Have you ever had a drink with ice in it? You’ve probably noticed that the ice melts slowly and keeps your drink nice and cold for a long time. Ice melts because the liquid around it is warmer than the ice itself. But ice exists in nature too, especially in colder climates and during the winter.
When ice forms on bodies of water, it too will melt when the water warms up. But does it melt faster in freshwater, like lakes or rivers, or does it melt faster in saltwater that makes up our oceans? The video above shows a fun way to answer this question and learn a little more about water. Here is how it’s done:
Materials
Ice tray
Water
Blue food coloring (or any color you would like to use)
2 clear plastic or glass cups
Salt
Spoon
Stopwatch (optional)
Adult supervision (Adult supervision at all times please)
Procedure
**Day before preparations
- Fill your ice tray with water.
- Add 3-4 drops of food coloring in each well and stir it in.
- Carefully place your ice tray in the freezer and make sure it is flat. Let the colorful ice cubes freeze overnight.
**Day of procedures
- Fill both of your cups at least half-full with tap water.
- To one of the water cups (not both), add 3-4 tablespoons of salt and stir it with the spoon until the salt is dissolved.
- Now you are ready to add your ice cubes. Try to add an ice cube to both cups at the same time. If you are using a stopwatch to track how long it takes the ice to melt, press start when you add the ice to each cup.
- Try not to disturb the cups, but watch what happens to the ice cubes. Do you notice one of them melting faster than the other? What happens to the food coloring as the ice melts?
Concept
Were the results what you expected? You might have thought the ice would melt faster in the salt water since we put salt on our sidewalks and roads in the winter to melt the snow faster.
When the ice is melting into water though, interesting things happen. First, saltwater is more dense than freshwater, and the ice cubes are made from freshwater. As the freshwater cube melts into the freshwater cup, the water all mixes together and the colder water sinks.
This keeps the water around the cube warmer so it melts at a faster rate. In the saltwater cup, the cold freshwater melting from the ice cube floats on top of the saltwater because it is less dense. This keeps the cold water layered around the ice, slowing its melting rate.
*BONUS Activity
Make saltwater ice cubes to test whether they melt faster in freshwater or saltwater. Mix up some saltwater, add food coloring, and pour it into the ice tray. These ice cubes may take longer to freeze, so be sure to plan ahead. Repeat the same steps with the saltwater ice cubes.
