Definition and properties
The boiling point is the characteristic temperature at which a substance passes from the liquid to the gaseous state. The liquid and gaseous phases are in equilibrium at this point. A typical example is water, which starts boiling at 100 °C and becomes water vapor. The boiling point depends on the pressure. For example, at a lower atmospheric pressure – in the Alps, for example – water begins to boil just a few degrees below 100 °C, depending on the altitude. Evaporation, by the way, is not the same as boiling. Evaporation can cause liquids far below the boiling point to pass from the surface of the substance into the gas phase. The boiling point depends on the strength of the intermolecular interactions. The higher these are, the higher the boiling point. For example, sodium chloride (table salt) has a high boiling point of 1465 °C due to the strong ionic interactions. The boiling point of water is much lower due to the weaker hydrogen bonds. Another influencing variable is the molecular mass.
Areas of application
The boiling point is used, among other things, in analytics, for identification, characterization and for quality assurance.
Examples
Melting point of selected substances at normal pressure:
- Tungsten: 5930 °C
- Sodium chloride: 1465 °C
- Olive oil: approx. 700 °C
- Glycerol: 290 °C
- Propylene glycol: 188 °C
- Water: 100 °C
- Gasoline: about 85 °C
- Ethanol: 78 °C
- Acetone: 56 °C
- Diethyl ether: 35 °C
- Sulfur dioxide: -10 °C
- Propane: -42 °C
- Hydrogen: -253 °C
