Boiling Point
Definition and meaning of Boiling Point in chemistry.
Boiling point is the temperature at which a liquid's vapor pressure equals the surrounding external pressure, causing the liquid to vaporize throughout its bulk rather than only at its surface.
In more detail
As a liquid is heated, molecules gain kinetic energy and its vapor pressure rises; boiling begins the moment that vapor pressure matches the external pressure, allowing bubbles of vapor to form within the liquid itself. Because atmospheric pressure varies with altitude, boiling point is not a fixed property of a substance but depends on the pressure applied to it, lower pressure lowers the boiling point, which is why the "normal boiling point" is defined specifically at 1 atm (101.325 kPa). Boiling point trends reflect intermolecular forces: substances with stronger hydrogen bonding, dipole-dipole attraction, or greater molar mass generally boil at higher temperatures.
Key facts
| Field | Physical Chemistry |
|---|---|
| Normal boiling point of water | 100 °C (212 °F) at 1 atm |
| Formula (water example) | H2O |
| Pressure dependence | Boiling point decreases as external pressure decreases |
Water (H2O) has a normal boiling point of 100 degrees Celsius (212 degrees Fahrenheit) at 1 atm, but at the reduced atmospheric pressure atop Mount Everest it boils at only about 70 degrees Celsius.
Frequently asked questions
Why does water boil at a lower temperature at high altitude?
Atmospheric pressure is lower at high altitude, so water's vapor pressure reaches that lower value at a lower temperature, causing it to boil below 100 °C.
Is boiling point the same as evaporation?
No. Evaporation occurs only at a liquid's surface and can happen at any temperature, while boiling occurs throughout the liquid and only at the specific temperature where vapor pressure equals external pressure.