Pure substances, solutions, colloids, solubility curves, and all 7 separation techniques — sorted! 🧪
A pure substance contains only one type of particle (element or compound). It has sharp, fixed melting and boiling points. A mixture contains two or more substances not chemically combined.
| Property | Pure Substance | Mixture |
|---|---|---|
| Composition | Fixed & constant | Variable |
| Melting/Boiling Point | Sharp & fixed | Broad range |
| Separation | Cannot be separated physically | Can be separated by physical means |
| Examples | Water (H₂O), NaCl, O₂ | Air, sea water, ink |
Element: A pure substance made of only one type of atom. Cannot be broken down by chemical means. E.g. Fe, O₂, Cl₂.
Compound: A pure substance formed from two or more elements chemically bonded in fixed proportions. Has different properties from its elements. E.g. H₂O, NaCl, CO₂.
Uniform composition throughout — you cannot see the individual components. Also called solutions.
Non-uniform — you can tell the components apart.
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Particle Size | <1 nm | 1–1000 nm | >1000 nm |
| Visible to Eye? | No | No | Yes |
| Settles? | No | No | Yes |
| Light | Passes through | Scattered (Tyndall) | Blocked |
| Examples | Salt water, vinegar | Milk, fog, jelly | Muddy water, chalk in water |
Solubility = the maximum mass (in grams) of solute that will dissolve in 100 g of solvent at a specific temperature. Units: g per 100 g water.
A saturated solution contains the maximum dissolved solute at that temperature. Any extra solute added remains undissolved.
mass of solute = (solubility × mass of solvent) / 100When a saturated solution is cooled, the solubility decreases and excess solute crystallises out.
Separates: Insoluble solid from liquid | Based on: Particle size
Separates: Dissolved solid from solution (solvent is lost) | Based on: Volatility
Separates: Pure dissolved solid from solution | Based on: Differential solubility with temperature
Separates: Solvent from a solution | Based on: Different boiling points
Separates: Miscible liquids with similar boiling points | Based on: Different boiling points
Separates: Immiscible liquids | Based on: Different densities
Separates: Dissolved substances, especially dyes | Based on: Solubility in solvent AND attraction to paper
Rf = distance moved by substance / distance moved by solvent front| Technique | What it Separates | Property Used | What You Get |
|---|---|---|---|
| Filtration | Insoluble solid from liquid | Particle size | Residue + Filtrate |
| Evaporation | Dissolved solid from solution | Volatility | Solid only |
| Crystallisation | Pure dissolved solid | Solubility vs temp | Pure crystals + solution |
| Simple Distillation | Solvent from solution | Boiling point | Pure liquid + solid |
| Fractional Distillation | Miscible liquids (similar b.p.) | Boiling point | Separate liquids |
| Separating Funnel | Immiscible liquids | Density | Separate liquids |
| Chromatography | Dissolved substances/dyes | Solubility & attraction | Chromatogram + Rf values |
Watch dyes separate as the solvent front travels up the chromatography paper. Each dye has a different Rf value based on its solubility and attraction to the paper.
Animated solubility curves for three common salts. Observe how solubility changes with temperature and use the tool below to calculate crystallisation.
How many grams of crystals form when you cool a saturated solution?
Click the card to flip it! Use the arrows to navigate.
Problem: The solubility of potassium nitrate (KNO₃) is 110 g per 100 g water at 60°C and 31 g per 100 g water at 20°C. A saturated KNO₃ solution contains 250 g of water at 60°C. It is then cooled to 20°C.
Type the answer (number only, or type show to reveal):
Using the solubility formula: mass = (solubility × mass of water) / 100
At 60°C, solubility = 110 g/100 g water. Mass of water = 250 g. What mass is dissolved?
At 20°C, solubility = 31 g/100 g water. Same 250 g of water. What mass stays in solution at 20°C?
The crystals that form = mass dissolved at 60°C minus the mass that remains at 20°C. Calculate this.
After cooling, the solution at 20°C is still _______. Type the word that describes the solution.
Click a term on the left, then click its matching description on the right. Green = correct, Red = try again!
Past-paper style questions with full mark schemes. Click to reveal!
1 Filter the mixture through filter paper in a funnel.
2 Sand is collected as the residue on the filter paper; rinse with distilled water and dry in an oven. (Dry sand ✓)
3 The filtrate (salt water) is collected in a beaker.
4 Heat the filtrate gently in an evaporating dish until saturated (small crystals appear at edges), then remove from heat.
5 Allow to cool slowly — salt crystals form.
6 Filter off the crystals, pat dry with filter paper and leave to dry. (Pure salt ✓)
Award: 1 mark per correct step (max 6). Accept evaporation to dryness for step 4–6 but note crystals may decompose.
1 At 80°C: mass dissolved = (55 × 200) ÷ 100 = 110 g ✓
2 At 20°C: mass that remains = (20 × 200) ÷ 100 = 40 g ✓
3 Mass of crystals = 110 − 40 = 70 g ✓
4 Units: grams (g) ✓ [accept g of CuSO₄]
1 Solution: Homogeneous mixture; particle size <1 nm; particles do not settle; transparent. Example: salt water / vinegar. ✓✓
2 Colloid: Particle size 1–1000 nm; particles do not settle; scatters light (Tyndall effect). Example: milk / fog. ✓✓
3 Suspension: Particle size >1000 nm; particles settle on standing; opaque. Example: muddy water / chalk in water. ✓✓
Award 2 marks each: 1 for characteristic, 1 for example.
1 Rf = distance moved by substance ÷ distance moved by solvent front ✓
2 Rf = 7.5 ÷ 12.5 = 0.60 ✓
3 The Rf of the dye (0.60) matches the known pure dye (0.60) ✓
4 Conclusion: The unknown dye is (likely) the same substance as the known pure dye. ✓
Note: Same Rf in same solvent = likely same substance, but not 100% conclusive — use multiple solvents for certainty.
1 (i) Fractional distillation — because ethanol and water are miscible liquids with close but different boiling points (78°C and 100°C). ✓✓
2 (ii) Using a magnet (magnetic attraction) — iron is magnetic; sand is not. ✓✓
3 (iii) Separating funnel — because kerosene and water are immiscible liquids with different densities; they form two separate layers. ✓✓
mass = (solubility × mass solvent) / 100
Solubility in g per 100 g water
Rf = dist (substance) / dist (solvent front)
Always between 0 and 1
crystals = dissolved(T₁) − dissolved(T₂)
T₁ > T₂ (cooling down)
Colloids scatter light → beam visible
Solutions are transparent
Filter · Evap · Crystal · S.Dist · F.Dist · Sep.Funnel · Chrom
Know what each separates!
Solution < 1nm · Colloid 1-1000nm · Suspension >1000nm
Particle size determines type