In our March 2022 activity, we investigated what happens when you pour salt on ice. This month, let’s look at how other substances affect the properties of water and ice.

Materials

• Dixie® disposable paper cups, 4
• Freezer
• Measuring cup, one-quarter cup size
• Sand
• Salt
• Spoon or other stirring device
• Sugar
• Tablespoon
   

Experiment

Add a tablespoon of salt to one paper cup, sugar to another, and sand to a third. Pour a quarter cup of water into each of the three cups and also to another fourth cup, which will be our control. Using a spoon, stir the contents of the cups until the solids are dissolved as much as possible. Place all of the cups in the freezer and let them freeze for one day. When you’re ready to continue the experiment, remove the cups from the freezer and set them out in the open. Record the amount of time necessary for the ice in each cup to melt completely. Is there a difference among them? 

What’s happening?

When water freezes, it goes from the liquid state, where all the molecules move around very quickly, to a solid state, where the molecules are more or less locked into symmetrical positions and are simply vibrating in place. When a substance such as sugar or salt dissolves in water, those “foreign” particles (molecules or ions) surround the water molecules and prevent them from arranging themselves into a symmetrical pattern when the mixtures are frozen. These frozen mixtures thus melt more quickly when removed from the freezer. Because salt has two ions that bind to water molecules, salt disrupts the ice structure more than sugar and causes it to melt the fastest. Sand does not dissolve in water and thus does not disturb it from forming solid ice. The ice-sand mixture should melt at about the same time as pure ice. Sugar dissolves in water and disrupts the ice structure but not as much as salt because each sugar molecule is only a single particle.

Solid water (ice)
Photo credit: https://en.wikipedia.org/wiki/ Ice_XI#/media/

Extension

What other items could you use to see if they impact the freezing of the water, e.g., baking soda, commercial ice melting salt, honey, etc.?  Does the form of the substance, e.g., table sugar versus powdered sugar, influence the results?

Reference

https://www.youtube.com/watch?v=JkhWV2uaHaA

To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts

- PAUL BRANDT