Is Matter Around Us Pure? | Science Grade 9 CBSE

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Sorting out matter

Pure or mixed? Element or compound? Solution or colloid? Tap each term to see exactly what it means and how it differs from its neighbour.

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The three big ideas

A pure substance is made of only one kind of particle, with a fixed composition. It is either an element (cannot be broken into anything simpler — e.g. oxygen, gold, iron) or a compound (two or more elements chemically combined in a fixed ratio — e.g. water, H₂O).
A mixture contains two or more substances not chemically combined. Each keeps its own properties, the proportions can vary, and the parts can be separated again.
Homogeneous mixtures are uniform throughout (salt solution, air, brass); heterogeneous mixtures have visibly different parts (sand + water, oil + water).

Worked example. Is salt dissolved in water a mixture or a compound?

It is a mixture — specifically a solution. The salt and the water keep their own identities (the water still tastes salty, the salt is still there) and can be separated again by evaporation. Because they are not chemically bonded in a fixed ratio, it is not a compound.

Common mistake: calling a solution a "compound". A solution is not a compound — its components are not chemically bonded and are not present in a fixed ratio. You can make the salt water more or less salty, and recover both substances unchanged.

A solution is a homogeneous mixture of a solute (the substance dissolved) in a solvent (the dissolving medium) — e.g. salt in water. Its particles are too small to see or filter and do not scatter light.
A suspension is heterogeneous: large particles that you can see, that settle on standing and can be filtered out (e.g. chalk powder in water).
A colloid looks uniform but is heterogeneous: particles big enough to scatter a beam of light — the Tyndall effect — yet small enough not to settle (e.g. milk, fog, smoke).
Concentration = amount of solute in a given amount of solution (or solvent). For example, mass percentage = (mass of solute ÷ mass of solution) × 100.

Filtration — separates an insoluble solid from a liquid using filter paper (e.g. sand from water, tea leaves from tea).
Evaporation — recovers a dissolved solid by boiling off the liquid (e.g. salt from salt water).
Distillation — separates a liquid from dissolved solids, or two miscible liquids of different boiling points, by boiling then condensing the vapour (e.g. pure water from salt water).
Chromatography — separates dissolved substances that travel at different rates on a medium (e.g. the different dyes in a sample of ink).

Where you'll meet it

Mixtures, taken apart

Water purification

Making water safe to drink is a chain of separations: filtration removes mud and suspended solids, sedimentation lets heavy particles settle, and distillation (or boiling) plus disinfection removes dissolved and microbial impurities — the same ideas you just learnt.

Separating useful substances in industry

Refineries split crude oil into petrol, diesel and kerosene by fractional distillation; salt is harvested from sea water by evaporation; and chromatography separates and tests the dyes, medicines and food colours we use every day.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, a case study and an assertion–reason item, testing whether you can use the ideas, not just recall them.

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