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Q & A: evaporation of salt water and tap water

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Most recent answer: 10/22/2007
Q:
If you have a glass of tap water and a glass of tap water mixed with salt, which one will evaporate quicker?
- priscilla (age 15)
boyertown east jr. high, pa
A:
Assuming everything else (especially temperature but also breezes etc) is the same, the tap water will evaporate faster. We have an explanation on here somewhere for why the vapor pressure of near-pure water must be higher than that of water with solutes. That means that more water vapor will be found just above the tap water.
Try searching for ’vapor pressure’.

Mike W.

We also have an answer for someone who did exactly this experiment and observed the opposite of what we predicted, that the salt water evaporated more quickly. Try searching our Q and A site for the word "hygroscopic" (be sure to spell that exactly!). The vapor pressure argument is correct, but salt residue on the side of your glass can be an issue.

Tom

(published on 10/22/2007)

Follow-Up #1: Salt water evaporation rate

Q:
I think that salt water will evaporate quicker than tap water!
- stephanie (age 16)
springfield, Ohio US
A:
Hi Stephanie,
You have an interesting hypothesis but it needs to be verified.   Why don't you do an experiment and let us know the answer.  Try different amounts of salt concentrations and measure the evaporation rates.   Be sure they all start out at the same temperature.

LeeH

(published on 10/22/2007)

Follow-up on this answer.