Have you ever wondered why ice cubes in your cold drink become gradually smaller or why their surface becomes smoother as they melt? Does ice always melt this way?
In this activity you will use water balloons to create giant ice balls and observe how they melt. Can you predict the effect a bit of salt will have on your giant ice ball?
All matter is made up of tiny particles, and temperature is a measure of how much these tiny particles move. Even the particles in a solid like ice move—they vibrate. But because they are cold they do not wiggle much and can hold onto one another. In the case of water the particles arrange themselves in a regular lattice.
When ice is surrounded by air or liquid, at room temperature it absorbs heat from its surroundings. As a result the tiny particles in the ice start to vibrate more. Those at the edge might break loose and flow freely over and alongside one another and the ice. At this stage water and ice coexist. This is an active process—some particles break loose whereas others attach to the solid. Because the surroundings are at a higher temperature, more break loose than reattach so we see the ice melt. For pure water this coexistence of water and ice happens at 32 degrees Fahrenheit (0 degrees Celsius). When the surroundings are cooler heat will flow to the surroundings, cooling down the water particles. As a result more particles attach to the lattice, and we see the ice grow.
When table salt (sodium chloride) is dissolved in water, its sodium and chloride atoms are added to the water particles. These make it more difficult for water particles to arrange themselves in a regular lattice and solidify into ice. That is why you need to cool saltwater well below 32 degrees F before it becomes ice.
When pure ice is surrounded by a saltwater solution at room temperature particles at the edge of the ice will absorb heat, break loose and flow freely. At the same time some water particles might try to reattach. Will the sodium and chlorine particles be in the way, making it more difficult for the water particles to reattach to the ice and causing the ice to melt faster? Do this activity to find out!
- At least two water balloons
- Oven mitt
- Small plates
- Table salt
- Water dropper, baster or syringe
- Food coloring, preferably liquid
- Workspace that can get wet
- Towel to wipe your workspace
- The day before you plan to do the activity fill a balloon with water, tie it with a knot and freeze it overnight. Freeze at least two balloons for each person doing the activity.
- Just before you plan to start the activity fill a cup with water and add food coloring.
- Choose an area that can get wet to do the activity.
- Put on oven mitts, and retrieve the two frozen water balloons from the freezer.
- Peel off the balloon skin so you are left with two ice balls. (Some ice balls might be in a pear shape rather than a ball—that is fine.)
- Place each ice ball on a small plate and place them next to each other. What do you think will happen if you sprinkle salt on the ice ball?
- Sprinkle about one-eighth teaspoon of salt on the top of each ice ball, add a few drops of colored water to moisten the salt and observe.
- Wait a few minutes. What happens? Is it what you predicted?
- Drip more colored water over the top of the ice ball on the left. Leave the other untouched. Do you think one ball will melt faster than the other? If so, which one and why?
- Observe, and intermittently drip water over the left ball, leaving the other untouched. Use your dropper, baster or syringe to suck up the water collected in the plate and drip it back over the left ice ball. You may need to discard water from the plate when it looks like it may overflow. Does one ball melt faster than the other does? Why do you think this is the case?
- Occasionally sprinkle more salt on top of both ice balls, followed by a few drops of colored water to wet the salt. What happens when you add more salt?
- Can you see patterns appear in the ice? If so, are the patterns on the two ice balls similar? Can you explain what you observe?
- Extra: Have fun adding food coloring, salt and water. Can you make beautiful patterns?
- Extra: Hold a flashlight behind your melting ice balls and see how the patterns light up.
- Extra: Explore other scenarios, such as sprinkling salt without adding a few drops of water or dripping saltwater over the ice balls. Do these changes impact the ice differently? Why do you think would this be the case?
Observations and results
Did you see how the ice melts faster where it is in contact with wet salt? Did streams of melted water appear on the left ice ball, and did deep caverns pierce through the ice on the other ball?
When ice melts, water and ice coexist. Because salt particles make it harder for water particles to freeze back onto the ice, the ice that is in contact with dissolved salt melts faster. When the saltwater flows over the surface it melts the ice on its way, creating channels, like rivers, over the surface of the ice ball. When the saltwater is trapped in one location, such as in the case of the ice ball on the right, it erodes a path down into the ice, creating sharp ridges or peaks. Food coloring makes these patterns more visible.
When salt is sprinkled over the ice without adding water, the salt will dissolve in meltwater and have the same effect—it only takes a little longer to see the results.
More to explore
What Makes Ice Melt Fastest, from Scientific American
Chemistry of Ice Cream–Making: Lowering the Freezing Point of Water, from Science Buddies
Homemade Slushies, from Science Buddies
Make Ice Cream in a Bag, from Science Buddies
Use Chemistry to Lift Ice Cubes, from Science Buddies
STEM Activities for Kids, from Science Buddies
This activity brought to you in partnership with Science Buddies