Evaporation

If you go outside after a rain shower on a sunny, warm day, you may notice puddles of water. If you return to the same location after a few hours, the puddles may be gone. This disappearance of the puddles is due to evaporation. Evaporation is the process that changes a substance from a liquid to a gas at temperatures below the substance's boiling point.

Figure 21 shows what is happening as water evaporates from a small, shallow container. Some molecules near the surface are moving fast enough to escape the liquid and become water vapor. (A vapor is the gaseous phase of a substance that is normally a solid or liquid at room temperature.) The greater the surface area of the container, the faster the water evaporates.

What happens if the water is in a closed container? As the water evaporates, water vapor collects above the liquid. The pressure caused by the collisions of this vapor and the walls of the container is called vapor pressure. The vapor pressure of water increases as the temperature increases. At higher temperatures, more water molecules have enough kinetic energy to overcome the attractions of other molecules in the liquid.

Figure 21 Evaporation takes place at the surface of a liquid.

Boiling

As you heat a pot of water, both the temperature and the vapor pressure of the water increase. When the vapor pressure becomes equal to atmospheric pressure, the water boils. The temperature at which this happens is the boiling point of water.

The kinetic theory explains what happens when water boils. As the temperature increases, water molecules move faster and faster. When the temperature reaches 100°C, some molecules below the surface of the liquid have enough kinetic energy to overcome the attraction of neighboring molecules. Figure 22 shows that bubbles of water vapor form within the liquid. Because water vapor is less dense than liquid water, the bubbles quickly rise to the surface. When they reach the surface, the bubbles burst and release water vapor into the air.

Figure 22 Boiling takes place throughout a liquid. Applying Concepts Explain why the temperature of water does not rise during boiling.