What molecular processes are involved when water cycles through the earth?
Water cycles the earth through molecular processes called the water cycle. The cycle consists of three molecular processes. First is called evaporation, second is condensation and third precipitation. Here is a model of the water cycle:
From the picture, you can see that as water molecules evaporate, they spread out a lot more and become lighter. And as water molecules condense, they become much closer and heavy. Now, let’s discuss each phase in the water cycle.
In evaporation how do water molecules behave?
Water molecules on earth surface (oceans, seas, rivers, lakes, plants, ground water) absorb energy from the sun increasing their kinetic energy. This increase in kinetic energy causes some water molecules to overcome the attractive forces between other water molecules. Overcoming these attractive forces causes the highly energetic water molecules to escape into the atmosphere as water vapor. As these highly energetic water molecules rise in the atmosphere, they spread out a lot more, decreasing their pressure. As the pressure of these water molecules decrease, their temperature also falls. As their temperature continues to fall as these molecules rise, they reach a certain point where the water molecules condense on floating dust particles into liquid water droplets. These dust particles generally consist of dust, smoke, and other air molecules. And they act as support to help water condense on them to form water droplets just like how leaves act as support to help water molecules condense on them to form dew.
In condensation how do water molecules behave?
As temperature of rising water molecules falls, their kinetic energy also falls, causing the attractive forces between water molecules to grow stronger. As the attractive forces between water molecules grow stronger than their kinetic energy, the water molecules condense to form snow or water in clouds. As snow and water droplets become heavy, they precipitate.
In precipitation how do water molecules behave?
As water droplets in clouds collide with other water droplets, they collect and become larger. As these droplets grow in size, they become heavy. As they become heavy, they fall as rain, snow or hail. This process of falling water or snow is called precipitation.
Let’s use the following concept map to summarize what we just discussed:
As you can tell, as water cycles through the earth, the amount of water in it remains the same before and after the cycle. Therefore, the amount of water on earth is conserved.
Also;
In the evaporation phase, energy is absorbed from the surroundings, while in the condensation phase energy is released to the surroundings. For these reasons, the evaporation phase is usually called an endothermic process, while the condensation phase is called an exothermic process.
To learn more about phase changes click here.