Can freezing desalt saltwater?

[vastad]

My online research indicates you can remove salt from saltwater by freezing it; however at temperatures colder than I can obtain at home. Will Dry Ice work and will it effect the project? Also, Dry Ice is hard to come by. Any suggestions? Thanks, Debbie

[donnahardy2]

Hi Debbie,

This is an interesting project! You have probably already learned that as salt water starts to freeze, it excludes the salt from the salt crystals, so if you collect the ice crystals just as they start to freeze, the salt concentration will be lower than the remaining liquid. I don't know how many sequential freeze/thaw cycles would be required to completely purify the water.

If you are in an urban location, you should be able to find a source of dry ice. The following website includes a list of stores worldwide that sell dry ice, and it includes important safety information that you should follow when working with dry ice. There are suggestions for science demonstrations for kids also. I'm sure other dry ice companies will have a similar directory.

http://www.dryiceinfo.com/science.htm

If you can't find dry ice, you could investigate other ways of purifying water, for example by distillation by boiling the water or by deionization with a mixed bed ion exchange resin.

Do you have a way to measure the concentration of salt in the water as you purify it? This would be a better project if you could do this. Please let us know if you need more information on this project.

Donna Hardy

[vastad]

Donna, Thanks for your quick reply. Unfortunately we are locked into this method of purifying saltwater. I have a few more questions, we were going to use ocean water - would you recommend that or make our own saltwater? I was concerned with making our own, that the salt wouldn't completely dissolve and effect the project. Also, by using dryice to freeze the water, will the rapid freezing affect the outcome, i.e. separating the salt molecules from the water to make crystals? I was under the impression that saltwater cannot freeze, so wouldn't all the salt be removed and develop crystals during the first freezing?

Thanks so much! Debbie

[donnahardy2]

Hi,

Salt decreases the freezing point of water, but does not keep it from freezing. If you make your own saltwater, you can use a lower concentration of salt compared to ocean water, thereby increasing the freezing point. You can do a preliminary experiment in your home freezer, which should go down to minus 20 degrees Centigrade and make sure the concentration of sodium chloride you use will freeze at the temperature you have available. I don’t know the background information on your project, but it might be better to use pure salt (sodium chloride) because ocean water contains a mixture of other elements with the sodium chloride.

Here is a website that allows you to calculate freezing point based on salinity.

http://www.csgnetwork.com/h2ofreezecalc.html

Removing the salt from salt water by freezing will require several steps. When the ice crystals are forming, the crystalline structure of the ice partially excludes the sodium chloride, but if the entire sample freezes, the sodium chloride concentration will not be reduced. So you have to partially freeze the sample, filter out the ice (which now has a reduced concentration of sodium chloride) and discard the liquid portion of the sample, (which now contains a higher concentration of sodium chloride). You will have to repeat this process until the sodium chloride in the ice is reduced to the levels you want.

Donna Hardy