The complete lecture — but told through everyday things you already know. As you scroll, the panel on the right plays each idea as a real scene: sugar in tea, salt on an icy road, a fizzy drink going flat, a torch beam in fog. Read the text; the picture keeps pace.
1 — Solute, solvent & the solution
Stir a spoonful of sugar into a cup of tea and it disappears. It has not vanished — it has dissolved, breaking up into particles too small to see and spreading out until every sip tastes the same. That uniform mixture is a solution.
- Solution — a homogeneous mixture: the same throughout, with no settling and no visible boundary between the parts.
- Solute — the substance in the smaller amount, the thing that dissolves (the sugar).
- Solvent — the substance in the larger amount, the thing that does the dissolving (the tea, mostly water).
"Like dissolves like": polar solvents such as water dissolve polar and ionic solutes (sugar, salt); non-polar solvents dissolve non-polar solutes. That is why oil refuses to mix into water and floats on top.
2 — Solubility & saturation
Now keep adding sugar, spoonful after spoonful, and stirring. At first each one disappears — but there comes a point where the next spoonful simply will not dissolve, no matter how long you stir. It gathers as a pile at the bottom of the glass.
- Solubility — the maximum mass of solute that dissolves in a fixed amount of solvent (per 100 g) at a given temperature.
- Saturated solution — one holding that maximum; any extra solute stays undissolved at the bottom.
| Solution | Meaning |
|---|
| Unsaturated | can still dissolve more solute |
| Saturated | holds the maximum; excess settles out |
| Supersaturated | holds more than the maximum (unstable) |
Warm the tea and more sugar dissolves: for most solids, solubility rises with temperature.
3 — Concentration units (molarity & molality)
"Dissolved" is not enough for a chemist — we must say how much. A nurse mixing a drip or a lab making a reagent needs an exact strength. The everyday tool is a volumetric flask: weigh the solute, dissolve it, then top up with solvent to the etched mark on the neck.
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 expands with heat); molality does not, because mass is fixed. ppm = parts per million, used for very dilute solutions.
4 — Effect of temperature & pressure on gases
Open a cold fizzy drink and it bubbles gently for a long time. Leave it to go warm and it goes flat fast — the dissolved carbon dioxide rushes out. Gases behave the opposite way to solids.
- For most solids, solubility increases with temperature.
- For gases, solubility decreases with temperature (the warm drink goes flat).
- Gas solubility increases with pressure — this is Henry's law.
Henry's lawsolubility of a gas ∝ its partial pressure
A fizzy drink is bottled under pressure to force gas in; twist the cap and the pressure drops, so the gas hisses back out.
5 — Colligative properties & freezing-point depression
In winter, gritters spread salt on icy roads and the ice melts even below 0 °C. The salt does not heat the road — it dissolves into the thin film of water and lowers its freezing point, so the ice can no longer stay frozen.
- Colligative property — one that depends on the number of dissolved particles, not on what they are.
The four colligative properties are: lowering of vapour pressure, elevation of the boiling point, depression of the freezing point, and osmotic pressure. A non-volatile solute lowers the solvent's vapour pressure (Raoult's law), so the solution boils higher and freezes lower than the pure solvent.
Boiling & freezing changeΔTb = Kb · m (boiling point up) · ΔTf = Kf · m (freezing point down)
Same idea, two uses: salt on icy roads (freeze lower) and antifreeze in a car radiator (it stops the coolant both freezing in winter and boiling over in summer).
6 — Osmosis & osmotic pressure
Sprinkle salt on a fresh salad leaf and within minutes it turns limp and wet. Water has been pulled out of the crisp cells, across their thin skins, toward the salty outside. That movement of solvent through a membrane is osmosis.
- Osmosis — the net flow of solvent through a semi-permeable membrane from a dilute to a concentrated solution.
- Osmotic pressure (π) — the pressure you would have to apply to the concentrated side to stop that flow.
van 't Hoffπ = C R T (C = molarity, R = gas constant, T = temperature)
The same flow, reversed, keeps a plant firm: its roots draw water from the soil, and a vegetable left in fresh water grows turgid and crisp.
7 — Colloids & the Tyndall effect
Shine a torch through a glass of clear salt water and you see nothing — the beam passes straight through. Shine the same torch through a glass of milky water and the whole beam lights up. The difference is particle size.
| Mixture | Particle size | Example |
|---|
| True solution | < 1 nm | salt water (clear) |
| Colloid | 1 – 1000 nm | milk, fog, gel (never settles) |
| Suspension | > 1000 nm | muddy water (settles) |
- Tyndall effect — colloidal particles are large enough to scatter a beam of light, making its path visible; a true solution does not.
- Brownian motion — the random zig-zag jiggle of colloid particles, bombarded by the medium's molecules.
You see the Tyndall effect every day: headlights cutting visible cones through fog, and sunbeams streaming through dusty or misty air.
8 — Worked numericals & exam recap
molarity
Molarity of 4 g NaOH dissolved to make 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
Volume of 2 M HCl to make 100 cm³ of 0.5 M?
M₁V₁ = M₂V₂ → 2 × V₁ = 0.5 × 100 → V₁ = 25 cm³
- Solute / solvent; "like dissolves like".
- Solubility & saturation; temperature & pressure (Henry's law).
- Concentration units — molarity, molality, mole fraction, %.
- The four colligative properties; ΔTb, ΔTf; osmosis & osmotic pressure.
- Colloids vs solutions / suspensions; the Tyndall effect.