MacRoscopic Properties: The World We See
Phases of Matter and Intensive Properties
How do the boiling point and melting point of a substance change as a solute/impurity is added?
The addition of a solute typically raises the boiling point and lowers the melting point of a substance. These effects are appropriately named “boiling point elevation” and “melting point depression.”
Boiling points are raised when a nonvolatile solute (like NaCl) is added to a solution because the solute lowers the vapor pressure of the solution. That is a somewhat circular explanation though. It is important to know that this change in boiling point does not depend on what you add to the liquid, so there are no specific interactions going on here (like forming hydrogen bonds, etc.). As long as the solute has lower vapor pressure (remember we said it was nonvolatile, so its vapor pressure is essentially zero), this effect will be present. It is perfectly correct to think about it as just lowering the vapor pressure of the mixture (if you add something with very low vapor pressure, the average vapor pressure of whatever liquid you’re adding it to will go down).
Melting (or freezing) points are usually lowered when a solute is added to a liquid. The best explanation for this effect is based on entropy (see “Physical and Theoretical Chemistry”). When a molecule of solvent moves from the liquid to the solid phase (freezes), the amount of liquid solvent (i.e., its volume) is reduced. This means the same amount of solute is in a smaller space, which reduces their entropy (or raises their energy). This raising of energy means that you have to take even more energy out of the system for each molecule that joins the solid phase. Less energy means lower temperature, so adding a solute lowers the freezing point. An alternative way to look at this is that any impurity will disrupt the crystal lattice, raising its energy, relative to the liquid phase. This also contributes to lowering the freezing point as solutes are added.