Colligative Properties & Osmosis
Solvent crosses a membrane and raises a column until osmotic pressure balances the flow.
About this simulation
A U-tube separated by a semipermeable membrane: solvent flows into the solution and the column rises until its extra pressure — the osmotic pressure π = i·c·R·T — stops the net flow. Students vary concentration and temperature and switch the solute between glucose (i = 1), NaCl (i = 2) and CaCl₂ (i = 3), watching π, the boiling-point elevation and the freezing-point depression all scale with the number of dissolved particles. It makes the central idea concrete: colligative properties count particles, not identity — so the van't Hoff factor is what really drives them.
How to use it
- Start with the solute set to 'Glucose · i = 1' and watch solvent particles cross the dashed semipermeable membrane while the solution arm rises.
- Increase 'Concentration' and see the π bracket between the two liquid levels grow, along with the π value in atm printed beside it.
- Raise 'Temperature' and notice that the π value rises while the 'i = … mol/L of particles' line at the bottom does not change.
- Switch the solute to 'NaCl · i = 2' and then 'CaCl₂ · i = 3' at the same concentration and watch the solute dots crowd the right arm and π — with the ΔTb and ΔTf lines at the top — double and triple.
- Read the line at the bottom, such as 'i = 3 · 1.50 mol/L of particles', for each solute: i·c is the particle count that every colligative property depends on.
What to notice
- Only the solvent crosses the membrane; the solute dots stay on the solution side, and that one-way flow is what raises the solution column.
- The height difference between the two arms is the osmotic pressure made visible: the column stops rising when its extra pressure balances the flow.
- Glucose, NaCl and CaCl₂ at the same molarity give osmotic pressures in the ratio 1 : 2 : 3, because NaCl and CaCl₂ split into 2 and 3 ions.
- Boiling-point elevation and freezing-point depression follow the same particle count, so they also double and triple with the van't Hoff factor.
The formulas
- π = i · c · R · T
- Osmotic pressure π (atm) from van't Hoff factor i, molar concentration c (mol/L), gas constant R = 0.0821 L atm mol⁻¹ K⁻¹ and temperature T (K).
- ΔTb = i · Kb · m
- Rise in boiling point from van't Hoff factor i, ebullioscopic constant Kb (0.52 K kg/mol for water) and molality m.
- ΔTf = i · Kf · m
- Fall in freezing point from van't Hoff factor i, cryoscopic constant Kf (1.86 K kg/mol for water) and molality m.
Questions
What is osmosis and what is osmotic pressure?
Osmosis is the net flow of solvent molecules through a semipermeable membrane from pure solvent (or a dilute solution) into a more concentrated solution. Osmotic pressure is the extra pressure that must be applied on the solution side to just stop this flow. In the simulation, the rising column on the solution side supplies that pressure, and the flow stops when the π bracket reaches its full height.
Why are colligative properties said to depend on the number of particles, not their nature?
Colligative properties arise because solute particles lower the solvent's tendency to escape, whatever those particles are. One mole of glucose, one mole of urea or one mole of any non-electrolyte gives the same effect. What changes the effect is how many particles each formula unit gives. Switch between glucose, NaCl and CaCl₂ at one concentration and watch the π value scale 1 : 2 : 3.
What is the van't Hoff factor and why is it 2 for NaCl?
The van't Hoff factor i is the ratio of the number of particles actually present in solution to the number of formula units dissolved. NaCl dissociates into Na⁺ and Cl⁻, so ideally i = 2; CaCl₂ gives Ca²⁺ and two Cl⁻, so i = 3. Real solutions give slightly smaller values because ions attract each other, but i = 2 and 3 are the ideal limits used here.
Why does osmotic pressure increase with temperature but freezing-point depression does not?
Osmotic pressure behaves like a gas pressure, π = i c R T, so at a fixed concentration it rises in proportion to the absolute temperature. Freezing-point depression and boiling-point elevation are measured at the solvent's own freezing and boiling points and depend only on the constants Kf, Kb and the molality. Move the Temperature slider: the π value changes while the particle count printed at the bottom stays the same.
Why is osmotic pressure preferred for finding the molar mass of proteins and polymers?
For a large molecule even a few grams per litre gives a very small molar concentration, so the boiling-point rise or freezing-point drop is too tiny to measure. The osmotic pressure of the same solution is still a measurable column of liquid, and it can be measured at room temperature, which protects biomolecules that would be damaged by heating.
