitchen thermometer.