Concentration units, solubility, the colligative properties, and colloids with the Tyndall effect — exam-focused for the BIEK / Sindh Board paper. Read it, or open the interactive lecture and make a solution of your own concentration.
1 — Solutions
- Solution — a homogeneous mixture of two or more substances.
- Solute — the substance present in the smaller amount (dissolves). Solvent — the substance present in the larger amount (does the dissolving).
"Like dissolves like": polar solvents (water) dissolve polar/ionic solutes; non-polar solvents dissolve non-polar solutes.
2 — Concentration units
The common unitsMolarity (M) = moles of solute / volume of solution (dm³)
Molality (m) = moles of solute / mass of solvent (kg)
Mole fraction = moles of component / total moles
% w/w = (mass solute / mass solution) × 100
Molarity depends on temperature (volume changes); molality does not (mass is constant). ppm = parts per million.
3 — Solubility & saturation
- Solubility — the maximum mass of solute that dissolves in 100 g of solvent at a given temperature.
| Solution | Meaning |
|---|
| Unsaturated | can dissolve more solute |
| Saturated | holds the maximum (excess stays undissolved) |
| Supersaturated | holds more than the maximum (unstable) |
4 — Effect of temperature & pressure
- For most solids, solubility increases with temperature.
- For gases, solubility decreases with temperature (warm fizzy drinks go flat).
- Gas solubility increases with pressure — Henry's law.
Henry's lawsolubility of a gas ∝ its partial pressure
5 — Colligative properties
- Colligative property — a property that depends on the number of solute particles, not their nature.
The four colligative properties are: lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.
6 — Vapour pressure, boiling & freezing point
A non-volatile solute lowers the vapour pressure of the solvent (Raoult's law). As a result the solution boils at a higher temperature and freezes at a lower temperature than the pure solvent.
Boiling & freezingΔTb = Kb · m (elevation) · ΔTf = Kf · m (depression)
This is why salt is spread on icy roads (lowers the freezing point) and antifreeze is added to car radiators.
7 — Osmosis & osmotic pressure
- Osmosis — the net flow of solvent through a semi-permeable membrane from a dilute to a concentrated solution.
- Osmotic pressure (π) — the pressure that must be applied to stop osmosis.
van 't Hoffπ = C R T (C = molarity)
8 — Colloids
Particle size separates the three kinds of mixture:
| Mixture | Particle size | Example |
|---|
| True solution | < 1 nm | salt water |
| Colloid | 1 – 1000 nm | milk, fog, gel |
| Suspension | > 1000 nm | muddy water (settles) |
- Colloid — a mixture with particles of intermediate size dispersed (the dispersed phase) through a dispersion medium; it does not settle.
9 — Types of colloids
| Dispersed phase | Medium | Type | Example |
|---|
| solid | liquid | sol | paint, ink |
| liquid | liquid | emulsion | milk |
| gas | liquid | foam | whipped cream |
| liquid | gas | aerosol | fog, mist |
| solid | solid | solid sol | coloured glass |
10 — Properties of colloids
- Tyndall effect — colloidal particles scatter a beam of light, making the path visible (true solutions do not).
- Brownian motion — the random zig-zag movement of colloidal particles (bombarded by medium molecules).
- Coagulation — colloid particles clump and settle when their charge is neutralised (e.g. adding an electrolyte).
Dialysis purifies a colloid by removing dissolved ions through a semi-permeable membrane (used in artificial kidneys).
11 — Worked numericals
molarity
Find the molarity of a solution made by dissolving 4 g NaOH in 500 cm³ of solution.
moles = 4/40 = 0.1; volume = 0.5 dm³
M = 0.1/0.5 = 0.2 M
mass needed
Mass of glucose (M = 180) for 250 cm³ of 0.1 M solution?
moles = 0.1 × 0.25 = 0.025; mass = 0.025 × 180 = 4.5 g
dilution
What volume of 2 M HCl makes 100 cm³ of 0.5 M?
M₁V₁ = M₂V₂ → 2 × V₁ = 0.5 × 100 → V₁ = 25 cm³
12 — Exam recap
- Solute/solvent; "like dissolves like".
- Concentration units (molarity, molality, mole fraction, %).
- Solubility & saturation; temperature/pressure effects (Henry's law).
- The four colligative properties; Raoult's law; ΔTb, ΔTf, osmotic pressure.
- Colloids vs solutions/suspensions; types of colloids.
- Tyndall effect, Brownian motion, coagulation, dialysis.