Which of the following best explains why adding salt to water causes it to freeze at a lower temperature?

• A. Salt makes water molecules move faster, causing them to freeze more quickly.
• B. Salt reacts chemically with water to produce heat, lowering the freezing point.
• C. Salt disrupts the formation of ice crystals, lowering the freezing point.
• D. Salt increases the boiling point, which also lowers the freezing point.

Answer: (C)

Verified step by step guidance

1. Understand the concept of freezing point depression, which is a colligative property. Colligative properties depend on the number of solute particles in a solvent, not their identity.
2. Recognize that when salt (a solute) is added to water (the solvent), it dissolves into ions that interfere with the ability of water molecules to organize into a solid ice structure.
3. Know that the presence of these dissolved ions disrupts the formation of the regular lattice of ice crystals, making it harder for water to freeze at its normal freezing point (0°C).
4. Recall the freezing point depression formula: \(\Delta T_f = i \cdot K_f \cdot m\), where \(\Delta T_f\) is the decrease in freezing point, \(i\) 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 the correct explanation is that salt disrupts the formation of ice crystals, lowering the freezing point, rather than salt making water molecules move faster, reacting chemically to produce heat, or increasing the boiling point.