Colligative properties – freezing point, boiling point, vapour pressure and osmosis

What it shows

Some properties of a solution depend only on how many solute particles are dissolved, not on what they are. A dissolved solute lowers the vapour pressure of the solvent (Raoult's law), so the solution boils at a higher temperature and freezes at a lower one: ΔTb = i·Kb·m and ΔTf = i·Kf·m. Across a membrane that lets only the solvent through, the solvent flows into the solution until the extra pressure equals the osmotic pressure π = i·c·R·T. Salts give more particles (i ≈ 2 or 3), while ethanoic acid in benzene pairs up (i ≈ 0.5).

How to use

Pick a tab, a Solvent and a Solute, and set the Solute mass w and Solvent mass W. Press Run: the cooling or heating curve is drawn and the result goes into the table. Repeat with other masses to plot ΔT against m. On the Osmosis tab, press Run, change the Applied pressure and press Record flow to find the pressure that stops the flow.

Parameters you can change

• Experiment Freezing point, Boiling point, Vapour pressure, Osmotic pressure
• Solvent Water, Benzene
• Solute Glucose, Urea, Ethylene glycol, Sodium chloride, Calcium chloride, Ethanoic acid, Unknown solute X, Naphthalene
• Mass of solute 0–10 g
• Mass of solvent 20–200 g
• Temperature (vapour pressure and osmosis) 10–60 °C
• Pressure applied to the solution 0–8000 kPa

Questions to explore

1. Why does 0.1 mol/kg calcium chloride lower the freezing point about three times as much as 0.1 mol/kg glucose?
2. How can a freezing-point measurement give the molar mass of an unknown solute?
3. Why does ethanoic acid in benzene give a van 't Hoff factor close to 0.5?