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Do Solutes Affect the Melting Point of Ice?

A solute is a substance that is dissolved into another substance called a solvent. In salt water for example, salt (NaCl) is the solute and it is dissolved into water (H2O), the solvent. Salt lowers the melting point, also known as the freezing point, of water. For this reason salt is spread on roadways to prevent freezing in the winter.

Vapor Pressure of Solutes and Solvents
- If the vapor pressure is lower for the solute than the solvent, then the melting point of the solvent will go down. On the other hand, if you add a solute with a higher vapor pressure, the melting point will go up. The lower the vapor pressure, the more heat required to bring to a boil.
Vapor Pressure of Water
- Frozen water at 0 degrees C has a vapor pressure of about 4.5840 P/mmHg.
  
  The vapor pressure of salt water is lower.
Polar Molecules
- Salt molecules are far more polar than water. The water will create strong bonds with the salt, and as stronger bonds appear, the vapor pressure decreases. Solutes that create strong bonds with solvents will lower melting points. The converse is true as well.
Non-Polar Molecules
- Non-polar molecules are less likely to create strong bonds, and would thus increase the melting point. For example, carbon disulphide is non-polar and would cause the melting point of water to increase.

Nature