A Closer Look at Phase Changes

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Since evaporation and condensation are such important phase changes for water day-to-day in the climate system, they deserve more of our attention. We defined them in the previous section, but now, let’s look closer at how these processes work.

Evaporation is when liquid water molecules break their bonds with neighboring molecules and transform into water vapor. You might recognize it as a natural cooling mechanism from sweating on a hot summer day as your body's way of regulating temperature. But what exactly causes this cooling effect?

Firstly, it's essential to understand that water molecules with the highest kinetic energy, those with the fastest vibrations, are the most likely to break their bonds with neighboring molecules and transition into vapor. Remember, the kinetic energy of a group of molecules is directly related to the material’s temperature. Kinetic energy refers to the speed at which molecules, or their constituents in a vibration, move. The faster they move, the higher the temperature. This removal of high energy water molecules during evaporation reduces the average kinetic energy of the remaining liquid water because the most energetic molecules have gone off and transformed into vapor. Since the most energetic molecules are gone, we are left with molecules not moving quite as fast. This reduction in kinetic energy leads to a decrease in the remaining water's temperature.

Secondly, the process of breaking bonds between liquid water molecules requires energy. Where does this energy come from? The surrounding air! Simply put, as water evaporates, it extracts kinetic energy from its surroundings, including the air. So it’s a bit of a double whammy -- both the fact that high-energy molecules leave less energetic (somewhat cooler) water behind and that evaporation “steals” energy from the air -- that overall gives us cooling during evaporative processes.

As water evaporates from your skin, it feels cooler, right? That's because the fastest-moving water molecules (the ones with the most energy) are 'jumping' off into the air, taking energy with them. Meanwhile, the water molecules left on your skin are slower-moving and less energetic, corresponding to a lower temperature. Or think about mist rising off a swimming pool on a warm day. That mist is water evaporating into the air and then condensing into small droplets, carrying energy away from the pool and cooling the surface in the process.

Overall, water’s phase changes involve either absorbing kinetic energy from or releasing kinetic energy to the surrounding environment, as demonstrated in the “energy staircase” diagram for ice, water, and water vapor below. While this diagram encompasses all possible phase changes of water, our primary focus will be on two of particular interest: evaporation and condensation.

Starting with liquid water, a select group of highly energetic water molecules can gradually break their bonds with neighboring molecules and transition to the vapor phase over time. To accomplish this transition, a specific amount of energy is necessary – 600 calories from every gram to move from the “liquid” stair to the “vapor” stair. This energy input is required to break all the bonds and facilitate the rapid transformation of all the water into the gaseous state of water vapor, representing the highest energy step. This process, in turn, leads to a cooling effect on the surrounding air.

The energy levels associated with ice, water, and water vapor can be considered a set of steps. Changing from one phase (solid, liquid, or gas) to another requires either an addition of energy (stepping up) or a release of energy (stepping down).

Credit: David Babb© Penn State University is licensed under CC BY-NC-SA 4.0

So, if evaporation is a cooling process, what about its reverse -- condensation (the process by which water vapor changes to liquid)? When water vapor condenses back into water, there's a step-down in energy levels, so if you think condensation is a warming process… well, you're correct! Indeed, the energy used to evaporate water in the first place is never lost (a consequence of the conservation of energy), so as water vapor condenses into liquid water and bonds form between molecules, energy is released (600 calories per gram -- identical to the amount required for evaporation) to keep the energy books balanced. The release of this energy, called “latent heat of condensation,” warms up the surrounding air. In a way, you can think of condensation like a campfire. As the wood burns, it releases heat, warming up the surrounding air. Similarly, when water vapor condenses into a liquid, the energy that was used to evaporate it is now released back into the environment, warming it up.

So, any time a phase change (such as evaporation) causes water to go “up the energy staircase,” energy is required to break bonds between molecules (just as climbing requires effort), which cools the surrounding air. Any time a phase change (such as condensation) causes water to go “down the energy staircase,” energy is released -- after all, it is much easier to go downstairs! This warms up the surrounding air.

The warming that occurs with condensation is not easily noticeable to humans, but I bet you've noticed the impacts of evaporational cooling. When you step out of the shower, you sometimes feel a chill as the water evaporates off your skin, even in a warm room. Now you know that the cooling sensation directly results from evaporation pulling energy away from your body.

It's intriguing to realize that evaporation and condensation continually unfold in your surroundings, if their effects remain imperceptible at the macroscopic level. These dynamic processes operate on the molecular scale. Observable phase changes become apparent when there is a “net” condensation event, signifying that the rate of condensation surpasses that of evaporation, resulting in the formation of liquid water droplets. Conversely, when “net” evaporation occurs (assuming an initial presence of liquid water), it implies that the evaporation rate exceeds the condensation rate. A notable instance of net evaporation can be witnessed during the descent of raindrops. In this scenario, small raindrops diminish or completely vanish as the rate of evaporation outpaces that of condensation.

One last thing I want you to take home -- phase changes of water are critical for energy. Water can store energy and give off energy through these transitions. If water vapor moves around (for example, moist air from the Gulf of Mexico traversing up to New England it eventually rains out), it acts as a powerful transporter of energy! Water's ability to transport energy through phase changes is essential not only for weather but also for large-scale climate patterns. For example, as water evaporates over the warm oceans, it transforms kinetic energy into potential energy between the water vapor molecules that is carried up into the atmosphere. Winds can then move this water vapor and the potential energy associated with it across continents. When the water vapor eventually condenses to form liquid drops and precipitation,  the potential energy between water molecules is transformed to kinetic energy and the environment warms up. We'll talk more about this soon!