Gaining Salt in Evaporating Water

Most recent answer: 01/18/2016

I did an experiment where I evaporated salt water. I ended up with more salt than I started with. Why did I have more salt?
- Maeve (age 11)

Nice question! Let's think about the evidence that you gained some salt. I guess it was that the crystals that were left weighed a little more than the ones you started with.  Here's something to wonder about: how do you know those crystals were really made of just the same thing as the ones you started with?

What I suspect is that the final crystals had some water in them. Another possibility is that the tap water you used had some salts (probably mostly calcium salts) in it, and those added to the salt you started with.

How can you test ideas like that?

a) The crystals pick up some water that's hard to get rid of by evaporation. If that's true then if you take those crystals and dissolve them again and again evaporate the water the same way, what's left shouldn't gain any more weight. In other words, that business of picking up some extra water would be a one-time deal.

b) Each time you evaporate the water you're left with some salts that were in it to start with. In this case, the weight would go up on each repeated evaporation, opposite to what we described in (a). Also, if you used distilled water instead of tap water this gain wouldn't happen. As another test, if you took the water and evaporated it without adding salt, you'd be left with some of those salts that were already in the water.  I know that there's at least some of this going on, if you used tap water, because just a few minutes ago I cleaned out a tea kettle that had many crystals in it left by evaporating tap water.

Combining these experiments you can figure out how much of each effect contributes.

Mike W. (posted without vetting until Lee returns)

p.s. We love questions like this where we can help figure out how to do experiments to find out what's happening.

(published on 01/18/2016)