Salt and Ice

In our part of the world, temperatures are low enough in winter for snow and ice to form. This can make driving and walking dangerous as roads and sidewalks become slippery. In western New York, rock salt is often used to melt icy roads. The interaction of salt and ice is an interesting thing to explore.

Put two ice cubes in two glasses. Put a teaspoon of table salt on one ice cube. Which ice cube melts faster, the one with the salt or the one without the salt? Why does this happen?

The ice cube without salt melts because the air around it is warmer than 32 degrees F. The salted cube melts faster. When you add salt it dissolves into the water of the ice cube. Salt water freezes at a lower temperature than the 32 degrees F at which freshwater freezes. The difference between the air temperature and the freezing point of salt water is bigger than the difference between the air temperature and the freezing point of freshwater. This makes the ice with salt on it melt faster.

Try putting ice in a glass of cold water. Leave it there for 10 minutes or so. What temperature is the water? Recheck the temperature every few minutes. Does it get any colder?

Next, add a tablespoon of table salt to the water. Stir the ice, salt and water together. Does the temperature change? Keep stirring. What temperature can you get it to be?

Here is why. The salt dissolves in the water. Fresh water freezes at 32 degrees F (0 degrees C). Salt water freezes at lower temperatures, depending on how much salt is in the water. As the ice turns to water, the change from solid to liquid requires heat. This is just like when water changes from liquid to gas, for example when evaporating sweat cools your skin. As ice melts, the heat is taken from the ice and water around it. Both get colder. How cold can you get the ice, water, and salt? When the Fahrenheit temperature scale was first made, 0 F was the lowest temperature that could be made using salt, water, and ice.

For directions on how to make ice cream in a plastic bag, click here. This is one of several links on a page about using salt to melt ice found on How Stuff Works.
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