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Rajasthan (RBSE)Class 9 Science★ 6–8 marks · RBSE Class 9 Science (new NCF — Curiosity)

Exploring Mixtures and their Separation

The new NCF Class 9 Science chapter on mixtures. Pure substances vs mixtures; homogeneous and heterogeneous; solutions, suspensions and colloids; concentration and saturation; the Tyndall effect; and the full toolkit of separation methods — evaporation, filtration, distillation, chromatography, centrifugation and more.

⏱ 30 min readMedium difficulty✓ Free · NCERT-aligned

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The chapter at a glance

Every concept and exam-favourite branch — revisable in 5 minutes.

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Study Guide

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Key points, glossary and the questions most likely to show up.

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By the end of this chapter you'll be able to…

1Distinguish pure substances (elements, compounds) from mixtures
2Classify mixtures as homogeneous or heterogeneous
3Define solution, solute, solvent and calculate mass % concentration
4Compare true solutions, colloids and suspensions and explain the Tyndall effect
5Choose the correct separation method for a given mixture
6Explain why separating a mixture is a physical change

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Why this chapter matters

This chapter builds the core chemistry vocabulary — pure substance, mixture, solution, colloid — and the separation toolkit used in every lab and in Class 10 chemistry. Separation-method questions and the solution/colloid/suspension comparison are reliable board scorers.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Materials Around Us (Class 6 Curiosity)

Soluble/insoluble and basic properties of materials.

Exploring Mixtures and their Separation (RBSE Class 9 · Science)

Almost nothing around you is pure — the air you breathe, the sea, your morning milk are all mixtures. Chemistry begins by asking two questions: what is mixed in here? and how do I pull it apart? This chapter answers both.

RBSE note (2026-27). Class 9 uses the new NCF (Curiosity) Science textbook. Exploring Mixtures and their Separation is the new book's treatment of what older books called "Is Matter Around Us Pure?". BSER (Ajmer) sets the exam.

1. Pure substances and mixtures

A pure substance has a fixed composition and a single kind of particle — an element (e.g. copper, oxygen) or a compound (e.g. water, salt).
A mixture contains two or more substances not chemically combined, in any ratio, each keeping its own properties.

Classifying mixtures

Homogeneous — uniform throughout; you cannot see the parts (salt solution, air, brass).
Heterogeneous — non-uniform; parts are visible or separable (sand + iron filings, oil + water).

2. Solutions

A solution is a homogeneous mixture of a solute (dissolved, smaller amount) in a solvent (dissolving medium, larger amount). In salt water, salt = solute, water = solvent.

Concentration = amount of solute in a given amount of solution. A common measure: $mass % of solute = \frac{mass of solute}{mass of solution} \times 100$

A saturated solution can dissolve no more solute at that temperature; an unsaturated one can dissolve more.
Solubility usually increases with temperature for solids.

Suspensions and colloids

Type	Particle size	Visible?	Settles?	Example
True solution	< 1 nm	no	no	salt water
Colloid	1–1000 nm	not to eye	no (stable)	milk, fog
Suspension	> 1000 nm	yes	yes	muddy water, chalk in water

3. The Tyndall effect

When a beam of light passes through a colloid, the particles scatter the light and the beam becomes visible — the Tyndall effect. It is why a torch beam shows up in fog and sunlight streams through a forest canopy or a dusty room. A true solution shows no Tyndall effect.

4. Separating the components of a mixture

Choose the method by the property that differs between the components:

Evaporation — recover a dissolved solid (salt from salt water) by boiling off the solvent.
Filtration — separate an insoluble solid from a liquid (sand from water).
Sedimentation and decantation — let heavy particles settle, then pour off the liquid.
Centrifugation — spin fast to settle fine particles (cream from milk, blood components).
Using a separating funnel — separate two immiscible liquids (oil and water) by density.
Sublimation — separate a solid that sublimes (ammonium chloride, camphor) from one that does not (salt).
Distillation — separate a solvent from a solution (or two miscible liquids) by boiling and condensing. Fractional distillation separates liquids with close boiling points (e.g. components of air, petroleum).
Chromatography — separate dissolved substances that travel at different rates on paper (dyes in ink).

Rule of thumb: insoluble solid → filter; dissolved solid → evaporate; miscible liquids → distil; immiscible liquids → separating funnel; coloured mixtures → chromatography.

5. Physical vs chemical change (why mixtures matter)

Separating a mixture is a physical change — no new substance forms and the change is usually reversible (evaporated salt is still salt). Breaking a compound needs a chemical change. This is the key difference between a mixture (physically separable) and a compound (chemically bonded).

6. Quick recap

Pure substance (element/compound) vs mixture (two+ substances, not combined).
Mixtures are homogeneous (uniform) or heterogeneous (non-uniform).
Solution = solute in solvent; concentration = solute per solution; saturated = no more dissolves.
True solution < colloid < suspension by particle size; colloids show the Tyndall effect, true solutions don't.
Pick a separation method by the differing property: filter (insoluble solid), evaporate (dissolved solid), distil (miscible liquids), separating funnel (immiscible liquids), chromatography (dyes), sublimation, centrifugation.
Separating a mixture is a physical change.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Mass % concentration

(mass of solute / mass of solution) × 100

Mass of solution = solute + solvent.

Particle-size order

true solution < colloid < suspension

<1 nm, 1–1000 nm, >1000 nm.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Calling a colloid a true solution because it looks clear

✓ A colloid scatters light (Tyndall effect) and has larger particles (1–1000 nm); a true solution does not scatter light.

WATCH OUT

✗ Using filtration to get salt back from salt water

✓ Salt is DISSOLVED, so it passes through filter paper. Use EVAPORATION (or distillation to also recover the water).

WATCH OUT

✗ Confusing solute and solvent

✓ Solute is the dissolved substance (usually smaller amount); solvent does the dissolving (usually larger amount).

WATCH OUT

✗ Forgetting mass of solution includes the solvent

✓ Mass of solution = mass of solute + mass of solvent; use that as the denominator in mass %.

WATCH OUT

✗ Thinking distillation and evaporation are identical

✓ Evaporation discards the vapour to recover the solid; distillation CONDENSES the vapour to recover the liquid (solvent) too.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

In-text (mixtures & solutions)

Homogeneous/heterogeneous; solute/solvent; concentration

Questions

In-text (colloids & Tyndall)

Solution vs colloid vs suspension; Tyndall effect

Questions

Activity

Tyndall with a torch; chromatography of ink; separating funnel

Questions

End-of-chapter Ex

End-of-chapter exercise

Choosing separation methods; concentration sums

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Classify

Is air a homogeneous or heterogeneous mixture?

▶ Show solution

Air is a homogeneous mixture of gases (uniform composition).

✦ Answer: homogeneous.

Q2EASY· Solution

In a sugar solution, name the solute and the solvent.

▶ Show solution

Solute = sugar (dissolved); solvent = water (dissolving medium).

✦ Answer: sugar (solute), water (solvent).

Q3EASY· Separation

Which method separates sand from water?

▶ Show solution

Sand is insoluble, so use filtration.

✦ Answer: Filtration.

Q4MEDIUM· Concentration

5 g of salt is dissolved in 45 g of water. Find the mass % of salt.

▶ Show solution

Step 1 — mass of solution = 5 + 45 = 50 g.

Step 2 — mass % = (5/50) × 100 = 10%.

✦ Answer: 10%.

Q5MEDIUM· Tyndall

Why is a beam of light visible when it passes through fog but not through clear air?

▶ Show solution

Step 1 — Fog is a colloid; its particles (1–1000 nm) scatter light — the Tyndall effect — making the beam visible.

Step 2 — Clear air is essentially a true solution of gases with particles too small to scatter visible light.

✦ Answer: colloidal fog scatters light (Tyndall); clear air does not.

Q6MEDIUM· Compare

Give two differences between a true solution and a suspension.

▶ Show solution

1) Particle size: true solution < 1 nm; suspension > 1000 nm.

2) Stability: a true solution never settles; a suspension settles on standing and can be filtered.

✦ Answer: particle size and settling/stability.

Q7HARD· Method choice

How would you separate a mixture of salt, sand and iron filings?

▶ Show solution

Step 1 — Use a magnet to remove the iron filings.

Step 2 — Add water and stir: salt dissolves, sand does not. Filter — sand stays on the filter paper.

Step 3 — Evaporate the filtrate (salt water) to recover the salt.

✦ Answer: magnet → dissolve + filter → evaporate.

Q8HARD· Distillation

Why is distillation, not simple evaporation, used to obtain pure water from sea water?

▶ Show solution

Step 1 — Evaporation would lose the water as vapour and leave only the salts.

Step 2 — Distillation boils the sea water AND condenses the steam back to liquid, collecting pure water while salts stay behind.

✦ Answer: distillation recovers the water (by condensing the vapour); evaporation does not.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Pure substance (element/compound) vs mixture (not chemically combined).
•Homogeneous (uniform) vs heterogeneous (non-uniform) mixtures.
•Solution = solute + solvent; mass % = (solute/solution) × 100; saturated = no more dissolves.
•Particle size: true solution (<1 nm) < colloid (1–1000 nm) < suspension (>1000 nm).
•Tyndall effect: colloids scatter light (visible beam); true solutions don't.
•Separation by property: filter (insoluble solid), evaporate (dissolved solid), distil (miscible liquids / recover solvent), separating funnel (immiscible liquids), chromatography (dyes), sublimation, centrifugation.
•Separating a mixture is a physical change (reversible, no new substance).

Rajasthan (RBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 6–8 marks

Question type	Marks each	Typical count	What it tests
MCQ / Assertion–Reason	1	1–2	Classify mixture, Tyndall, solute/solvent
Short answer / numerical	2	2	Mass % concentration; solution vs colloid vs suspension
Short/Long answer	3	1	Multi-component separation; distillation reasoning

Prep strategy

Memorise the 'differing property → method' rule for separations
Practise mass % sums (remember solution = solute + solvent)
Make a 3-row table: true solution / colloid / suspension with size, Tyndall, settling
Prepare the salt+sand+iron separation as a model multi-step answer

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Getting salt from the sea

Coastal salt pans use evaporation to recover common salt from sea water — exactly this chapter's method.

Water purifiers

Filtration and distillation/RO separate impurities from drinking water.

Petroleum refining

Fractional distillation splits crude oil into petrol, diesel and kerosene by boiling point.

Blood tests

Centrifugation separates plasma from blood cells in the lab.

Forensics

Chromatography separates dyes and chemicals to analyse inks and samples.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Name the differing property first, then state the matching separation method.
For concentration sums, compute mass of solution (solute + solvent) before dividing.
Distinguish evaporation vs distillation by whether the vapour is recovered.
Use the size/Tyndall/settling table to answer any solution–colloid–suspension comparison.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Solubility curves and how they predict crystallisation on cooling.
Types of colloids (sol, gel, emulsion, aerosol) and the dispersed phase/medium.
Molarity and molality as more advanced concentration units.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

RBSE Class 9 Annual (BSER Ajmer)Very high — separation methods + a concentration sum each year

NTSE / NMMSMedium — mixture-classification MCQs

JEE/NEET FoundationHigh — base for Class 11 'Some Basic Concepts of Chemistry'

Science Olympiad (NSO)Medium — separation reasoning

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Yes in content. Class 9 (2026-27) uses the new NCF NCERT 'Curiosity' Science book; 'Exploring Mixtures and their Separation' covers pure substances, mixtures, solutions/colloids/suspensions and separation methods. BSER Ajmer sets the RBSE paper.

Colloidal particles are small enough (1–1000 nm) to stay dispersed by constant collisions with the medium's molecules (Brownian motion), so they remain suspended; suspension particles are large and heavy enough to settle under gravity.

A compound's components are chemically combined in a fixed ratio and lose their own properties (water ≠ hydrogen + oxygen); a mixture's components keep their properties and any ratio, and can be separated physically.

To separate two or more dissolved substances (especially coloured ones, like dyes in ink) that move at different speeds on the paper as the solvent rises.

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