Salt and ice effect

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. Saltwater 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). Saltwater 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 the 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.


If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice.­ You may have also used salt on the ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases, the idea is to take advantage of the lower melting point.

Ice forms when the the­ temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C). On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the






If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.