Does an ice cube melt faster in fresh or salt water?

This simple science demonstration uses only a few basic materials, but demonstrates several scientific concepts and looks super impressive. Showing children the end result first is a great way to get them to start questioning and thinking about what is happening and why.

There are two different things that children can observe with this activity.

How quickly the ice cube melts in each glass.
What happens to the water in the ice cube when it melts?

Adding salt to water increases its density. This means that the melted water from the ice cube should be on top of the salt water, since unsalted water has a lower density.

If you have ever done ice cream in a bagYou know that salt lowers the freezing point of water, so you might expect the salt water to melt the ice cube faster, but in our case, this didn't happen.

Does ice melt faster in fresh or salt water?

You will need to

Two tall glasses

Water

Salt

Ice tray

Spoon

Freezer

Colorful food

Instructions

Fill two segments of an ice cube tray with water and food coloring and place in the freezer until frozen.

Fill each glass to within 3cm of the rim with room temperature water.

Add two tablespoons of salt to a glass and stir until the salt has dissolved and the water is clear.

Place an ice cube in each glass and watch what happens.

two glasses full of water. One contains salt and both contain a blue ice cube.

You can see that in the glass containing salt, the ice cube melted more slowly and most of the water in the melted ice cube remained near the top of the glass.

This is because the salt water underneath is denser than the water in the ice cubes and food coloring, so the water in the ice cubes sits on top of the salt water.

We expected that the ice cube would melt faster in salt water, but this did not happen. However, we noticed that the colored water mixed quickly with the fresh water, forming a convection current. The colder water in the ice cube sank to the bottom and the warmer, less dense water rose, creating a small current as the ice melted. This did not happen in salt water. Some of the water in the colored ice cubes sank, but most of it stayed on top, so it didn't get as hot as the ice cube in fresh water.

Additional challenges

Repeat the activity using containers with one tablespoon of salt, two tablespoons of salt, three tablespoons of salt, and no salt.

Challenge children to find out which glass of water contains the most salt out of four containers, each containing different amounts of dissolved salt.