Put some water in a closed flask and wait. Molecules at the surface with enough energy escape into the gas, and gas molecules that hit the surface stick. The forces from Comparing Intermolecular Forces decide how easily they escape, and the result is a pressure you can measure, the same kind of Gas Pressure you met before.
Key idea
In a closed container, evaporation and condensation reach a dynamic equilibrium; the pressure of the vapor then is the vapor pressure. It depends only on the substance and the temperature: weaker forces between molecules or higher temperature mean a higher vapor pressure. A liquid boils when its vapor pressure equals the pressure above it; the normal boiling point is the temperature where that pressure is 1 atm.
Why it works
At first the vapor is empty, so molecules only leave. As vapor builds up, more of them hit the surface and return, until the two rates match. The vapor pressure doesn't depend on how much liquid there is, because both rates are per unit of surface: more liquid doesn't change how fast molecules leave each square centimeter. Heating gives more molecules enough energy to escape, so the vapor pressure climbs steeply. Boiling is bubbles of vapor forming inside the liquid; a bubble survives only if its vapor pressure can push back against the pressure on the liquid.
| Temperature | 20 °C | 40 °C | 60 °C | 80 °C | 90 °C | 100 °C |
|---|---|---|---|---|---|---|
| Vapor pressure of water (mmHg) | 17.5 | 55.2 | 149 | 355 | 525 | 760 |
Worked example: Boiling in the mountains
At a high-altitude camp the air pressure is about $525\ \text{mmHg}$. At what temperature does water boil there?
- Water boils when its vapor pressure equals the pressure above it.
- From the table, water's vapor pressure reaches $525\ \text{mmHg}$ at $90^\circ\text{C}$.
- So water boils at about $90^\circ\text{C}$, and food cooks more slowly. (A pressure cooker does the reverse: at about 2 atm, water boils near $120^\circ\text{C}$.)
Answer: About $90^\circ\text{C}$.
Worked example: Which evaporates faster?
At $25^\circ\text{C}$, which has the higher vapor pressure, water or ethanol? Their normal boiling points are $100^\circ\text{C}$ and $78^\circ\text{C}$.
- The lower normal boiling point means ethanol reaches 1 atm of vapor pressure at a lower temperature.
- That reflects weaker forces between ethanol molecules than between water molecules (water forms more hydrogen bonds per molecule).
Answer: Ethanol (about 59 mmHg against 24 mmHg).
Common mistake
Two common beliefs. First, that more liquid in the container gives a higher vapor pressure; it doesn't, because evaporation and condensation both happen per unit of surface. Second, that water always boils at $100^\circ\text{C}$; that is only true at 1 atm. Check: the boiling point is wherever the vapor pressure curve meets the outside pressure.