Evaporation |
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In a closed container Molecules leave and re-enter the surface of a liquid enclosed in a container, in equilibrium with its own vapor, at equal rates. The temperature remains constant. In an open container In an open container more energetic molecules leave more often and the remaining liquid has less internal energy per kilogram. ie. it is cooled. Cooling by evaporation is one of the most convenient and common ways of lowering temperature. |
| Note: a liquid boils when the vapor pressure reaches the pressure of the atmosphere above it and bubbles of vapor form throughout the liquid. Heating water in a microwave oven in a very smooth clean container is dangerous, because the added pressure at the bottom of the liquid may prevent the formation of bubbles of vapor and the water may become superheated. Any small disturbance then causes sudden catastrophic conversion of water to vapor. |