Chemistry Regents Practice Test

How do the boiling point and freezing point of NaCl(aq) compare to those of H2O(ℓ) at standard pressure?

1. The boiling point of NaCl(aq) is lower, freezing point is higher

2. The boiling point of NaCl(aq) is higher, freezing point is lower

3. The boiling and freezing points are the same

4. The boiling point of NaCl(aq) is equal, freezing point is unequal

The correct answer is: The boiling point of NaCl(aq) is higher, freezing point is lower

To understand why the boiling point of NaCl(aq) is higher and the freezing point is lower than that of water (H2O), consider the effects of solutes on colligative properties. When a solute like sodium chloride (NaCl) is dissolved in water, it dissociates into its ions, Na⁺ and Cl⁻. The presence of these ions affects the colligative properties of the solution. Specifically, the addition of NaCl increases the boiling point of the solution compared to pure water due to the phenomenon known as boiling point elevation. This occurs because the solute particles disrupt the ability of the solvent (water) molecules to evaporate, requiring a higher temperature to achieve the vapor pressure necessary for boiling. On the other hand, the freezing point of the NaCl solution is lower than that of pure water, which is referred to as freezing point depression. The solute ions interfere with the formation of the ordered crystal structure of ice, thereby requiring a lower temperature to freeze the solution compared to pure water. Thus, NaCl(aq) has a higher boiling point and a lower freezing point relative to H2O(ℓ) at standard pressure, making the assertion that the boiling point is higher