The states of water
Solid, liquid and gas — and the transitions between them, which move more energy around the planet than almost anything else.
Water is the only substance that occurs naturally on Earth’s surface in all three states. That is not a coincidence of chemistry so much as a coincidence of temperature: the planet happens to sit near water’s phase boundaries, and almost every weather system is a consequence.
The phase transitions
| Transition | From → to | Energy | Where it matters |
|---|---|---|---|
| Melting | Solid → liquid | 334 kJ/kg absorbed | Spring snowmelt; ice keeping drinks cold |
| Freezing | Liquid → solid | 334 kJ/kg released | Frost protection in orchards; pipes bursting |
| Vaporisation | Liquid → gas | 2,260 kJ/kg absorbed | Evaporative cooling; the energy driving weather |
| Condensation | Gas → liquid | 2,260 kJ/kg released | Cloud formation; the energy powering storms |
| Sublimation | Solid → gas | 2,838 kJ/kg absorbed | Snow disappearing without melting in cold dry air |
| Deposition | Gas → solid | 2,838 kJ/kg released | Frost forming on a clear night |
Values at standard pressure. Note that vaporisation takes nearly seven times the energy of melting — which is why evaporative cooling works so well and why a boiling pan takes so long to boil dry.
The fixed points
- Melting point
- 0 °C at standard atmospheric pressure. Falls slightly under pressure, which is unusual — for almost every other substance it rises.
- Boiling point
- 100 °C at standard pressure. Falls with altitude: about 93 °C in Denver, about 71 °C at the summit of Everest, which is why cooking takes longer at height.
- Triple point
- 0.01 °C and 611.657 Pa, where solid, liquid and gas coexist in equilibrium. It is so precisely reproducible that it defined the kelvin until 2019.
- Critical point
- 373.95 °C and 22.06 MPa. Above this there is no distinction between liquid and gas — supercritical water is an aggressive solvent used industrially to destroy organic waste.
- Maximum density
- 3.98 °C, not the freezing point. This is why lakes stratify and why they freeze from the top down.
Why boiling point falls with altitude
A liquid boils when its vapour pressure equals the pressure of the atmosphere pressing on it. At altitude there is less atmosphere above, so less vapour pressure is needed, so boiling happens at a lower temperature.
This matters practically: boiling water to make it microbiologically safe works because the pasteurisation temperatures that kill waterborne pathogens are all well below boiling point even at considerable altitude. Reaching a rolling boil is the signal that the water got hot enough, not the mechanism.
- One litre of water, so one kilogram
- Starting at 12 °C, a typical UK cold mains temperature
- 100% heating efficiency, which an electric kettle approaches closely
- Heat it to boiling
1 kg × 4.184 kJ/kg/K × 88 K = 368 kJ - In kilowatt-hours
368 ÷ 3,600 = 0.102 kWh - Now boil it dry — the latent heat
1 kg × 2,260 kJ/kg = 2,260 kJ = 0.628 kWh
Heating the water to boiling takes about 0.1 kWh. Actually turning it all to steam would take a further six times that. The latent heat of vaporisation dominates completely.
Real kettles lose some heat to the room and to the kettle body. The comparison between the two figures is the point, not the absolute values.
States that are not quite states
- Supercooled water: liquid below 0 °C, common in clouds. It freezes instantly on contact with a nucleation site, which is what causes aircraft icing and freezing rain
- Amorphous ice: frozen without a crystal structure, formed by extremely rapid cooling. It is probably the most abundant form of water in the universe, in interstellar dust
- At least seventeen crystalline ice phases exist at various pressures. Ordinary ice is ice Ih; the others require pressures not found naturally on Earth’s surface
Sources
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain
- International Association for the Properties of Water and Steam — IAPWS formulations for the properties of water. Public but restricted · Publisher terms — public access, reuse not clearly granted