Class 9 — Is Matter Around Us Pure? Mixtures and Separation
Almost nothing you touch in daily life is a pure substance — tea, seawater, milk and air are all mixtures. NCERT's "Is Matter Around Us Pure?" chapter asks you to classify mixtures correctly, tell solutions, suspensions and colloids apart, and pick the right separation technique for a given situation. This is a heavily scenario-based chapter in exams, so this guide works through the classification rules and the concentration calculations together.
Pure substances vs mixtures
A pure substance has a fixed, definite composition throughout — it is either an element (made of one kind of atom, e.g. copper, oxygen) or a compound (two or more elements chemically combined in a fixed ratio, e.g. water, carbon dioxide). A mixture contains two or more substances physically combined in any proportion, and each substance retains its own properties.
Solutions, suspensions and colloids
All three are mixtures of a solute-like component dispersed in a solvent-like medium, but they differ sharply in particle size, and that single difference explains every other property.
| Property | True solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | Less than 1 nm | 1 nm to 1000 nm (1 µm) | Greater than 1000 nm (1 µm) |
| Appearance | Clear, transparent | Generally translucent | Opaque, often visibly cloudy |
| Settling | Never settles | Does not settle on standing | Settles on standing |
| Filtration | Passes through filter paper | Passes through ordinary filter paper | Does not pass through filter paper |
| Tyndall effect | Does not scatter light | Scatters light visibly (Tyndall effect) | Scatters/blocks light, but is not stable |
| Example | Sugar solution | Milk, ink, fog | Chalk-water mixture, muddy river water |
The Tyndall effect — the scattering of a beam of light as it passes through a colloid, making the light's path visible — is the standard experimental test used to distinguish a colloid from a true solution, which does not scatter light because its particles are too small.
Concentration of a solution
ICSE/CBSE Class 9 uses two simple mass-based ways to express how concentrated a solution is:
Mass by volume percentage = (mass of solute ÷ volume of solution) × 100
Worked example 1 — mass by mass percentage
15 g of sugar is dissolved in 135 g of water. Find the mass by mass percentage concentration of the solution.
Mass of solution = mass of solute + mass of solvent = 15 + 135 = 150 g
Mass by mass % = (15 ÷ 150) × 100 = 10%
Worked example 2 — mass by volume percentage
25 g of common salt is dissolved in water and the solution is made up to 250 mL. Find its mass by volume percentage concentration.
Mass by volume % = (25 ÷ 250) × 100 = 10%
Separation techniques — matching the method to the mixture
| Situation | Correct technique | Why |
|---|---|---|
| Common salt from sea water | Evaporation | Solid solute left behind as the liquid solvent is driven off |
| Two miscible liquids with a large boiling-point gap | Simple distillation | Lower-boiling liquid vaporises and is collected first |
| Crude oil / petroleum, or liquids with close boiling points | Fractional distillation | A fractionating column separates components whose boiling points are too close for simple distillation |
| Mixture of two or more dyes, or plant pigments | Chromatography | Components travel at different rates through a stationary medium based on solubility/adsorption |
| Ammonium chloride mixed with sand | Sublimation | Ammonium chloride passes directly to gas and is re-collected as a solid, leaving sand behind |
| Iron filings mixed with sulphur powder | Magnetic separation | Iron is attracted to a magnet; sulphur is not |
| Cream from milk | Centrifugation | Denser and less dense components separate rapidly under fast rotation |
Worked example 3 — choosing a separation method
A student is given a mixture of naphthalene balls crushed together with common salt. How should the two be separated?
Naphthalene sublimes on gentle heating (solid directly to gas), while common salt does not. Heating the mixture in a china dish covered with an inverted funnel lets naphthalene vapour rise and re-solidify on the cooler funnel, leaving salt behind in the dish — sublimation is the correct method, not filtration or evaporation, since neither of those separates two solids from each other.
Physical change vs chemical change
Separation techniques work because mixing is a physical change — no new substance forms, and the original components can be recovered by physical means. This is why mixtures (unlike compounds, which form through a chemical change) can always, in principle, be separated back into their components.
Common mistakes that cost marks
- Calling a colloid a "solution." Milk and ink look uniform to the eye but are colloids, distinguishable from a true solution by the Tyndall effect and by particle size, not by appearance alone.
- Choosing filtration for a colloid. Colloidal particles are small enough to pass through ordinary filter paper, just like a true solution — only a suspension is retained by filter paper.
- Confusing simple and fractional distillation. Fractional distillation is specifically for liquids whose boiling points are close together; simple distillation suffices when the gap is large.
- Forgetting to add solute and solvent masses when finding "mass of solution" for a mass by mass percentage calculation.
Where this chapter appears in exams
| Exam | Typical use |
|---|---|
| CBSE Class 9 | Classifying a given mixture, concentration numericals, matching separation techniques to scenarios |
| CBSE Class 10–11 | Foundation for solubility, solutions and colligative-properties chapters later |
Check concentration calculations as you revise. The free calculator suite has dedicated tools for concentration and dilution once you reach the Class 11–12 level of this topic.
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