Chapter Overview & Weightage
“Is Matter Around Us Pure?” is Chapter 2 of Class 9 Science (Chemistry section). It covers the classification of matter into pure substances and mixtures, and methods of separation. This chapter carries 8–10 marks in CBSE Class 9 exams and connects directly to Class 10 chapters on acids, bases, and chemical reactions.
| Question Type | Marks | Topics |
|---|---|---|
| MCQ / Fill in the blank | 1–2 | Definitions, classification, separation methods |
| Short Answer | 2–3 | Differences between types, properties of solutions |
| Long Answer | 4–5 | Separation techniques with diagrams |
Separation techniques — especially fractional distillation, chromatography, and crystallisation — appear almost every year. Know the principle behind each method and when to use it.
Key Concepts You Must Know
Pure Substance: Has a fixed composition and fixed properties throughout. Two types: elements (only one kind of atom/molecule — e.g., copper, nitrogen) and compounds (two or more elements chemically combined in a fixed ratio — e.g., water H₂O, table salt NaCl).
Mixture: Two or more substances physically combined. Composition is variable. Two types:
- Homogeneous mixture (Solution): Uniform composition throughout — e.g., salt water, air, brass
- Heterogeneous mixture: Composition varies from part to part — e.g., soil, salad, sand in water
Solution: A homogeneous mixture of solute (dissolved) and solvent (does the dissolving). The one present in larger amount is the solvent.
- Concentration = (mass of solute / mass of solution) × 100
- Saturated solution: No more solute can dissolve at that temperature
Colloid: Heterogeneous mixture where particles are between 1 nm and 1000 nm. Shows Tyndall effect (scattering of light beam). Examples: milk, fog, paint.
Suspension: Particles >1000 nm. Visible to naked eye, settles on standing. Example: chalk water.
Tyndall Effect: When a beam of light passes through a colloid, the path of light becomes visible due to scattering by colloidal particles. Does NOT occur in true solutions.
Important Formulas
Example: 10 g NaCl in 90 g water → 10/100 × 100 = 10% solution.
Separation Techniques
| Method | Principle | Used For |
|---|---|---|
| Evaporation | Solvent evaporates, solute remains | Salt from sea water |
| Distillation | Differences in boiling points | Acetone-water mixture |
| Fractional Distillation | Small differences in boiling points | Petroleum fractions, liquid air |
| Crystallisation | Solubility varies with temperature | Purifying copper sulphate |
| Sublimation | One component sublimes | Ammonium chloride + salt |
| Chromatography | Different affinities for stationary phase | Dyes in ink, medicines |
| Centrifugation | Density differences under centrifugal force | Cream from milk, blood components |
| Magnetic separation | One component is magnetic | Iron filings from sulphur |
Solved Previous Year Questions
PYQ 1 — Tyndall Effect (2 marks)
Q: What is the Tyndall effect? In which type of mixture is it observed?
Solution: When a beam of light passes through a colloidal solution, the particles scatter the light, making the beam visible. This scattering of light is called the Tyndall effect.
It is observed in colloids (e.g., milk, smoke, fog) but NOT in true solutions (e.g., salt water) — because solution particles are too small to scatter light.
PYQ 2 — Separation Method Selection (3 marks)
Q: Which method would you use to separate: (a) ammonium chloride from salt, (b) dyes in a black ink, (c) iron from sulfur?
Solution: (a) Sublimation — ammonium chloride sublimes on heating; NaCl does not. (b) Chromatography — dyes travel different distances on the stationary phase based on their solubility. (c) Magnetic separation — iron is magnetic, sulphur is not.
PYQ 3 — Concentration Calculation (3 marks)
Q: 15 g of common salt is dissolved in 85 g of water. Calculate the mass percent of the solution.
Solution: Mass of solution = 15 + 85 = 100 g
PYQ 4 — Compound vs Mixture (4 marks)
Q: Distinguish between a compound and a mixture with two examples of each.
Solution:
| Property | Compound | Mixture |
|---|---|---|
| Composition | Fixed ratio | Variable |
| Properties | New properties, different from constituents | Properties of individual constituents |
| Separation | Only by chemical means | By physical methods |
| Formation | Chemical change, heat/energy change | Physical mixing, no energy change |
Examples of compounds: Water (H₂O), Carbon dioxide (CO₂) Examples of mixtures: Air, soil
Difficulty Distribution
| Difficulty | % of Questions | Types |
|---|---|---|
| Easy | 45% | Definitions, Tyndall effect, identifying pure/mixture |
| Medium | 35% | Separation method selection, concentration calculation |
| Hard | 20% | Multi-step separation, explaining why a method works |
Expert Strategy
For separation technique questions, always state the principle first (why it works) before the steps. CBSE awards 1 mark for the principle. “Fractional distillation separates components with different boiling points” — this single sentence is worth at least 1 mark.
For compound vs mixture questions, make a comparison table — it signals structured thinking and earns full marks efficiently.
The Tyndall effect and Brownian motion are the two properties that specifically characterise colloids in exams. Learn both: Tyndall = light scattering; Brownian motion = zig-zag movement of colloidal particles due to collisions with solvent molecules (observable under microscope).
Common Traps
Trap 1 — Homogeneous ≠ pure: Air is homogeneous (same composition throughout) but it is a mixture. Students confuse “uniform appearance” with “pure substance.” A pure substance has a fixed chemical composition — not just uniform appearance.
Trap 2 — Distillation vs fractional distillation: Use simple distillation when boiling points differ by more than 25°C. Use fractional distillation when boiling points are close. Students sometimes use both interchangeably — be precise.
Trap 3 — Crystallisation vs evaporation: Both separate solute from solvent. But crystallisation gives pure crystals by controlled cooling (used for purification); evaporation leaves a crude residue (used for bulk salt production). The question’s context tells you which to write.