How dissolved particles change the properties of solvents
A solution is a homogeneous mixture of a solute in a solvent. Concentration can be expressed as mass percentage, mole fraction, molarity (moles per litre of solution) or molality (moles per kilogram of solvent). Molality does not change with temperature, which makes it useful for colligative properties. Henry’s law states that the solubility of a gas in a liquid is proportional to its partial pressure, p = KH x.
Raoult’s law states that the partial vapour pressure of each volatile component equals its mole fraction multiplied by its pure vapour pressure. Ideal solutions obey this law, while non-ideal solutions show positive or negative deviations and can form azeotropes. Colligative properties depend only on the number of solute particles, not their identity: relative lowering of vapour pressure, elevation of boiling point (ΔTb = Kb m), depression of freezing point (ΔTf = Kf m) and osmotic pressure (π = CRT). The van’t Hoff factor i corrects these for solutes that dissociate or associate.
Why it matters
Salt is spread on icy roads to lower the freezing point of water, and antifreeze in car radiators works the same way. Saline drips in hospitals must have the right osmotic pressure to protect blood cells, and reverse osmosis purifies drinking water. Divers face the bends because gas solubility changes with pressure, as described by Henry’s law.
Key terms
- Molality
- The number of moles of solute dissolved per kilogram of solvent.
- Raoult’s law
- The partial vapour pressure of a component equals its mole fraction times its pure vapour pressure.
- Colligative property
- A property of a solution that depends on the number of solute particles, not their type.
- Osmotic pressure
- The pressure needed to stop solvent from flowing into a solution through a semi-permeable membrane.