What is the boiling point of a liquid?

The boiling point of a liquid is the temperature at which its vapor pressure equals the pressure of its surroundings. At that point, bubbles of vapor can form throughout the liquid. Pure water boils at 100 °C (212 °F) at sea level, but at a lower temperature at high altitude, where air pressure is lower.

Boiling Point of a Liquid: Vapor Pressure and Altitude

What vapor pressure means

Molecules at a liquid’s surface are always escaping as vapor. The push of that vapor is the vapor pressure, and it rises as the liquid heats up. When it matches the pressure of the air pressing down, vapor bubbles can form inside the liquid and rise. That’s boiling.

Altitude changes everything

Place	Elevation	Water boils at (approx.)
Sea level	0 m	100 °C (212 °F)
Denver, Colorado	1,609 m	95 °C (203 °F); see the Mile High City
La Paz, Bolivia	3,640 m	88 °C (190 °F)
Summit of Everest	8,849 m	About 70 °C (158 °F)

At high altitude, pasta and beans take longer to cook, because the boiling water isn’t as hot. Some recipes include special high-altitude instructions.

Raising the boiling point

A pressure cooker traps steam, raising the pressure inside to about twice normal air pressure. Water then boils at around 120 °C, cooking food much faster. The relationship between gas pressure and volume is explained in Boyle’s law.

Adding salt raises water’s boiling point too, but only slightly: a typical pinch for pasta changes it by a fraction of a degree. Salt mainly adds flavor.

Boiling points of common substances

Substance	Boiling point at sea level
Liquid nitrogen	−196 °C
Ethanol (alcohol)	78 °C
Water	100 °C
Mercury	357 °C; see which metal is liquid at room temperature
Iron	About 2,860 °C

Boiling vs evaporation

Evaporation happens at any temperature, only at the surface, like a puddle drying. Boiling happens at one specific temperature, throughout the liquid.

At the other end of the scale, see what temperature water freezes at.