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Exploring Mixtures and Their Separation – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 5. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Matter can be a pure substance (element or compound) or a mixture. Mixtures can be separated by physical methods based on differences in properties.

Pure substances and mixtures

Element: one kind of atom (iron, oxygen). Compound: elements chemically combined in a fixed ratio (water, H₂O).
Mixture: substances mixed in any ratio, keep their own properties.
Homogeneous mixture (solution: salt in water); heterogeneous (sand in water).

Solutions, suspensions, colloids

Solution: particles < 1 nm, clear, do not settle, pass through filter paper.
Suspension: large particles, settle on standing (muddy water).
Colloid: medium particles, scatter light (Tyndall effect) — milk, fog, smoke.
Concentration = mass of solute / mass of solution × 100.

Separation methods

Evaporation (salt from water), filtration (sand from water), centrifugation (cream from milk).
Distillation (liquids with different boiling points), fractional distillation (air, petroleum).
Sublimation (camphor, ammonium chloride), chromatography (dyes in ink), separating funnel (oil and water), magnet (iron filings).

Key terms

Solute: the substance dissolved
Solvent: the substance that dissolves the solute
Tyndall effect: scattering of a light beam by colloid particles
Sublimation: change of a solid directly into vapour

Formulas

Concentration (%) = (solute / solution) × 100

Important questions with answers

How will you separate a mixture of salt and sand?2 marks

Add water and stir so the salt dissolves; filter to remove sand; evaporate the filtrate to get the salt back.

Distinguish between a true solution, a colloid and a suspension.3 marks

True solution: particles smaller than 1 nm, transparent, stable, no Tyndall effect (salt in water). Colloid: particles 1–1000 nm, appears uniform, stable, shows the Tyndall effect (milk). Suspension: particles larger than 1000 nm, opaque, settles on standing, shows the Tyndall effect (chalk in water).

Explain how you would separate (i) dyes in black ink, (ii) acetone and water, (iii) ammonium chloride from salt, (iv) cream from milk, and (v) oxygen and nitrogen from air.5 marks

(i) Chromatography: dyes travel up the paper at different speeds. (ii) Distillation (or fractional distillation): acetone boils at 56 °C and water at 100 °C. (iii) Sublimation: on heating, ammonium chloride turns directly to vapour and collects on a cool surface. (iv) Centrifugation: spinning separates lighter cream from the milk. (v) Fractional distillation of liquefied air: nitrogen (−196 °C) boils off before oxygen (−183 °C).

Quick revision

Milk is a colloid.
Solutions do not show the Tyndall effect.
Sublimation separates camphor from salt.

Exam tips

Compound properties differ from its elements; a mixture keeps its components' properties.
Chromatography separates colours because they move at different speeds with the solvent.

Practice MCQs

Salt water is an example of a
1. compound
2. heterogeneous mixture
3. homogeneous mixture
4. element
Show answer
Answer: (c) homogeneous mixture. Uniform solution.
Cream is separated from milk by
1. filtration
2. sublimation
3. centrifugation
4. distillation
Show answer
Answer: (c) centrifugation. Spinning fast.
Which is an element?
1. water
2. oxygen
3. salt
4. sugar
Show answer
Answer: (b) oxygen. One kind of atom.
Which shows the Tyndall effect?
1. salt solution
2. sugar solution
3. milk
4. copper sulphate solution
Show answer
Answer: (c) milk. Colloids scatter light.
Different coloured components of ink are separated by
1. magnet
2. evaporation
3. filtration
4. chromatography
Show answer
Answer: (d) chromatography. Chromatography.
20 g salt is dissolved in 180 g water. Concentration (mass %) is
1. 11.1%
2. 10%
3. 20%
4. 9%
Show answer
Answer: (b) 10%. 20/200 × 100 = 10%.
Gases of air are separated by
1. fractional distillation of liquid air
2. simple distillation
3. sublimation
4. chromatography
Show answer
Answer: (a) fractional distillation of liquid air. Different boiling points.
Which is NOT a property of a compound?
1. fixed composition
2. new properties
3. components keep their properties
4. formed by chemical change
Show answer
Answer: (c) components keep their properties. That is a mixture property.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.