Home >> Science & Nature >> Science

How Is Mass Conserved When Water Evaporates?

It may seem like water disappears into thin air when it evaporates, but that does not mean it has been destroyed. Evaporation is a type of phase change in which liquid water transforms into water vapor, an invisible substance that retains many of the original properties of liquid water. Most importantly, mass is neither created nor destroyed during a phase change, so mass is conserved when water evaporates.

Phases of Matter
- Matter occurs in one of three major phases: solid, liquid or gas. Matter is solid when the molecules that make it up are joined in a rigid configuration with strong molecular bonds. Unlike other phases, solids retain their shape regardless of their environment. Liquids have weaker bonds than solids, which gives them much less rigidity. Because of this, a liquid will mold its shape to fit the bottom of a container. Gases have the weakest bonds and will expand to completely fill a container.
Phase Changes
- A phase change occurs when a substance is heated or cooled to a temperature that causes its bonds to become significantly weaker or stronger. Heating a substance weakens the bonds by causing the molecules to become more energetic. Conversely, cooling a substance strengthens the bonds by lowering the molecule's energetic state. A liquid such as water, for instance, has a boiling point of 212 degrees Fahrenheit. At that temperature, water changes phase to become a gas that is nearly invisible to the naked eye. This gas is called water vapor.
Evaporation
- When water evaporates, it appears to lose mass as it effectively disappears into the air. This is because water vapor (water in a gas state) is lighter than air, causing it to rapidly rise and disperse. Furthermore, water vapor has much weaker molecular bonds than water, so there are several orders of magnitude fewer molecules of water vapor in a cubic inch of air than molecules of water in the same space. Because the vapor quickly rises and is nearly invisible, it may seem as though mass is disappearing, but it is merely changing state.
Conservation of Mass
- The law of conservation of mass states that matter cannot be created or destroyed. This law can been verified experimentally for evaporating water. If you set up an apparatus to measure the exact molar mass of a flask of water, heat the beaker so the water partially or completely evaporates, and use a condenser to collect 100 percent of the resulting water vapor in a separate flask, you will find that 100 percent of the mass from the first flask is present in the second flask.

Science