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2.21: Vaporization
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2.21: Vaporization

Vaporization changes a liquid substance into a gaseous or vaporous substance. To achieve this, 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.

Methods

Adding heat to a liquid until it reaches its boiling point is one method of vaporization. Boiling is a type of vaporization that 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 100 oC (212 oF)—this sea level temperature is called the normal or atmospheric boiling point. At higher elevations, water requires less energy to boil. On Mount Everest, water boils at about 71 oC (160 oF). In space, which lacks an atmosphere but is also extremely cold, water will first boil and then freeze—a consequence of the high heat capacity of water.

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. As water molecules are released from pores on the leaves and evaporate, they attract water molecules below them upward through molecular adhesion. On an environmental scale, water evaporation is the engine that drives the water cycle and much of the Earth’s weather and climate. About 71% of the Earth’s surface is water, so it is important to understand the mechanisms and power of water evaporation.

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