Boiling point elevation and freezing point depression are colligative properties, which means they depend on the number of dissolved solute particles, not on the chemical identity of those particles. When a nonvolatile solute such as salt or sugar is added to a solvent, the solution boils at a higher temperature and freezes at a lower temperature than the pure solvent. These effects matter in everyday systems such as salted roads, car antifreeze, cooking water, and laboratory solution design.
They also show how microscopic particle behavior changes measurable macroscopic properties.
At the molecular level, solute particles interfere with the ability of solvent particles to escape into the vapor phase and to arrange into an ordered solid. Boiling point elevation occurs because the solution has a lower vapor pressure, so a higher temperature is needed for its vapor pressure to equal external pressure. Freezing point depression occurs because solute particles disrupt crystal formation, so a lower temperature is needed for the solid and liquid phases to be in equilibrium.
The size of both changes is calculated using molality, the van't Hoff factor, and the solvent's colligative constants.
Key Facts
- Boiling point elevation: ΔTb = iKb m
- Freezing point depression: ΔTf = iKf m
- New boiling point: Tb,solution = Tb,pure + ΔTb
- New freezing point: Tf,solution = Tf,pure - ΔTf
- Molality is m = moles of solute / kilograms of solvent
- The van't Hoff factor i estimates how many dissolved particles form per formula unit, such as i ≈ 2 for NaCl and i ≈ 1 for sugar.
Vocabulary
- Colligative property
- A solution property that depends mainly on the number of dissolved solute particles rather than their chemical identity.
- Molality
- The concentration unit equal to moles of solute divided by kilograms of solvent.
- Van't Hoff factor
- The factor i that represents the effective number of dissolved particles produced by each solute formula unit.
- Boiling point elevation
- The increase in a solvent's boiling point caused by dissolving a nonvolatile solute.
- Freezing point depression
- The decrease in a solvent's freezing point caused by dissolving a solute.
Common Mistakes to Avoid
- Using molarity instead of molality, which is wrong because colligative formulas use kilograms of solvent rather than liters of solution.
- Forgetting the van't Hoff factor, which gives answers too small for ionic solutes because salts often produce more than one dissolved particle per formula unit.
- Adding freezing point depression to the pure freezing point, which is wrong because ΔTf is the amount the freezing point decreases.
- Using the mass of the entire solution as the solvent mass, which is wrong because molality only uses the mass of the solvent in kilograms.
Practice Questions
- 1 A solution is made by dissolving 0.50 mol of glucose in 1.00 kg of water. For water, Kb = 0.512 °C/m and glucose has i = 1. What is the boiling point of the solution at 1 atm?
- 2 A student dissolves 0.20 mol of CaCl2 in 0.500 kg of water. Assume i = 3 and Kf = 1.86 °C/m for water. What is the new freezing point of the solution?
- 3 Road salt lowers the freezing point of water on roads, but it becomes less effective at very low temperatures. Explain this using freezing point depression and the idea of dissolved particles.