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Q & A: salt and water evaporation rates

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Most recent answer: 03/15/2015
Q:
Hello, So we have investigated our own physics experiment and it was related to thermodynamics. We decided to investigate the effect of change in concentration of salt on the rate of evaporation of water. We used 0.5, 1.0, 1.5, 2.0 and 2.5 grams of salt and dissolved it in water, then using a Bunsen burner, we heated up the water and recorded the time it took for the water to evaporate. Our result showed that as the concentration of salt increases, the time it took was shorter. However the problem is according to my research, the less salty water evaporates faster. I would be really thankful if you could describe the reasoning behind this!
- Kiana (age 16)
Dubai, United Arab Emirates
A:

That's a very ineresting observation. I don't know what the reason for it is, but can give you a gues that you can test.

You're right that the dissoved salt lowers the vapor pressure of water at any temperature. That should make the saltier solutions evaporate slower. What could revere that effect?

Perhaps as the water evaporates, salty deposits are left on the sides of the beaker. Water could wick up into these salty regions, giving a bigger area from which to evaporate. Can you see any salty regions like that? 

Does the effect change at all if you use different types of glasses for the solution? What happens if you put each of the glasses on a heated metal plate at the same time, to try to make sure that they are each at the same temperature? How well-stirred were the salt solutions?

We'll be happy to follow up to help figure this out.

Mike W.

p.s. It looks like others have seen something similar: .


(published on 03/15/2015)

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