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Boiling Point Elevation and Freezing Point Depression

High schoolChemistryDilute-limit water freezing depression, boiling elevation and ideal osmotic pressure with a prescribed electrolyte dissociation factor

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About this experiment

Dissolve a nonvolatile solute and compare its solution with pure water in a controlled bath. Measure how solute amount, solvent mass and dissociation affect freezing and boiling onset temperatures.

Learning goals

  • Use molality rather than total solution mass in freezing and boiling shifts.
  • Relate dissociation to effective dissolved-particle count.
  • Distinguish an onset temperature from a calculated phase fraction or rate.

How to use it

  1. 1Compare glucose, sodium chloride and calcium chloride at the same molality.
  2. 2Change electrolyte dissociation fraction and observe the effective particle factor.
  3. 3Set the bath to each onset and compare the pure-water and solution markers.
  4. 4Add water to a fixed solute amount and record the reduced temperature shifts.

The model

Partial electrolyte dissociation changes the effective particle count.
Water constants are in kelvin kilograms per mole at one atmosphere.
The dilute osmotic estimate approximates solution volume by the initial water volume.

Assumptions

  • The solute is nonvolatile and remains dissolved. Ideal dilute-limit equations neglect activity coefficients, eutectics and solubility limits.
  • Bath temperature is imposed. Ice and bubble markers identify possible onset conditions, not a computed amount frozen, boiling duration or evaporation rate.
  • Water density is approximated as one kilogram per litre for the osmotic concentration. A warning marks molality above 0.5 mol per kilogram as a concentrated extrapolation.

Try this in class

  1. Q1Why do equal solute masses not give equal freezing-point depressions?
  2. Q2Why can real electrolyte solutions differ from the fully dissociated ideal prediction?

Frequently asked questions

Is a boiling-point elevation a prediction that the liquid can heat indefinitely?

No. It is an equilibrium onset relative to pure water at the same pressure. The bath control is not a thermal energy or evaporation calculation.

Why does undissociated glucose still change the freezing point?

Each dissolved glucose molecule is a solute particle. Dissociation is not required for a colligative effect.