A solution is prepared by dissolving NaCl in water, causing the freezing point to decrease. If pure water freezes at 0°C, at which temperature does the solution freeze?

• A. At 0°C
• B. Above 0°C
• C. Below 0°C
• D. It does not freeze

Answer: (C)

Verified step by step guidance

1. Understand that when a solute like NaCl is dissolved in a solvent such as water, it causes a colligative property called freezing point depression, which lowers the freezing point of the solution compared to pure solvent.
2. Recall that pure water freezes at 0°C, so adding NaCl will disrupt the formation of the solid ice lattice, requiring a lower temperature to freeze.
3. Recognize that the freezing point of the solution will be below 0°C because the presence of dissolved ions interferes with the freezing process.
4. Note that the freezing point depression can be quantitatively described by the formula \(\Delta T_f = i \cdot K_f \cdot m\), where \(\Delta T_f\) is the freezing point depression, \(i\) is the van't Hoff factor (number of particles the solute dissociates into), \(K_f\) is the freezing point depression constant of the solvent, and \(m\) is the molality of the solution.
5. Conclude that since NaCl dissociates into two ions (\(Na^+\) and \(Cl^-\)), the freezing point of the solution will be lower than 0°C, meaning the solution freezes below 0°C.