Solids, liquids and gases, in 3D

What the simulation shows

• Heating: the crystal of molecules shakes loose and becomes a liquid that pools at the bottom.
• More heat: the molecules break free and fly around, filling the whole box as a gas.
• Cooling: the gas slows down and condenses back into a liquid.
• Cooling further: the molecules lock into a crystal again (a solid).
• Heat added steadily: the molecules speed up (colours go from blue to red).
• A piston pushes down to half the space: the molecules hit the walls more often, the pressure rises.
• The two halves are coloured differently and mix by themselves, with no stirring.
• A callout: air molecules move at about 500 metres a second.
• Cooled to absolute zero (−273.15 °C): the molecules almost stop moving.

The real numbers it runs on

The simulation is built on these facts, so the narration and the picture always agree:

• Everything is made of molecules that never stop moving; temperature is how fast they move.
• In a solid they're locked in place and only vibrate; in a liquid they slide past each other; in a gas they fly freely.
• Heating gives them more energy until they break free of each other: melting, then boiling. Cooling does the reverse: condensing, then freezing.
• Water melts at 0 °C and boils at 100 °C at sea level.
• Air molecules at room temperature move at about 500 metres a second, and each hits another billions of times a second.
• Gas pressure is molecules hitting the walls; squeeze a gas into half the space and the pressure doubles.
• Molecules spread out and mix on their own (diffusion); that's how a smell crosses a room.
• Absolute zero, −273.15 °C, is the coldest possible temperature: the point where molecules would almost stop.

The script this video was made from

“What heat really is”: the narration of the video above. Each highlighted moment in the simulation happens on the word that names it.

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