Definition
Boiling point (abbreviated BP) is the temperature at which a liquid’s vapor pressure equals the external pressure surrounding the liquid, causing the liquid to change phase to vapor throughout the bulk of the material rather than only at the surface. At this temperature, bubbles of vapor form within the liquid and rise to the surface.
Scientific Context
- The boiling point is dependent on ambient pressure. Standard boiling points are reported at a pressure of 101.325 kPa (1 atm).
- For pure substances, the boiling point is a characteristic physical property; for mixtures, a range of temperatures may be observed (boiling point elevation or depression).
- The Clausius‑Clapeyron relation quantitatively describes how the boiling point varies with changes in pressure.
Typical Values (at 1 atm)
| Substance | Boiling Point (°C) |
|---|---|
| Water | 100 |
| Ethanol | 78.37 |
| Acetone | 56.05 |
| Mercury | 356.7 |
Factors Influencing Boiling Point
- Pressure: Increasing external pressure raises the boiling point; decreasing pressure lowers it (e.g., water boils at ~70 °C at 0.4 atm).
- Solutes: Dissolved substances elevate the boiling point of a solvent (boiling point elevation).
- Molecular Structure: Intermolecular forces such as hydrogen bonding, dipole–dipole interactions, and van der Waals forces affect the temperature required for vaporization.
Metaphorical Use
Beyond the scientific meaning, “boiling point” is used figuratively to denote a critical moment when tension or anger reaches an intolerable level, often expressed as “reaching a boiling point” in social or psychological contexts.
Etymology
The noun “point” derives from Latin punctum (“a sharp tip”), while “boil” originates from Old English bylgan meaning “to bubble, to seethe.” The compound term has been used in English scientific literature since at least the early 19th century to describe the temperature at which a liquid boils.
References
- Physical Chemistry textbooks (e.g., Atkins & de Paula, Physical Chemistry, 10th ed., 2014) for thermodynamic description.
- International Union of Pure and Applied Chemistry (IUPAC) Gold Book entry on “boiling point.”
