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Is Evaporation a Result of a Chemical Change?

Evaporation is a process by which the surface layer of a given liquid reaches a temperature where the molecules have enough energy to turn into a vapor. Most notably, evaporation is part of the water cycle, where oceans and other bodies of water warm to the point that molecules at the surface rise into the sky to form clouds. Yet, while the form of the water changes, at no point does its chemical composition alter.

Chemical Change
- A chemical change is when the chemical make-up of a substances is changed, the result of which is irreversible. Often such chemical changes also create a physical change, such as an alternation in color, odor or form. Such actions as the digestion of food, explosion of fireworks or color change of leaves are all examples of chemical changes. In each of these changes, the original matter that undergoes the change cannot regain its original form; the food is converted to energy, the fireworks burn away and the leaves never become green again.
Physical Change
- Some substances, such as water, undergo a physical change in which the form and density of their molecules change but the actual chemical composition of the molecule does not. In the case of water, heat may cause the physical structure to change from solid (ice) to liquid (water) to gas (steam) but never does the molecular structure alter from two hydrogen atoms to one oxygen atom. Once the heat is gone, the evaporated water will return to its liquid form. Thus evaporation is the result of physical change and not a chemical change.
Water Cycle
- In the water cycle, water can begin as a liquid somewhere on the surface of the planet, it can be in an ocean, lake, river, in the soil or any other such place where the sun can reach it. As the water absorbs heat from the light the molecules become more active and start to move and collide with each other, eventually gaining enough energy to form into a vapor that is lighter than air. At a certain altitude, the water cools enough to condense and gather into clouds. The water then falls back to the ground in its liquid form. In this process the water changes from a liquid to a gas back to a liquid, but at no time does it cease to be water.
Observation
- While boiling isn't the same as evaporation in that the entire liquid is heated to the point of steam and not just the surface molecules, the same physical change that causes evaporation can be seen in boiling. When boiled, a liquid becomes hot enough that it converts from that form into a gas, and if gathered and cooled, that gas reverts to its liquid form. For example, if a pot of boiling water is covered to trap the steam, that steam gathers on the inside of the cover where it again becomes water. This not the result of any chemical change in the water, just a change in its physical form.

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