Melting Ice with Salt

The origin for this activity was one of those classic questions that kids come up with, "Momma, why do they put ice on the road in the winter?" The follow up question that my older daughter came up with was, "Why do they use salt, though?  Couldn't they use sugar?"  For those of you lucky enough to live in a warmer climate where that isn't a question in your household, a similar one might be, "Why do you put salt in the pasta water?"  (I'll describe how you can do an experiment with second question in the Extensions section below.) 

Materials Needed:

• 3 small plastic cups
• crushed ice or ice cubes
• measuring cups
• spoon
• salt
• sugar

You could this activity a variety of ways depending on what type of ice you have available.  If you only have cubes, the best way would probably be to have your child count out the same number of ice cubes for each cup.  For crushed ice, you could either do a volume measurement (using the measuring cups) or, if you have a kitchen scale, you could measure out the same mass/weight of ice for each cup.

Process and Pictures:
Measure out an equal amount of ice for each of the cups, either by number of cubes, volume of crushed ice using a measuring cup or mass of crushed ice using a kitchen scale.  We used 1/2 cup for each of our cups but use what works for your cups.

Add about 1/4 cup of salt to one of the cups, 1/4 cup of sugar to another cup and don't add anything to the third cup.  Ask your child to predict which of the cups will melt the fastest.

Use the spoon to stir each of the cups.  Make sure to rinse the spoon between cups so that you are not contaminating the ice with salt or sugar from another cup.  Or use two different spoons.

Wait - the hardest part for the little ones.  If you are using science journals, then this would be a good time for your child to draw the experimental set-up in their journal.  Record how long it takes for each of the cups to melt.  This takes a while (1-2 hours +) so you will have to go do some other things and come back and check every 15 minutes or so.

From L to R: plain ice, ice with table salt, ice with sugar

Revisit the prediction your child made at the end regarding which cup had the ice that melted the fastest.

What's Happening:
Water typically freezes at 32 degrees Fahrenheit.  When table salt (NaCl - sodium chloride) is added to ice, it depresses the melting point; a 10% table salt solution will freeze at 20 degrees Fahrenheit and a 20% table salt solution will freeze at 2 degrees Fahrenheit.   Sodium chloride is an ionic solid which means that in water it will dissolve into two different ions, a positively charge sodium ion and a negatively charged calcium ion.  The ions interfere with the structure of the ice, making it more difficult to freeze.  Similarly, when you add table salt to boiling water it increases the boiling point of water making the pasta cook faster.  Other salts will similarly depress the melting point of ice; sodium chloride will actually only work well to about 15 degrees Fahrenheit whereas other salt (magnesium chloride, calcium chloride) is capable of bringing the freezing point down to a lower level. Interestingly, the sugar will also hasten the melting of the ice because it also interferes with the structure of the ice - it just won't melt it as fast because it lacks the ionic structure of salt.

Extensions:
A great extension to this activity is to make homemade ice cream.  Here's a good link with directions for kids to make them in plastic bags. Kids love this process, but be forewarned to have the kids use mittens or to hold the bags as they get cold enough to cause frostbite!

If you have a thermometer that will measure below 32 degrees Fahrenheit use this to measure the temperature of the different mixtures.  You could also add different types of salt (epsom salt, rock salt, table salt, etc.) and record the different temperatures that each mixture drops to.  Unfortunately, most kitchen thermometers do not go below 32 degrees Fahrenheit, but you may be able to find one at a teaching supply store.  A good place to find some cheap, classroom-style thermometers (and tons of other lab supplies) is American Science & Surplus.  I think their "small, two-way thermometer" is a great choice for kitchen science experiments.  Similarly, for older kids you could alter the experiment by exploring how different substances affect the boiling point of water.