Summarize the six processes by which water changes from one state of matter to another. For each, indicate whether energy is absorbed from or released to the environment.

Water is the only substance that naturally exists on Earth as a solid (ice), liquid, and gas (water vapor). Because all forms of water are composed of hydrogen and oxygen atoms that are bonded together to form water molecules (H2O), the primary difference among water’s three phases is the arrangement of these water molecules (Figure 4.3).

Ice, Liquid Water, and Water Vapor

Ice is composed of water molecules that have low kinetic energies (motion) and are held together by mutual molecular attractions (hydrogen bonds). These molecules form a tight, orderly network and are not free to move relative to each other; rather, they vibrate within their fixed sites. When ice is heated, the molecules oscillate more rapidly. When the rate of molecular movement increases sufficiently, the hydrogen bonds between some of the water molecules are broken, resulting in melting.

In the liquid state, water molecules are still tightly packed but move fast enough to be able to easily slide past one another. As a result, liquid water is fluid and will take the shape of the container that holds it.

When liquid water gains heat from the environment, some of the molecules acquire enough energy to break their hydrogen bonds and escape from the water surface to become water vapor. Water vapor molecules are widely spaced compared to liquid water and exhibit very energetic and random motion.

These processes are reversed when water vapor returns to its liquid state and when water freezes. When water changes state, hydrogen bonds either form or are broken.

Latent Heat

Whenever water changes state, energy is exchanged between water and its surroundings (see Figure 4.3). For example, heat is required to evaporate water. The heat involved when water changes state is measured in units called calories. One calorie is the amount of energy required to raise the temperature of 1 gram of water 1°C (l.8°F). Thus, when 10 calories of heat are absorbed by 1 gram of water, the molecules vibrate faster, and a 10°C (18°F) temperature increase occurs. (In the International System of Units [SI], joules [J] are used to denote energy; 1 calorie = 4.184 J.)

During a phase change, energy may be added to a substance without an accompanying rise in temperature. For example, when the ice in a glass of ice water melts, the temperature of the mixture remains a constant 0°C (32°F) until all the ice has melted. If the added energy does not raise the temperature of ice water, where does this energy go? In this case, the added energy breaks the hydrogen bonds that once bound the water molecules into a crystalline structure.

Because the energy used to melt ice does not produce a temperature change, it is referred to as latent heat. (Latent means hidden, like the fingerprints hidden at a crime scene.) The energy, stored in liquid water, is released to its surroundings when the water freezes. In fact, your refrigerator runs more frequently when making ice cubes to counteract the additional energy released during the freezing process.

Melting 1 gram of ice requires 80 calories (334 J), an amount termed the latent heat of melting. The reverse process, freezing, releases these 80 calories (334 J) per gram to the environment as the latent heat of fusion. We will consider the importance of latent heat of fusion in Chapter 5, in the section on frost prevention.

Evaporation and Condensation

Latent heat is also involved in evaporation, the process of converting a liquid to a gas (vapor). The energy absorbed by water molecules during evaporation is used to give them the motion needed to escape the surface of the liquid and become a gas. This energy is referred to as the latent heat of vaporization and varies from about 600 calories (2500 J) per gram for water at 0°C to 540 calories (2260 J) per gram at 100°C. (Notice in Figure 4.3 that it takes much more energy to evaporate 1 gram of water than it does to melt the same amount of ice.) During the evaporation process, the faster-moving molecules escape the surface. As a result, the average molecular motion (temperature) of the remaining water is lowered—hence the expression “evaporation is a cooling process.” You have undoubtedly experienced this cooling effect when you step, dripping wet, out of a swimming pool or shower. In this situation, the energy used to evaporate water comes from your skin—hence, you feel cool.

Condensation, the reverse process, occurs when water vapor changes to the liquid state. During condensation, water vapor molecules release energy (latent heat of condensation) in an amount equivalent to what was absorbed during evaporation. When condensation occurs in the atmosphere, it results in the formation of fog or clouds (Figure 4.4A).

Latent heat plays an important role in many atmospheric processes. In particular, when water vapor condenses to form cloud droplets, latent heat is released, which warms the surrounding air, making it less dense and buoyant. When the moisture content of air is high, this process can spur the growth of towering storm clouds. In addition, the evaporation of water over the tropical oceans and the subsequent condensation at higher latitudes results in significant energy transfer from equatorial regions to more poleward locations. On a smaller scale, when condensation occurs on the outside of a glass filled with ice, the condensation heats the glass and eventually melts the ice.

Sublimation and Deposition

You are probably least familiar with the last two processes illustrated in Figure 4.3—sublimation and deposition. Sublimation is the conversion of a solid directly to a gas, without passing through the liquid state. Examples you may have observed include the gradual shrinkage of unused ice cubes in a freezer and the rapid conversion of dry ice (frozen carbon dioxide) to wispy clouds that quickly disappear.

Deposition is the reverse process: the conversion of a vapor directly to a solid. An example is water vapor deposited as ice on solid objects such as grass or a window pane (Figure 4.4B). These deposits are called white frost or simply frost. As shown in Figure 4.3, the process of deposition returns the combined energy released by condensation and freezing to the environment.

• condensation: The change of state from a gas to a liquid.

• deposition: The process whereby water vapor changes directly to ice, without going through the liquid state.

• evaporation: The process by which a liquid is transformed into gas.

• latent heat: The energy absorbed or released during a change of state.

• sublimation: The process whereby a solid changes directly to a gas, without going through the liquid state.

• Water can change from one state of matter (solid, liquid, or gas) to another at the temperatures and pressures experienced on Earth.

• The processes involved in changes of state include evaporation, condensation, melting, freezing, sublimation, and deposition. During each change, latent (hidden, or stored) heat is either absorbed or released.