Boiling point: definition in chemistry
Boiling point: The temperature at which a liquid's vapor pressure equals the pressure around it, so bubbles of vapor form throughout the liquid.
The normal boiling point is measured at standard atmospheric pressure (1 atm), where water boils at 100 °C. Lower pressure lowers the boiling point, so water boils at about 70 °C on top of Mount Everest, while a pressure cooker raises it to about 120 °C and cooks food faster. Boiling points depend on the strength of the attractions between particles, so polar and hydrogen-bonded liquids boil higher than nonpolar ones of similar size.
Examples
- Water — 100 °C at sea level
- Ethanol — about 78 °C
- Liquid nitrogen — about −196 °C
- Helium — about −269 °C, the lowest of any element
On the periodic table
Boiling points of the halogens rise down group 17, so fluorine and chlorine are gases, bromine is a liquid and iodine is a solid at room temperature.