Class 9 › Science › Chapter 5

Exploring Mixtures and their Separation

Homogeneous and heterogeneous mixtures, concentration and solubility, separation techniques (crystallization, distillation, chromatography, separating funnel, sublimation, centrifugation, coagulation), colloids and the Tyndall effect.

1. Classifying Mixtures

Homogeneous

Solution

Uniform composition throughout; always stays homogeneous. Eg: sugar in water, vinegar (acetic acid in water), soda (CO₂ in water).

Heterogeneous

Non-uniform

Components are visible or settle with time. Eg: sand in water, oil and water, smoke, fog, dust in air, most solid-solid mixtures.

Activity 5.1: three mixtures, three observations

A (salt + water): particles not visible; laser beam path not visible; no residue on filtering; does not settle.

B (chalk powder + water): particles visible; beam path visible; residue on filter paper; settles.

C (a few drops of milk + water): looks homogeneous; beam path visible; passes through filter paper; does not settle.

So they are three different types: solution, suspension, colloid. Never look directly into a laser beam.

Gas mixtures: gas particles move freely, so most gas-gas mixtures are homogeneous (eg: hydrogen + oxygen). With solid or liquid particles in air (smoke, fog, dust) they are heterogeneous.

2. Solutions and Concentration

Solute = substance that dissolves. Solvent = substance that dissolves the solute. Solution $=$ solute $+$ solvent. Concentration = amount of solute dissolved in a given amount of solvent or solution.

Why proportion matters: ORS needs specified amounts of salt and sugar in a fixed amount of water (not every sugary drink is ORS). Pesticide sprays: too little does not protect crops, too much damages crops, soil and environment.
Dilip Mahalanabis: Indian paediatrician who developed and implemented ORS treatment for dehydration caused by diarrhoea and cholera; it saved millions of lives after WHO popularised it.

Method	Formula	Meaning	Used for
Mass by mass (% m/m or % w/w)	$\dfrac{\text{mass of solute}}{\text{mass of solution}}\times100$	g of solute in 100 g of solution	Homogeneous mixtures; also milk powder, spice mixes, composition labels on packaged food
Mass by volume (% m/v or % w/v)	$\dfrac{\text{mass of solute}}{\text{volume of solution}}\times100$	g of solute in 100 mL of solution	Where volume is easier to measure: medicines, laboratories (5% glucose solution)
Volume by volume (% v/v)	$\dfrac{\text{volume of solute}}{\text{volume of solution}}\times100$	mL of solute in 100 mL of solution	Two miscible liquids: perfumes, cosmetics, vinegar

Mass of solution = mass of solute + mass of solvent (not just the solvent). Volume in % m/v and % v/v is the total volume of solution.
Industries commonly use % w/w; numerically % m/m and % w/w are equal.

Worked examples

5.1: 10 g salt in 90 g water: solution $= 100$ g; $\frac{10}{100}\times100 = 10\%$ m/m.

5.2: 5 g glucose made up to 100 mL: $\frac{5}{100}\times100 = 5\%$ m/v.

5.3: 1 mL pesticide made up to 100 mL: $\frac{1}{100}\times100 = 1\%$ v/v.

Reverse use: 4% m/m zinc oxide in 300 g talcum powder: mass $= \frac{4}{100}\times 300 = 12$ g.

Saline drip: 0.9% m/v sodium chloride, i.e. 0.9 g salt in 100 mL; safe for blood and replaces lost fluids.
Vinegar: about 5% v/v acetic acid; glacial acetic acid is 100% acetic acid, so it is diluted with water.

3. Solubility and Solubility Curves

Solubility: maximum amount of solute that dissolves in a fixed quantity of solvent (100 g or 100 mL) at a given temperature. Saturated solution: cannot dissolve any more solute at that temperature.

Solids in liquids: solubility generally increases with temperature. Gases in liquids: solubility generally decreases with temperature.
Solubility curve: graph of solubility (g per 100 g water, y-axis) against temperature (°C, x-axis).
Table 5.4 pattern: potassium nitrate rises sharply (21 g at 10°C to 167 g at 80°C); sodium chloride hardly changes (36 to 37 g); potassium chloride and ammonium chloride rise moderately. A steeper curve means a bigger drop in solubility on cooling.
Saturated solution amount: eg: KNO₃ at 40°C dissolves 62 g in 100 g water, so 31 g in 50 g water.

Solubility curves (Fig. 5.6, schematic). B: 287 g at 60°C, 241 g at 40°C.

4. Crystallization

Crystal: a solid whose particles are arranged in a regular geometric pattern. Crystallization: forming crystals from a saturated solution (often by cooling). Principle: solubility differs at different temperatures.

Natural crystals: rock salt, candy sugar (mishri), snowflakes, frost on windows (water vapour turning to ice crystals), quartz, cave formations (eg: Mawsmai Cave, Sohra).
Uses: separate two solids when one is present in a small quantity and both dissolve in the same solvent; purify solids; separate a desired pure product from impurities.

Example (Fig. 5.6): compound B

Saturated solution at 60°C holds 287 g in 100 g water. Cooled to 40°C, only 241 g can stay dissolved.

Solid separating out $= 287 - 241 = 46$ g (crystals).

Compound with a steeper curve deposits more solid on cooling.

Activity 5.3: crystals of copper sulfate

1. Dissolve copper sulfate in water with a drop of dilute sulfuric acid (teacher adds it; helps pure crystals), heat gently in a water bath, add more until saturated.

2. Filter hot to remove insoluble impurities; cover with a watch glass.

3. Cool slowly, undisturbed: larger, shiny, well-shaped crystals. Rapid cooling (ice-cold water) gives smaller, less well-formed crystals.

4. Filter the crystals, rinse with cold water, dry on a watch glass.

Copper sulfate is toxic: use under adult supervision, no bare hands. Sulfuric acid is needed only for some salts.
Salt from seawater: seawater → saturated solution (evaporation) → salt crystals. India: panga salt by boiling concentrated brine; karkatch salt by evaporation of seawater.
Evaporation vs crystallization: evaporation drives off the solvent (solvent lost); crystallization forms pure crystals from a saturated solution and leaves impurities in the liquid. They are not the same process.

5. Distillation

Distillation: heating a mixture until the liquid with the lower boiling point vaporises, then cooling the vapour in a condenser back to liquid (distillate).

Separates: two miscible liquids whose boiling points differ by at least about 25°C; also recovers a liquid (solvent) from a solution containing dissolved solids (the solid stays in the flask).
Apparatus: distillation flask with thermometer, burner on wire gauze and tripod, water condenser (water enters at the lower inlet, leaves at the upper outlet), collecting conical flask.
Eg: acetone (bp about 56°C) and water (bp 100°C): acetone vaporises first, condenses and is collected; water remains.
Fractional distillation: separates components with small boiling-point differences (less than 25°C); used in petroleum refineries to split crude oil into petroleum gas (LPG), petrol, kerosene (aviation fuel), diesel, lubricating oil and bitumen.
Kannauj (UP): perfume capital of India; Mitti ka Ittar (earthy fragrance) made by the traditional Deg-Bhapka distillation method.

6. Paper Chromatography

Paper chromatography: separates components of a mixture using differences in how they interact with the solvent and the paper; the solvent carries them up the paper at different rates. Word origin: Greek chroma (colour) + graphein (to write).

Activity 5.5: black ink

1. Draw a pencil line 2 cm from the bottom of a 3 cm wide strip; put an ink spot at the centre of the line.

2. Dip the lower end in a thin layer of water; the water level must be below the spot.

3. As water rises, the ink separates into different coloured spots: the ink is a mixture of dyes.

Uses: separate pigments of spinach leaves or flower petals, dyes in food colours. Water does not work for every sample; alcohol or a solvent mixture may be needed.
Pencil, not ink, is used for the base line (it would run with the solvent).

7. Separating Heterogeneous Mixtures

Immiscible liquids: separating funnel

Immiscible: liquids that do not mix and form separate layers (oil and water). Separation is based on different densities.
Mustard oil + water: oil (less dense) forms the upper layer, water the lower layer. Open the stopcock slowly to drain water, close it when almost drained, discard the small mixed portion, then collect the oil.
If two immiscible liquids had the same density, clear layers would not form.

Sublimation

Sublimation: a solid changes directly into vapour on heating (below its melting point) without becoming liquid. Deposition: vapour cools and changes directly back to solid.

Activity 5.7: camphor + sand in a china dish, covered by an inverted funnel plugged with cotton. On gentle heating, camphor sublimes and deposits as white solid on the funnel wall; sand stays (it does not sublime).
Other sublimable solids: naphthalene, dry ice (solid CO₂, used for ice-cream storage).
Sublimation vs evaporation: sublimation is solid → vapour (no liquid stage); evaporation is liquid → vapour.

Alloys

Alloy: homogeneous mixture of two or more metals, or a metal and a non-metal; made by melting and mixing, then cooling. Physical methods cannot separate the components.
Made to be stronger, more rigid or more corrosion-resistant. Brass: about 80% copper, 20% zinc. Bronze: about 80% copper, 20% tin. Stainless steel: iron with carbon (about 0.03–0.8%), chromium (16–18%), nickel (10–14%), molybdenum (2–3%).

8. Suspensions: Centrifugation and Coagulation

Suspension: heterogeneous mixture in which solid particles do not dissolve but stay suspended in the medium. Particles are larger than in a solution and visible to the naked eye. Eg: sand, sawdust or tea leaves in water, muddy water.

Muddy water: heavy mud settles (sedimentation); filtering through cloth or filter paper removes larger particles but water often stays cloudy. Then use centrifugation and/or coagulation.

Centrifugation

Spin at high speed

Centrifugal (outward) force pushes heavier particles outward, so they settle at the tube bottom while the lighter liquid stays on top. Eg: separating RBCs and plasma of blood; many chemical industries.

Coagulation

Clump, settle, remove

A coagulant (powdered alum, fitkari) makes fine suspended particles clump; clumps settle by gravity (sedimentation) and are removed by decantation or filtration. Eg: paneer from milk using lemon juice or vinegar.

Paperfuge: low-cost hand-powered centrifuge (based on a toy) that works without electricity; helps detect diseases like malaria and anaemia in remote areas.
Blood donation: donated blood is tested, blood group identified, and separated into plasma, platelets, white and red cells; the body replaces donated blood in a few weeks. After centrifugation, red blood cells are at the bottom, plasma on top.
Mini centrifuge model (Activity 5.8): cardboard disc and thread; shows heavier particles moving outwards.

9. Colloids and the Tyndall Effect

Colloid: a mixture that looks homogeneous but has particles larger than in a solution; particles stay uniformly dispersed and do not settle. Eg: milk, blood, tomato sauce, ice cream, butter, clouds. Blood is neither a true solution nor a true suspension.

Property	Solution	Suspension	Colloid
Nature	Homogeneous	Heterogeneous	Appears homogeneous (heterogeneous at fine scale)
Particle size	Less than 1 nm	More than 1000 nm	1–1000 nm
Visibility	Not visible; transparent	Visible to naked eye	Not visible to naked eye
Settling	Does not settle	Settles on standing	Does not settle
Filtration	Cannot be separated by filtration	Separated by filtration	Cannot be separated by ordinary filtration
Tyndall effect	No	Yes	Yes
Examples	Salt in water, vinegar, brass	Sand/mud in water, chalk in water	Milk, blood, smoke, butter

Tyndall effect: scattering of light by particles, making the path of a light beam visible. Named after John Tyndall. Shown by colloids and suspensions, not by true solutions.

Everyday examples: a fine beam of light entering a dark room (scattered by dust and smoke), floodlights in a stadium, bright rays of sunlight through gaps in dense tree leaves, hazy cities with smoke and dust.
Components of a colloid: dispersed phase (the solute-like dispersed particles) and dispersion medium (what they are dispersed in). In blood, the cells are dispersed in plasma.
Emulsion: colloid with both phases liquid. Oil-in-water: milk, vanishing cream. Water-in-oil: butter, body lotion, cold cream. Emulsifying agents (proteins in milk, soap) stabilise emulsions. Some medicines are made as emulsions to be palatable and less greasy.

10. Quick Revision

Mixture to separate	Method	Principle
Insoluble solid from liquid (sand, mud)	Sedimentation, decantation, filtration	Particle size, density
Very fine suspended particles	Coagulation (alum) or centrifugation	Clumping; density and spinning
Blood components	Centrifugation	Density
Pure solid from saturated solution	Crystallization	Solubility changes with temperature
Solvent from solute (solid in liquid)	Distillation (solid only: evaporation)	Boiling point
Miscible liquids, bp difference ≥ 25°C	Simple distillation	Boiling point
Miscible liquids, bp difference < 25°C	Fractional distillation	Boiling point
Immiscible liquids (oil + water)	Separating funnel	Density
Sublimable solid + non-sublimable solid	Sublimation (camphor, naphthalene)	Sublimation
Dyes, plant pigments	Paper chromatography	Different rates of movement
Magnetic + non-magnetic solids	Magnet	Magnetic property
Alloy components	Cannot be separated by physical methods	Homogeneous solid solution

Concentration

Three percentages

% m/m: solute g / solution g. % m/v: solute g / solution mL. % v/v: solute mL / solution mL. All $\times 100$.

Remember

Tyndall effect

Colloid and suspension: yes. Solution: no.

Daily life

Separation everywhere

Kidneys remove wastes from blood; sewage treatment uses sedimentation, coagulation and filtration; dry waste is recycled, wet waste composted; lithium is recovered from old batteries.

Common mistakes:
• Using the mass of the solvent instead of the mass of the solution in % m/m.
• Saying a colloid is a suspension, or that colloids settle or are removed by ordinary filtration.
• Saying solutions show the Tyndall effect.
• Using distillation for liquids with the same (or very close) boiling points; fractional distillation is needed for less than 25°C difference.
• Saying evaporation and crystallization are the same, or that sublimation is the same as evaporation.
• Placing the water level above the ink spot in chromatography.
• Expecting solid solubility to fall with rising temperature (it generally rises; gases fall).
• Thinking brass or other alloys can be separated by simple physical methods.