12.4Evaporation and Condensation

The idea

• Describe evaporation, vapor pressure, and boiling.
• Explain dynamic equilibrium in a closed container.

In a liquid, speeds vary. A molecule at the surface with enough kinetic energy can escape into the gas. That is evaporation, and it is endothermic. In an open dish the vapor drifts away and the liquid disappears. In a closed container, vapor molecules also hit the surface and rejoin the liquid. When the rates of evaporation and condensation match, the pressure of the vapor is constant. That pressure is the vapor pressure. It rises steeply with temperature because more molecules have the energy to escape.

Boiling is not just fast evaporation at the surface. It happens when the vapor pressure equals the external pressure, so bubbles of vapor can form inside the liquid and survive. At 1 atm, water boils at 100 °C. At lower pressure, on a mountain or in a vacuum pump, it boils at a lower temperature. A pressure cooker raises the boiling point by raising the pressure. Volatile liquids have high vapor pressures and low boiling points.

Worked path

A liquid’s vapor pressure is 80 kPa at some temperature, and the room pressure is 100 kPa. Is the liquid boiling?

1. Observe

   Compare vapor pressure with external pressure.

Check yourself