Solid, liquid, gas: states of matter

Everything around you is built from particles that never stop moving. How fast they move, and how tightly they cling to each other, is the only thing that decides whether you call it a solid, a liquid, a gas or a plasma.

Solid

In a solid the particles are locked into a fixed pattern and can only vibrate on the spot, so it keeps both its shape and its size. Water ice is one of the rare solids that is less dense than its own liquid, which is exactly why it floats.

Melting

Heat the solid and those vibrations grow until the bonds shake apart. Here is the odd bit: while ice is melting the temperature stops climbing. It sits at zero degrees Celsius and every scrap of added heat goes into breaking bonds instead of getting warmer.

Liquid

Now the particles slide past each other while still touching, so a liquid keeps its volume but takes the shape of whatever holds it. Squeeze as hard as you like and it barely shrinks, because water is thousands of times harder to compress than air.

Evaporating

A puddle dries up without ever boiling. At any temperature a few surface particles happen to be moving fast enough to escape, and they carry their energy away with them, which is precisely why sweating cools you down.

Boiling

Keep heating and at one hundred degrees Celsius bubbles of vapour form throughout the liquid, not just at the top. The temperature sticks again until the last drop is gone, and turning that water into steam takes more than five times the energy it took to heat it from freezing to boiling.

Gas

In a gas the particles have broken free completely and travel in straight lines until they collide with something. Water turning to steam expands to about one thousand six hundred times the space it filled as liquid, and that expansion is what pushed every steam engine in history.

Ionising

Heat a gas to thousands of degrees and the collisions turn violent enough to knock electrons clean off the atoms. The gas stops being electrically neutral and starts carrying charge, which changes almost everything about how it behaves.

Plasma

That charged soup is plasma, the fourth state. It glows, it conducts electricity and it can be shaped by a magnet. It is also the most common state in the universe, because every star is made of it, including the one warming your face.

Recombining

Let a plasma cool and the loose electrons snap back onto the atoms, each one shedding its spare energy as a flash of light. Every glowing gas works this way, from the green curtains of an aurora to the fading channel of a lightning bolt and the tube of a neon sign.

Condensing

Cool a gas and the particles slow until attraction finally wins. The colder the air, the less water can stay as vapour, so a cold glass pulls droplets straight out of the room around it. That is dew, cloud, mist on a window and the fog inside your car in winter.

Freezing

Cool the liquid further and the particles lock back into a pattern, releasing exactly the heat that melting had absorbed. Freezing and melting happen at the very same temperature, and the only difference between them is which direction the energy is travelling.

Sublimating

Some solids skip the liquid stage altogether. Frozen carbon dioxide, better known as dry ice, turns straight into gas at about minus seventy eight degrees Celsius, which is why it smokes on a stage and never leaves a puddle behind.

Depositing

And the same shortcut runs in reverse. Water vapour can turn directly into ice crystals on a cold window or a blade of grass, never once being liquid. That is frost, and it is how a snowflake is built too. Ice grows in a six sided lattice, so every flake ends up with six arms.