Salt Water Evaporation

I did an science project and also wanted to find out which would evaporate faster, salt water or water witout salt. I put a cup of water in a pot without salt and then put another pot with water and salt and in the four tests that I ran, the salt water evaporated faster. Only once did the water without salt evaporated faster. Can you tell me what happened? I read the other answers about evaporation and you mentioned that water without salt evaporates faster. Thanks.
- Zaira (age 9)
Wildomar, California

Zaira- Thanks for doing this careful experiment. We can't tell you for sure what happened, but we can make an educated guess. As the water starts to evaporate, the salty water leaves behind a sort of crust inside the glass. This crust pulls water up into it, exposing more water to the atmosphere than in the plain water glass. That helps the salt water evaporate faster. There's another effect- that salt tends to hold water in solution- which goes the other way, but I guess it's not as important in this case.

Mike W.

(published on 02/21/2010)