The simple definition of boiling point is that it is the temperature at which a liquid boils. For example, the boiling point of water at sea level is 100 °C or 212 °F. The formal definition in science is that boiling point is the temperature where the vapor pressure of a liquid equals the vapor pressure of its environment. At this temperature, the liquid changes into the vapor (gas) phase.
Difference Between Boiling and Evaporation
In both boiling and evaporation, a liquid transitions into a vapor. The difference is that all of the liquid starts changing into a vapor at the boiling point. The bubbles you see forming within a boiling liquid are this vapor. In evaporation, in contrast, only liquid molecules at the surface escape as vapor. This is because there isn’t enough liquid pressure at the interface to hold these molecules. Evaporation occurs over a wide range of temperatures, but it is fastest at higher temperature and lower pressures. Evaporation stops when the gas is saturated with vapor. For example, water stops evaporating when the air is at 100% humidity.
Factors That Affect Boiling Point
The boiling point is not a constant value for a substance. The main factor it depends on is pressure. For example, you see high-altitude cooking directions on recipes because water boils at a lower temperature at a higher altitude, where atmospheric pressure is lower. If you drop the pressure to a partial vacuum, water boils at room temperature.
Another key factor affecting boiling point is purity. Contaminants or other nonvolatile molecules in a liquid increase its boiling point in a phenomenon called boiling point elevation. The impurities lower the vapor pressure of the liquid and increase the temperature at which it boils. For example, dissolving a bit of salt or sugar in water raises its boiling point. The increase in temperature depends on how much salt or sugar you add.
In general, the higher the vapor pressure of a liquid, the lower its boiling point. Also, compounds with ionic bonds tend to have higher boiling points than compounds with covalent bonds, with larger covalent compounds having higher boiling points than smaller molecules. Polar compounds have higher boiling points than nonpolar molecules, assuming other factors are equal. The shape of a molecule slightly affects its boiling point. Compact molecules tend to have higher boiling points than molecules with large surface area.
Normal Boiling Point vs Standard Boiling Point
The two main types of boiling points are the normal boiling point and the standard boiling point. The normal boiling point or the atmospheric boiling point is the boiling point at 1 atmosphere of pressure or sea level. The standard boiling point, as defined by the IUPAC in 1982, is the temperature at which boiling occurs when the pressure is 1 bar. The standard boiling point of water is 99.61 °C at 1 bar of pressure.
Boiling Points of the Elements
This periodic table shows the normal boiling point values of the chemical elements. Helium is the element with the lowest boiling point (4.222 K, −268.928 °C, −452.070 °F). Rhenium (5903 K, 5630 °C, 10,170 °F) and tungsten (6203 K, 5930 °C, 10706 °F) have extremely high boiling points. The exact conditions determine which of these two elements has the highest boiling point. At standard atmospheric pressure, tungsten is the element with the highest boiling point.
References
- Cox, J. D. (1982). “Notation for states and processes, significance of the word standard in chemical thermodynamics, and remarks on commonly tabulated forms of thermodynamic functions”. Pure and Applied Chemistry. 54 (6): 1239–1250. doi:10.1351/pac198254061239
- DeVoe, Howard (2000). Thermodynamics and Chemistry (1st ed.). Prentice-Hall. ISBN 0-02-328741-1.
- Goldberg, David E. (1988). 3,000 Solved Problems in Chemistry (1st ed.). McGraw-Hill. ISBN 0-07-023684-4.
- Perry, R.H.; Green, D.W., eds. (1997). Perry’s Chemical Engineers’ Handbook (7th ed.). McGraw-Hill. ISBN 0-07-049841-5.