Class 9 Science Chapter 5: Exploring Mixtures and their Separation — Notes, Examples & Practice

A pure substance has a fixed composition and fixed properties, such as a definite melting and boiling point. Elements (iron, oxygen) and compounds (water, sodium chloride) are pure substances.

A mixture contains two or more substances physically combined in any proportion. Each component keeps its own properties, and the components can be separated by physical methods. Sugar water can be sweeter or less sweet; it is still sugar water.

A homogeneous mixture has the same composition throughout and no visible boundaries (salt water, air, brass). A heterogeneous mixture has visibly different parts (sand in water, oil and water, a salad).

A solution is a homogeneous mixture. The substance present in the larger amount (usually the liquid) is the solvent; the substance dissolved in it is the solute. Solutions can be solid (alloys such as brass), liquid (salt in water) or gas (air).

Solute particles are extremely small (less than about 1 nm), so a solution is clear, does not scatter light, does not settle, and passes through filter paper.

Concentration tells how much solute is present: mass by mass percentage $= \dfrac{\text{mass of solute}}{\text{mass of solution}} \times 100$. Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a given temperature; a solution holding that much is saturated. Solubility of most solids increases with temperature.

A suspension is a heterogeneous mixture with particles big enough to see (larger than about 100 nm, often much larger). It looks cloudy, the particles settle on standing, and they can be removed by filtration (muddy water, chalk in water).

A colloid has particles of intermediate size (about 1–1000 nm). It looks homogeneous but is actually heterogeneous; the particles do not settle and pass through filter paper, but they scatter light — the Tyndall effect. You see it when sunlight streams through a dusty room or a forest canopy, or a torch beam shines through fog or milk-water.

A colloid has a dispersed phase (the particles) and a dispersion medium (what they are spread in). Examples: milk (liquid in liquid, emulsion), fog (liquid in gas, aerosol), smoke (solid in gas), jelly (liquid in solid, gel), shaving foam (gas in liquid, foam).

Every separation exploits a difference in a physical property between the components. Ask: do they differ in particle size, solubility, density, magnetism, boiling point, or the ability to sublime?

Insoluble solid in liquid → sedimentation and decantation (pour off the liquid) or filtration. Very fine or light particles (cream in milk, blood cells) → centrifugation, which spins the mixture so denser particles collect at the bottom. Magnetic + non-magnetic solids → a magnet. A solid that sublimes (ammonium chloride, camphor, naphthalene) → sublimation.

Soluble solid from solution → evaporation (to dryness) or crystallisation. Crystallisation gives purer solid because impurities stay dissolved in the leftover liquid, and it does not decompose heat-sensitive solids like sugar.

Simple distillation separates a liquid from dissolved solids (pure water from salt water) or two miscible liquids whose boiling points differ a lot (more than about 25 °C). The liquid boils, its vapour is cooled in a condenser, and the pure liquid (the distillate) is collected. Use it when you want to keep the liquid.

Fractional distillation separates miscible liquids with close boiling points, using a fractionating column — this is how air (after liquefying) is separated into nitrogen, oxygen and argon, and how petroleum is refined.

A separating funnel separates immiscible liquids (oil and water): the denser layer is run off through the tap at the bottom. Paper chromatography separates coloured components dissolved in the same solvent: they travel up the paper at different speeds because they have different solubility in the solvent and different attraction to the paper. It separates dyes in ink, pigments in leaves and is used to detect drugs in blood.

Real mixtures usually need several methods in a sensible order. Plan by listing the components and their properties, then remove one component at a time.

Sand + salt + iron filings + camphor: (1) magnet removes iron; (2) heat gently in a covered dish — camphor sublimes and collects on the cool surface; (3) add water, stir — salt dissolves; (4) filter — sand stays on the paper; (5) evaporate or crystallise the filtrate to get salt.

Water treatment uses the same logic: sedimentation, loading with alum (it makes tiny particles clump together and settle), filtration through sand and gravel, then chlorination to kill germs (a chemical step, not a separation).