To many of us, the term ‘freezing point depression’ may make us think of how really cold weather makes us melancholy, but in fact, it is the answer to the question, ‘Why does salt melt snow and ice?’
We all know that when water freezes, it turns into ice, and although snow is not frozen water, it is made up of tiny ice crystals. The freezing point of water is 32 °F (or 0°C). When you add salt to the snow or ice, you lower the temperature at which it can freeze. This is called ‘freezing point depression.’
To understand this better, it is important to know more about the relationship between the physical properties of water and temperature.
When water reaches the freezing point it is balanced, meaning it is freezing just as fast as it is melting. Melting occurs in ice when surface molecules escape (or melt) into the water. Freezing occurs when molecules are captured (or are frozen) on the surface of the ice.
Think of a pan of water placed over the heat. As the liquid gets warmer, the molecules begin to move faster. The liquid begins to boil then turns from a liquid into a gaseous state in the form of steam, or vapor.
The opposite occurs when you place water in the freezer. The molecule movement slows and eventually stops, and the water is frozen. In warm temperatures, the molecules are moving all over the place but during the freezing process, they move slowly and begin to line up in an ordered structure, at this point, they are turning from a liquid state into a solid – ice, or ice crystals, in the case of snow.
*Salt forces the water molecules apart and makes it more difficult for them to arrange themselves in the correct way to form into a solid.
Temperature Determines What Type Of Salt Works Best
In order for salt to melt ice or snow there must be some amount of liquid present. That is why it can be too cold for salt to work, and why that thin layer of liquid over the ice or snow is so necessary.
When you mix salt with water, it dissolves and breaks down into particles. Different types of salt break down into different amounts of particles. Since any impurity will disturb the process of water forming into ice, the type of salt that dissolves into more particles will melt ice at lower temperatures than the salt with fewer particles.
Sodium Chloride works at 15 °F and up. When dissolved in water it breaks down into two parts. It evaporates slower and is more economical than other salts. Sodium chloride is another name for table salt, rock salt, or halite.
Magnesium Chloride works in temperatures from 5 °F to 15 °F and is also known as Epsom salt.
Calcium Chloride works down to -20 °F and breaks down into three parts when mixed with water.
Whether it is salt or some other foreign substance, like sugar or alcohol, when mixed with water, the freezing point will go down and any ice or snow above that temperature will melt. When you hear someone ask why salt melts snow and ice you can tell them it is because salt lowers (or depresses) the freezing point. You can also tell them that is how salt works to make homemade ice cream. But that is another story!
You can find more facts about how salt melts ice at antoine.frostburg.edu.