Change of state

Matter exists as solid, liquid or gas. A change from one to another is a change of state, and it happens by exchanging heat with the surroundings.

Melting (fusion) turns a solid into a liquid; freezing is the reverse. While ice melts the temperature stays fixed until all of it has melted: solid and liquid coexist in thermal equilibrium.

The melting point is the temperature at which the solid and liquid are in thermal equilibrium. It is a property of the substance and depends on pressure; at standard atmospheric pressure it is the normal melting point.

Regelation: a wire with a 5 kg block hanging from each end sinks through a slab of ice. The pressure under the wire lowers the melting point so ice melts there, and the water freezes again above the wire; the slab stays in one piece.

Skating works because the pressure under the skate melts a thin layer of snow or ice, and that water acts as a lubricant.

Vaporisation turns a liquid into vapour. While water boils, liquid and vapour coexist and the temperature stays fixed until all the liquid has changed; that temperature is the boiling point.

The boiling point rises with pressure. In a flask of water taken off the flame and closed, pouring cold water on the flask lowers the vapour pressure inside, and the water boils again at a lower temperature.

On hills the air pressure is lower, so water boils below 100 °C and food takes longer to cook. A pressure cooker raises the pressure, so water boils above 100 °C and food cooks faster. The normal boiling point is the value at standard atmospheric pressure.

Sublimation is a direct change from solid to vapour without melting; the reverse is also called sublimation. Dry ice (solid CO₂) and iodine sublime.

On a pressure–temperature (phase) diagram the sublimation, fusion and vaporisation curves separate the solid, liquid and vapour regions. They meet at the triple point, where all three phases coexist; for water it is 273.16 K at about 611 Pa (roughly 0.006 atm).