Evaporation

The evaporation process can be explained using kinetic theory. Like gas molecules, water molecules also move. The difference is that water molecules cannot be scattered because the attraction between molecules is still able to hold them together. Conversely, the attraction between the gas molecules is very weak, so that the gas molecules cannot fuse. When moving, water molecules have velocity. There are water molecules that have high speeds; there are also water molecules that have a small velocity. The distribution of the velocity of the water molecule resembles the Maxwell distribution.

Evaporation occurs when the speed of the water molecule is large enough so that the attraction between the water molecules is unable to hold it together. Similar to rockets moving into space, the speed of a rocket is large enough so that the gravitational force of the earth cannot hold it to stay on earth. Note that only molecules with large velocities can escape from the attraction between molecules. The molecules with small speeds remain together like water.

Water molecules have mass and velocity so that water molecules have kinetic energy (EK = 1/2 m v2). High-speed water molecules have higher kinetic energy than low-speed water molecules. Thus, it can states that water molecules that can escape from the attraction between molecules (water molecules that turn into vapor) have considerable kinetic energy. Usually, the kinetic energy of water molecules increases with increasing water temperature. If the water temperature is high enough, the kinetic energy of water molecules increases, thus, more water will turn into steam. Consistent with the results of research showing that the rate of evaporation is usually higher at high temperatures.

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When we dry wet clothes in the sun, the wet clothes absorb heat emitted by the sun. The addition of energy from the sun causes the kinetic energy of water molecules in clothing to increase. Because kinetic energy increases, water molecules move faster (the speed of water molecules increases). After the speed or kinetic energy reaches a specific value, the water molecules can escape the attraction between the water molecules, then turn to steam. Please note that drying of wet clothes is not only affected by the addition of heat from the sun. Wet clothes can also dry out because of the additional heat from the warm air around the clothes (heat moves by conduction from air to wet clothes).

During a hot day, the ground gets hotter. The land heats the air above it (in this case heat transfer by conduction). Hot air expands (the density decreases) and moves up. When passing wet clothes, air molecules pound water molecules inside clothes. Water molecules move faster, the kinetic energy of the water molecules increases. Water molecules that move quickly hit other water molecules. Because it is pounded continuously by air molecules, the water molecules move faster (the kinetic energy increases). Once the velocity or kinetic energy reaches a specific value, the fast-moving water molecules can escape the attraction between molecules and turn into steam. The kinetic energy of water molecules or air molecules is closely related to temperature. If the kinetic energy of the water molecules is large then at the same time the water temperature is high. Or vice versa, when the water temperature is high, the kinetic energy of water molecules must be large. Kinetic energy is also related to speed. The higher the kinetic energy of the molecule, the higher the speed of the molecule. Or conversely the higher the speed of the molecule, the higher the kinetic energy of the molecule.

What about hot water? Hot water has a high temperature. Because of the high water temperature, molecules in the water have a considerable average kinetic energy. Because of the considerable average kinetic energy of water molecules, many water molecules have high velocities. Water molecules have a high speed can escape the force of attraction between molecules and turn into steam. Low-velocity water molecules (water molecules with little kinetic energy) do not turn into steam. Thus, when water molecules whose high velocity turns into steam, the average kinetic energy of water molecules become smaller. The smaller the average kinetic energy, the lower the water temperature. Based on this brief description, can state that evaporation is a cooling process.

The cooling process due to evaporation is always experienced in everyday life. When the air is hot enough, a lot of heat is absorbed by the body. To keep the body temperature constant, the body usually releases heat through sweat. Because sweat gets extra heat from the sun and the air around it, the kinetic energy of sweat water molecules increases. Because of the kinetic energy of sweat water molecules increases, the speed of sweat water molecules increases. Sweat molecules turn into steam. When sweat evaporates, the body feels cool.

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Usually, after taking a bath, our body feels cool. This is because the water attached to the surface of the skin undergoes a process of evaporation.

The evaporation process described earlier always happens every day. Seawater, lake water, river water can also evaporate.

Vapor pressure

Evaporation 1What is meant by steam here is water vapor. Observe the figure. A closed container filled with water (suppose that the air inside the container has been removed). According to kinetic theory, water molecules always move. When moving, water molecules have moving speed and energy. Water molecules that have velocity and large kinetic energy to escape the force of attraction between water molecules and turn into steam.

The same process occurs in water molecules in a container. As time goes on, more and more water molecules turn into steam. Because the container is closed, the water molecules that have turned into steam cannot escape into the atmosphere (the molecules are trapped in the container). The amount of water molecules that turn into vapor is quite a lot, so there is a possibility of collisions between molecules with the walls of the container.

Some molecules that pound the walls of the container will be reflected towards the surface of the water and merge into the water. This process repeats continuously. As time goes on, more and more water molecules turn into steam (changing from liquid to vapor). At the same time, some of the molecules that crash into the walls of the container will change again to water (changing from steam to liquid). If the number of molecules changing from liquid to vapor is equal to the number of molecules that change form from vapor to liquid, there will be a balance. When a balance occurs, the top of the container containing the gas is said to be saturated. The vapor pressure in the saturated region is known as the saturated vapor pressure.

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The change in form from liquid to vapor is called evaporation. Whereas the change in form from steam to liquid is called condensation. Note that saturated vapor pressure depends only on temperature and does not depend on volume. If the water temperature increases, the kinetic energy of the water molecules increases.

The kinetic energy of water molecules increases so that the speed of water molecules must increase. Thus, more and more high-speed molecules will turn into steam (changing from liquid to steam). Because the volume of the container is fixed, the vapor pressure depends only on the number of molecules (N) and speed (v).

The more molecules turn into steam and the higher the speed of the molecules, the vapor pressure also increases. Thus, the balance will occur at higher vapor pressure. Therefore saturated vapor pressure is also higher. Saturated vapor pressure only exists when a balance occurs.

Vapor pressure depends on volume, but saturated vapor pressure does not depend on volume. If the volume of the container increases or decreases, a balance will occur. The illustration above only wants you to understand the saturated vapor pressure that happens in the atmosphere. The difference is, in the previous example we assume there is no air in the part of the container that does not contain water. Therefore the part of the tank that does not hold water is only occupied by water vapor. Conversely, the surface of the earth is always filled with air. The collision between steam molecules and other gas molecules only prolongs the balance. However, at some point, there will be a balance if the number of water molecules that turn into steam is equal to the number of vapor molecules that turn into water.

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