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Why Does Salt Melt Ice?

The reason ice melts when you spread salt on it is because the salt lowers the freezing point of water. Let me explain what that means.

Let's say it's 29°F outside and some ice forms on your sidewalk. The ice forms because the freezing point of water is 32°F and the outdoor temperature is colder than 32°F. If you dissolve some salt into the ice, the salt will lower the freezing point from 32°F to 15°F(1). That means the ice will not freeze unless the outdoor temperature cools down to 15°F or colder. But in our example we said the outdoor temperature is 29°F, so the ice will melt. By adding salt, you have reduced the freezing point! More importantly, you won't break a leg and make a fool of yourself slipping on the ice.

Although under lab conditions a saltwater solution will actually lower the freezing point to -6°F, salt used to melt ice on your neighborhood street is only really effective to temp of 15°F to 20°F (1) & (2). A couple reasons for this difference could be the purity of salt used on the streets and the concentration.

If salt reduces the freezing point of water to 15°F, what happens when it's 10°F outside?

Higher concentration of salt will help to lower the freezing point. So, if the air temp is 10°F, you might be able to melt ice by using more amounts of salt. But when temps are expected to dip below 15°F and especially below zero, road crews will switch over from rock salt (NaCl) to Calcium Chloride (CaCl). Magnesium Chloride (MgCl) is less common but can also be used. While rock salt reduces the freezing point to about 15°F, Calcium Chloride will effectively melt ice until -20°F! Magnesium chloride will effectively melt ice to about 5°F(1) .

Also, Calcium Chloride is a great deicing agent because it releases heat as it dissolves. This makes it a very quick acting deicer. However, calcium chloride is more expensive than rock salt, so it is typically only used when temperatures get near or below freezing (2).

Beware! In Depth Science Below

Let me explain further what is happening when salt is added to ice. On the surface of the ice there is actually a thin layer of water that you can't see. Water molecules (H2O) are constantly going back and forth between the liquid phase and the solid phase (ice). You can't see this happening because it's on such a small scale, but these H2O molecules are coursing back and forth from liquid to solid and solid to liquid.

If more of these H2O molecules are becoming solid rather than liquid then the water is freezing and the ice grows. But if more H2O molecules are going from solid to liquid, then the ice will melt into a puddle.

Let's say each second 100 ice molecules are melting and 10,000 liquid water molecules are freezing. Although some H2O molecules are melting, the water will remain in solid ice form and grow bigger because there are a LOT more molecules freezing than melting each second. Temperature plays a large role in determining if there is a greater amount of water freezing or melting.

At 32°F roughly the same number of H2O molecules are freezing as are melting. If the temperature goes above this point, more molecules are melting and the ice will melt into a puddle. If the temperature goes below 32°F more molecules are freezing and the ice will remain solid or grow bigger.

This lengthy explanation has finally led us to a point where I can explain why salt melts ice!

(Salt Mine Above)

When salt (NaCl) dissolves in water it breaks up into ions of Na and Cl, and these ions prevent ice crystals from forming. Basically, salt interferes with the process of freezing. The salt however does NOT interfere with melting. If we think back to our example, instead of 10,000 molecules freezing, the salt gets in the way and only 25 molecules freeze each second. (These numbers are just being used for the example). But since the salt does NOT interfere with melting, we still have 100 H2O molecules melting. With 100 molecules melting each second and 25 freezing each second, the ice will melt!

In order to get more freezing and less melting, the temperature needs to be lowered. Thus, you have just reduced the point at which water will freeze. Or to say it another way, you have reduced the freezing point of water. Notice that's exactly what the first line at the top of this page reads. But now you really know why!

See (3) for more info and a diagram, because this is a tough concept to grasp the first time around.