Friday, January 30, 2015

Exploring the Effects of Salt on Ice with Elementary Students

Exploring the Effects of Salt on Ice
With Elementary Students
January is a great month to explore the properties of water.  For my monthly themed class we explored the effects of salt on ice.  We started with an investigation.  This created an experience where they started to wonder what was going on.  Students investigated the question:  Does the amount of salt affect the ability of a piece of string to lift an ice cube?

Here are the directions to demonstrate lifting an ice cube with a piece of string.

What You Need
 Ice Cube
Cup of water
Piece of string
1.  Fill the cup to the top with water.
2.  Put one ice cube in the water.  It should float.
3.  Place the center of the string across the ice cube.  The ends should be hanging over the cup.
4.  Sprinkle one pinch of salt onto the string and ice cube.
5.  Let the string and salt sit on the ice cube for 30 seconds.
6.  Carefully pick up the ends of the string and try to lift the ice cube.  


Salt melts ice, right?  Why would the string stick? Isn’t the ice cube melting?  Ice begins to melt at 0 degrees Celsius.  Salt lowers the freezing point of water.  It actually gets colder when you put salt on the ice.  A thin layer of the ice melts, the string sinks into the ice, and a thin layer of water re-freezes around the string.  This is why we use salt to make ice cream.  The mixture of ice and salt around the container of milk and sugar causes the temperature to drop.  When the temperature drops the mixing milk and sugar begin to become a solid.  It isn’t magic that we get ice cream, it is all the science of freezing and melting points.  On very cold winter days salt will not work on ice because it is too far below the freezing point.  Here is the link to the two investigation journals I used.
Dangling Ice:  A 2nd/3rd Grade Experiment on Freezing and Dangling Ice:  A Science Experiment for 4th, 5th, or 6th Grade
 After we completed the science journal each student observed an ice cube with salt and an ice cube without salt.  They all noticed that the ice cube with salt was melting faster.  This made them even more curious because how could string stick to something that was melting.  I made sure to focus on how the ice cube with salt had cracks and indents where the salt was.  Those are the cracks the string sinks into.

Observing the Changing Temperature
Now is the time to show the students that the temperature is actually decreasing with the salt. 

Place a thermometer in each Styrofoam cup.  Fill the cups with ice.  For this experiment we used rock salt.  Put two spoonfuls of rock salt in one cup. Shake the cup to get the salt to mix with the ice.   Students  will observe the temperature for 5 minutes.  The temperature will drop faster in the with salt cup.  In the picture below the left picture contains salt.  It is at -10 Celsius.  It can actually drop even further.  We then discussed how the string sunk into the ice and because the temperature dropped some water refroze over the string.  This really helped the students understand.

It is hard to not do this lesson without showing how to make ice cream.  So that is exactly what we did next.  I simplified it a little by using chocolate milk. 

Zip lock Bag Ice Cream
1 Gallon freezer bag     1 Quart freezer bag     Chocolate Milk
                                                                       (or an actual recipe for ice cream)

Ice Cubes                      Rock  Salt                   Kitchen Towel   
                                                                                                                 ( Kids could wear gloves)

* I also did this with pudding.  I took vanilla pudding and mixed it with almond milk.  The pudding won't solidify as well with almond milk than regular milk.  I took the pudding/almond milk mixture and followed the same directions below.  The yellow looking ice cream pictures are the pudding.  My kids really liked it.  It reminded me of the pudding pops I had as a kid.


1.  Fill the gallon freezer bag half full of ice.
2.  Add about 1/4 to a 1/2 cup of rock salt. 
3.  In the 1 quart freezer bag add 1 cup of chocolate milk or the recipe you are using.
4.  Zip the 1 quart bag well. Make sure you get the air out. You can double up the bag if concerned about leakage.
5.  Place the 1 quart bag into the gallon bag with the ice.   Place the bag so that it is buried into the ice.  Zip the gallon bag. Make sure to get out the air.
6.   Wrap the bag in the towel.  It will be too cold to touch.
7.  Start shaking the bag.  Make sure to flip the bag over and upside down too.  You want to make sure the bag with the liquid is buried in the ice.
8.  Shake and flip the bag for 5 minutes.  It may take up to 10 minutes depending on your mixture.
9.  Open the bag and take out the ice cream bag.  Use the towel to wipe off any salt. 
10.  Open the bag.  This is the best part.  You can put the ice cream in a bowl or fold the bag over and eat out of the bag.

Enjoy the wonders of salt lowering the temperature causing our mixture to solidify into ice cream, ice milk, pudding pop, or whatever you tried.

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