2.21: Vaporization

The physical form of a substance changes by changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize (convert into vapor). The process is called vaporization—a surface phenomenon. For vaporization to occur, kinetic energy must be greater than the intermolecular forces that keep molecules bonded. The amount of energy needed to vaporize a quantity of liquid at a given pressure and a constant temperature is called the heat of vaporization. When liquid water is vaporized, it turns into steam.

Heating a liquid until it reaches its boiling point is one method of vaporization. Boiling occurs when vapor bubbles form beneath the surface of the liquid. The boiling point varies based on atmospheric pressure. With more atmospheric pressure, more energy is needed to reach the boiling point. At sea level, water boils at the normal or atmospheric boiling point (100 ⁰C or 212 ⁰F). At higher elevations, water requires less energy to boil. For instance, water boils at about 71 ⁰C (160 ⁰F) on Mount Everest.

Evaporation, another type of vaporization, occurs below the boiling point. In this process, water molecules with enough kinetic energy to surpass intermolecular forces escape the surface of the water as vapor. The remaining water molecules have lower kinetic energy. If this happens on a large scale, the overall kinetic energy of the liquid mass decreases, cooling the liquid. Sweating takes advantage of the phenomenon of evaporation to decrease body temperature. When perspiration evaporates off the body, the remaining sweat is cooler and helps to absorb heat from the body.

The evaporative properties of water are also used by plants to help move water up through the plant. On an environmental scale, water evaporation is the engine that drives the water cycle and much of the Earth's weather and climate.

Vaporization is the transition from a liquid to a gas or vapor. For example, when heat energy is added to water, it reaches its boiling point and becomes steam.

The amount of energy needed to vaporize a given mass of liquid at a constant temperature is called the heat of vaporization.

Vaporization can also occur at the surface of a liquid through evaporation. Outdoors, the kinetic energy is often contributed by the sun and wind. If the energy added exceeds that of the intermolecular forces, molecules can become vapor.

Evaporation of sweat is an important way for many animals to cool off. Sweat is mostly water. It can absorb heat from the skin, increasing the kinetic energy of the water molecules and causing it to evaporate from the skin's surface. This results in a cooling effect which helps maintain body temperature.

In plants, evaporation is essential during transpiration. Water evaporating from the surface of leaves helps draw up neighboring molecules, with the help of cohesion and adhesion, bringing water from the roots to the leaves.