States of Matter
Solid, liquid, and gas are not different kinds of stuff — they are the same atoms in different arrangements, held at different energies. The distinction between phases is a question of how strongly the atoms are bound to one another relative to how fast they are moving.
In a solid, atoms sit in a fixed lattice. Their thermal motion is present but limited to vibration around fixed positions. The bonds between neighboring atoms are strong enough that the thermal energy can't break them; the structure holds its shape. Solids are rigid.
In a liquid, atoms are still close together and still attracting each other, but the thermal energy is now high enough that they can slip past their neighbors. A liquid has no fixed shape but keeps a fixed volume — the atoms stay roughly the same distance apart on average even as they flow around one another.
In a gas, atoms have enough energy to fly apart and largely ignore each other. The dominant behavior is free motion punctuated by occasional collisions. Gas expands to fill any container because there is no binding force strong enough to hold atoms near one another.
The Transitions
The phase transitions between these states — melting, freezing, evaporation, condensation — happen when the balance between thermal energy and binding energy tips. Add heat to a solid and its atoms vibrate faster; at the melting point, they have enough energy to escape the lattice while still staying near each other. Add more heat and they escape each other entirely: the liquid becomes a gas.
Pressure matters too. At high pressure, atoms are forced close enough together that even fast-moving ones can't escape their neighbors' pull — this is why water can remain liquid above 100°C under a pressure cooker. At low pressure, the threshold flips: water evaporates at room temperature in a vacuum.
What This Resolves
The atomic picture of states of matter dissolves several apparent mysteries. Why does ice float on water? Because the hydrogen-bond lattice structure of ice is less dense than liquid water, so solid water takes up more space than liquid water — an unusual inversion of the general rule. Why does steam have so much energy relative to liquid water? Because vaporization requires breaking all the residual bonds, not just allowing slippage — you have to add the "latent heat of vaporization" on top of just bringing the temperature up.
Connections
- atomic-hypothesis — states are properties of atomic arrangements
- heat-as-atomic-motion — temperature determines which state is stable
- dynamic-equilibrium — the liquid-gas boundary is a dynamic balance, not a sharp wall