Melting, boiling, freezing points
A substance can be solid, liquid or gas. Its state can be changed by heating or cooling. The melting point of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends (usually slightly) on pressure and is usually specified at standard pressure.
When considered as the temperature of the reverse change from liquid to solid, it is called the freezing point. The freezing point is not considered as a characteristic property of a substance because of the ability of some substances to supercool, i.e. lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid.
The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. A liquid in a vacuum has a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high-pressure has a higher boiling point than when that liquid is at atmospheric pressure. In other words, the boiling point of a liquid varies depending upon the surrounding environmental pressure. For a given pressure, different liquids boil at different temperatures.
Melting and boiling points can be useful for identifying a substance because no two substances have the same melting and boiling points. They can also say whether a substance is pure or whether it has something else mixed with it because melting and boiling points change if a substance is contaminated.
Triple-point of water
The triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity. The single combination of pressure and temperature at which liquid water, solid ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C) and a partial vapour pressure of 611.73 pascals. At that point, it is possible to change all of the substance to ice, water, or vapour by making arbitrarily small changes in pressure and temperature. The gas–liquid–solid triple point of water corresponds to the minimum pressure at which liquid water can exist. At pressures below the triple point solid ice when heated at constant pressure is converted directly into water vapour in a process known as sublimation. Above the triple point, solid ice when heated at constant pressure first melts to form liquid water, and then evaporates or boils to form vapor at a higher temperature.
Itus1994