Colligative Properties Calculator

Colligative Properties Guide

What Are Colligative Properties?

Colligative properties depend only on the number of dissolved particles, not their identity. Adding any solute to a solvent: lowers the freezing point, raises the boiling point, reduces the vapour pressure, and creates osmotic pressure. The key formula uses molality (m) = moles of solute per kilogram of solvent (not molarity). ΔTf = i × Kf × m. ΔTb = i × Kb × m. Van't Hoff factor i accounts for dissociation: NaCl dissociates into Na⁺ + Cl⁻ so i = 2, meaning it has double the colligative effect of a non-dissociating solute like glucose at the same molal concentration. Colligative properties (freezing point depression, boiling point elevation, osmotic pressure, and vapour pressure lowering) depend only on the number of dissolved particles, not on what those particles are — this is what distinguishes them from most other physical properties of solutions.

What should I know about Freezing Point Depression?

Adding solute lowers the freezing point — the solute particles disrupt the formation of the crystal lattice structure. ΔTf = i × Kf × m. Water: Kf = 1.86 °C·kg/mol. Example: 10g NaCl (Mr 58.44) dissolved in 500g water: m = (10/58.44)/(0.5) = 0.342 mol/kg. ΔTf = 2 × 1.86 × 0.342 = 1.27°C. New freezing point = 0 − 1.27 = −1.27°C. Applications: road salt (lowers ice formation temperature), antifreeze in car radiators (50:50 ethylene glycol:water freezes at approximately −37°C), salting pasta water

What do I need to know about Boiling Point Elevation?

Adding solute raises the boiling point by reducing the vapour pressure of the solvent. ΔTb = i × Kb × m. Water: Kb = 0.512 °C·kg/mol. The boiling point elevation for domestic cooking salt concentrations is tiny (< 0.1°C for typical pasta water) and does not meaningfully reduce cooking time — this is a persistent cooking myth. Industrial application: solutions used as heat transfer fluids can be operated above the solvent's normal boiling point by adding solutes. Sugar solutions boil above 100°C, which is why sugar syrup recipes (fudge, caramel, jam) specify a target temperature well above 100°C — the boiling point rises as water evaporates and the sugar concentration increases, so the thermometer reading is a direct, reliable indicator of how much water remains and therefore how set the finished product will be.

What do I need to know about Osmotic Pressure and Biology?

Osmotic pressure π = iMRT. Where M = molarity, R = 0.08206 L·atm/mol·K, T = temperature in K. Osmosis: water moves through a semipermeable membrane from low solute concentration to high solute concentration (down the water potential gradient). Blood plasma osmolarity is approximately 285-295 mOsm/kg — intravenous fluids must be isotonic (same osmolarity) to prevent red blood cell lysis (hypotonic IV) or crenation (hypertonic IV). This is why 0.9% NaCl ('normal saline') is used for IV infusion rather than plain water, which would be strongly hypotonic and cause red blood cells to swell and burst (haemolysis) as water moves into the cells by osmosis. Isotonic IV fluids match blood plasma's osmolarity closely enough that cells neither gain nor lose water across the membrane.