Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: salt-free rain water

Learn more physics!

Most recent answer: 10/22/2007
Q:
I will like to know, if 97% of the earth’s water is salty and its recycled, why isn’t the rain salty? Thank You.
- Noah (age 8)
Ojus Elementary, North Miami, FL
A:
The rain water comes from water vapor that has gone into the atmosphere, mostly from oceans. 'Vapor' means molecules that are bouncing around in mostly empty space without spending much time in contact, unlike a liquid where they stick together. When the vapor cools enough, it condenses into liquid water that falls as rain.

So why does the water vapor lack salt? Dissolved salt consists of ions- electrically charged versions of atoms. In liquid water, the electrically charged parts of the water molecules arrange so that on average their positive parts are near the negative ions, and the negative parts nearer the positive ions. That greatly lowers the energy of the ions, compared to the energy they would have off on their own. In other words, the ions stick to the liquid water. They actually stick to it much better than the water molecules do. So water molecules evaporate off into vapor, leaving the salt behind.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.