Chemistry question about solutions

Question

Why do we use salt on ice in cold weather?

1. The combination increases the boiling point of the ice, making it melt faster.
2. The combination increases the vapor pressure of the ice, so it evaporates faster.
3. The combination makes salt water, which has a lower melting point than ice, so the ice melts.
4. The combination increases the osmotic pressure of the ice, so it melts faster.
5. The combination generates energy as the salt dissolves, which melts the ice.

Answer 1

This is a great question, and many people have probably been taught that it is because salt water has a lower freezing point than fresh water, so the ice melts. While it is true that salt water has a lower freezing point, that is not why the ice melts.

Salts are held together by ionic bonds, so salts are very soluble in water. Although the ice is frozen, it has a very thin layer of liquid water, so some of the salt will dissolve immediately. When the salt dissolves the ionic bonds break and the bond energy (heat) has to go somewhere. It goes into the water, and the heat dissolves more of the ice. So the answer is E, the combination releases energy which dissolves the ice.