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Mixtures & Solutions Made Simple (Y7–9)

Updated: Sep 26, 2025

1) Key definitions (clean and quick)


  • Pure substance: one type of particle (e.g., pure water, pure copper).

  • Mixture: two or more substances physically combined (no new substance made).

  • Homogeneous mixture (solution): looks uniform throughout (e.g., saltwater, air).

  • Heterogeneous mixture: not uniform; you can see different parts (e.g., sand in water).

  • Solution parts: solute (gets dissolved) + solvent (does the dissolving).


Everyday examples

  • Air = nitrogen + oxygen + other gases (solution of gases).

  • Seawater = water + salts (solution).

  • Alloys (e.g., brass) = solution of metals.


2) Solubility & saturation


  • Solubility = maximum amount of solute that dissolves in a given solvent at a set temperature.

  • Unsaturated: more solute can dissolve.

  • Saturated: no more will dissolve (at that temperature).

  • Supersaturated (extension): holds more than saturation—unstable; crystals form if disturbed.

Tips: Most solids dissolve more when warm; gases dissolve less as temperature rises.


3) Concentration (say how “strong” a solution is)


Qualitative: dilute vs concentrated.

Quantitative:

  • g/L (grams per litre)

  • % m/v 

  • % m/m or % v/v (for solids/solids or liquids/liquids)


4) Choosing the right separation method


Pick the method by the property that differs:

Goal

Property exploited

Method

Separate big from small solids

Particle size

Sieving

Separate insoluble solid from liquid

Particle size + filter

Filtration

Pour off liquid from settled solid

Density (settling)

Decanting

Recover solute from solution

Solubility + boiling

Evaporation/Crystallisation

Separate liquid from solution (keep liquid)

Boiling point

Simple distillation

Separate two miscible liquids (close b.p.)

Boiling point (small differences)

Fractional distillation (extension)

Separate dye colours in ink

Solubility + attraction to paper/solvent

Chromatography

Pull out iron filings

Magnetism

Magnetic separation

Speed up settling

Density with spin

Centrifuging (extension)

Quick sketches (describe for your cover image):

  • Filtration funnel with residue on paper and filtrate below.

  • Evaporation dish on tripod; crystals forming.

  • Distillation: flask → condenser → beaker (thermometer at the top).

  • Chromatography: pencil baseline, spots of ink, solvent front, separated bands.


5) Mini investigations (home/school friendly)


Follow school safety rules. Use goggles for heating. Adult supervision for any flame/hot plate.

  1. Ink chromatography (water-soluble markers)

    • Draw two small dots of black marker on a pencil line 1.5 cm from the bottom of a paper strip.

    • Stand the strip in water so the dots sit above the waterline.

    • Watch colours separate. Discuss: Which colour travelled furthest? Why?

  2. Crystallising salt

    • Stir salt into hot water until some remains undissolved (saturated).

    • Filter to remove undissolved bits; evaporate gently.

    • Leave the dish to cool → crystals form.

  3. Oil and water

    • Mix cooking oil and coloured water in a jar → layers form.

    • Use a pipette/syringe to decant the top layer.


6) Stretch idea (chromatography maths)


Rf value === distance travelled by the spot ÷ distance travelled by the solvent front (both from the baseline). Values are 0–1; higher Rf = moved further.


Practice Questions (with answers)


A. Core concepts (Years 7–8)

  1. Is brass a compound, element or mixture? Explain.

  2. Define solute and solvent using cordial in water as the example.

  3. State whether each is solution or heterogeneous mixture:a) muddy water b) lemonade c) salad dressing (shaken)

  4. Which method separates sand from saltwater if you want the salt at the end?

  5. Which separation method relies mainly on boiling point?

  6. Label residue and filtrate in filtration.


Answers (A)

  1. Mixture (alloy). Metals (copper + zinc) physically combined.

  2. Solute: cordial concentrate; solvent: water.

  3. a) heterogeneous  b) solution  c) heterogeneous (immiscible liquids)

  4. Filter out sand → evaporate filtrate to recover salt.

  5. Distillation (simple or fractional).

  6. Residue stays on the paper; filtrate passes through.


B. Apply & explain (Years 8–9)

  1. You have copper sulphate crystals in water and want pure water back. Outline the setup and why it works.

  2. Two colour dyes separate on paper; dye X has Rf = 0.70, dye Y has Rf = 0.25 (same solvent). Which is more strongly attracted to the paper? Explain.

  3. A student says, “Salt won’t dissolve—so it must be insoluble.” Give two other reasons this might happen.

  4. Pick the best method and justify: a) tiny iron filings in sand b) ethanol + water c) pebbles in rice


Answers (B)

  1. Simple distillation: Heat solution → water boils first (lower b.p.), vapour condenses in condenser → distillate (water) collected; solute remains.

  2. Dye Y (Rf 0.25) is more attracted to the paper (moved less with the solvent).

  3. Solution already saturated; water too cold; not enough stirring/time; large crystals (surface area too small).

  4. a) Magnet → magnetic separation. b) Fractional distillation (close b.p.s). c) Sieving (particle size).


Printable checklist


  •  Defined mixture vs pure substance

  •  Named solute + solvent and the solution type

  •  Chosen a separation method by property (size, b.p., solubility, magnetism)

  •  Sketched/labelled filtration, evaporation, distillation, chromatography

  •  Safety noted for heating and glassware


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