Which Ice Cube will melt faster? Freshwater or Saltwater?

Right guys, this is a fun and easy experiment that everyone can do at home! We are going to see if ice melts faster in a cup of fresh water, versus in a cup of saltwater. Jot down what you think before you start the experiment.

What you will need:

• 2 identical cups

• 2 ice cubes (roughly the same size)

• Table salt

• Fresh water

• [Optional] Food colouring

Here is the method:

1. Fill both cups with fresh water to an equal volume

2. Add the table salt (1 or 2 tablespoons) to one of the cups (try label the cup so you don’t get confused between the 2 cups)

3. Stir the salt so it is dissolved

4. *Wait 2-3 minutes for the saltwater cup to get back to room temperature

5. Add an ice cube into each cup and let the experiment begin!

6. [If you have food colouring] Add food colouring to the surface of the water on each cup

7. Simply observe which ice cube melts faster!

*Note: the reaction between salt and water (NaCl + H2O) is endothermic, meaning it absorbs energy from the surrounding environment. This means when you dissolve the salt in water, the temperature of the system will decrease. Therefore, in step 4 we wait for the temperature to go back to room temperature, so it is a fair test.

Results

So, were you right? Well, if you did everything correctly, what you will find, is that the ice cube in freshwater melts much, much quicker compared to the one in the saltwater. If you added food colouring, it should look something like this:

But why? This experiment is based off on the theory of convection and density. Salt water is denser than freshwater. This is because the mass of the water will increase when salt is added, while volume wont change by much. Thus, density (which is mass/volume) will increase.

This is what happens in the cup with fresh water:

When the ice melts, the water that is melted is colder than the surrounding water in the cup, so it sinks to the bottom. This means that the water in the cup (which is hotter) will rise to the top of the cup and continue to melt the ice cube – it’s a principal of convection!

If you used food colouring, what you will see is a streak of food colouring trailing from the ice cube to the bottom of the cup (as shown in the picture above), which is representative of the cold, newly melted water sinking to the bottom.

This is what happens in the cup of saltwater:

When the ice melts, the cold water that is melted is less dense than the salt water, hence it stays at the top of the water by the ice cube. As a result, it melts slower than the ice in the fresh water, because the cold, melted water does not sink to the bottom of the cup, but rather stays by the ice cube to slow the melting down.

If you used food colouring, what you will see is the food colouring stays concentrated at the top of the cup, because the cold, melted water is less dense than the salt water!

Well, I hope you enjoyed the experiment and learned a thing or two about how convection is everywhere!

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