Exploring Mixtures and their Separation – Class 9 Science Chapter 5 Notes & NCERT PDF

Class 9ScienceChapter 5NCERT book: Exploration - Science Class 9

Notes and a simple summary of Chapter 5 of the NCERT Class 9 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 5: Exploring Mixtures and their Separation

Think It Over

Why do particles settle in muddy water over time but not in milk? How is evaporation different from boiling? Why do you see bright rays of sunlight through gaps between the leaves of a dense tree?

Introduction

How do we get white sugar crystals from green sugarcane, or detect malaria from a few drops of blood? Both depend on the science of separating mixtures. This chapter classifies mixtures, expresses how concentrated a solution is, and explores separation techniques for homogeneous and heterogeneous mixtures.

Key Concepts

1. Classifying mixtures

Homogeneous mixture (solution): uniform composition throughout — sugar in water, vinegar (acetic acid in water), soda (CO₂ in water).

Heterogeneous mixture: non-uniform — sand in water, oil and water.

Activity 5.1: salt + water (A), chalk + water (B), a few drops of milk + water (C) are tested for visible particles, a laser beam's path and filtration — they turn out to be three different types of mixture. Safety: never look directly into a laser beam.

2. Concentration of a solution

Concentration is the amount of solute dissolved in a given amount of solvent or solution.

The right proportion matters: ORS needs fixed amounts of salt and sugar; too much pesticide harms crops and soil, too little does not protect them.

Meet a Scientist: Dilip Mahalanabis, an Indian paediatrician, first developed and implemented the ORS treatment for dehydration from diarrhoea and cholera; it has saved millions of lives.

Three percentage methods:

- % m/m (mass by mass) = mass of solute ÷ mass of solution × 100 — used for solutions and for milk powder, spice mixes, food labels. (% w/w used in industry is numerically the same.)

- % m/v (mass by volume) = mass of solute (g) ÷ volume of solution (mL) × 100 — medicines, e.g. 5% glucose, 0.9% saline drip (0.9 g salt in 100 mL).

- % v/v (volume by volume) = volume of solute ÷ volume of solution × 100 — two miscible liquids, e.g. perfumes, vinegar (5% v/v acetic acid).

3. Solubility

Solubility = maximum amount of solute that dissolves in 100 g (or 100 mL) of solvent at a given temperature; such a solution is saturated.

Solids generally dissolve more at higher temperature; gases dissolve less as temperature rises.

A graph of solubility vs temperature is a solubility curve (Activity 5.2, compounds A and B).

4. Separating homogeneous mixtures

Crystallization: cool a hot saturated solution; excess solute separates as pure crystals (particles in a regular geometric pattern). Book example: compound B — 287 g dissolves in 100 g water at 60 °C but only 241 g at 40 °C, so 46 g crystallizes on cooling. Used to purify solids; based on solubility changing with temperature. Examples: rock salt, mishri, snowflakes, frost. Traditional Indian salts: panga (boiling brine) and karkatch (evaporating seawater).

Distillation: heat a mixture of two miscible liquids whose boiling points differ by at least about 25 °C; the lower-boiling liquid vaporises, is cooled in a condenser and collected pure. Acetone (≈56 °C) and water (100 °C) separate this way. Also recovers a liquid from a solution of a solid. Kannauj's Mitti ka Ittar is made by the traditional Deg-Bhapka distillation method.

Fractional distillation (difference less than 25 °C) separates crude oil into petroleum gas, petrol, kerosene, diesel, lubricating oil and bitumen; the gas becomes LPG.

Paper chromatography ("writing with colour"): a pencil line 2 cm from the bottom of the paper, an ink spot on it, the solvent level below the spot. The solvent rises and separates the ink into colours because components interact differently with the solvent and paper.

5. Separating heterogeneous mixtures

Separating funnel for immiscible liquids (Activity 5.6): mustard oil (upper layer) and water (lower layer) form two layers; open the stopcock to drain water, discard the mixed middle part, then collect oil.

Sublimation (Activity 5.7): camphor turns directly from solid to vapour (below its melting point) and deposits on the cool funnel — deposition — leaving sand behind. Naphthalene and dry ice also sublime.

Alloys (homogeneous mixtures of metals, or metal + non-metal) cannot be separated by physical methods: brass ≈ 80% Cu + 20% Zn, bronze ≈ 80% Cu + 20% Sn, stainless steel = iron with chromium, nickel, carbon, molybdenum.

Suspensions: heterogeneous mixtures whose particles are visible, settle and can be filtered (sand in water, sawdust in water).

- Centrifugation: fast spinning pushes heavier particles outward/to the bottom; separates blood into plasma, platelets & WBCs, and RBCs. The hand-powered paperfuge does this without electricity to help detect malaria and anaemia.

- Coagulation: alum (fitkari) makes fine particles clump and settle (sedimentation), then decant/filter. Making paneer with lemon juice or vinegar is also coagulation.

6. Colloids and the Tyndall effect

Particle size decides the type: solution < 1 nm; colloid 1–1000 nm; suspension > 1000 nm.

Colloid particles do not settle and look uniform, but they scatter light — e.g. blood, milk, tomato sauce, ice cream.

Tyndall effect: scattering of light by particles (colloids and suspensions), first explained by John Tyndall — seen in sunbeams through a small hole, stadium floodlights.

A colloid has a dispersed phase and a dispersion medium. Emulsions have both as liquids: oil-in-water (milk, vanishing cream) and water-in-oil (butter, cold cream); proteins act as emulsifying agents.

Worked Numericals

% m/m: 10 g salt in 90 g water → 10 ÷ 100 × 100 = 10% m/m.

% m/v: 5 g glucose in 100 mL solution → 5% m/v.

Talcum powder: 4% m/m zinc oxide in 300 g → 0.04 × 300 = 12 g ZnO.

Orange juice: 2 tablespoons × 15 mL = 30 mL in 150 mL → 20% v/v.

Three students (exercise): A: 20 g in 80 g water → 20/100 = 20%; B: 20 g in 100 g → 20/120 = 16.67%; C: 30 g in 80 g → 30/110 = 27.27% (most concentrated).

Brass: 70% copper in 120 g → 84 g Cu and 36 g Zn.

KNO₃ (Table 5.4): 62 g dissolves in 100 g water at 40 °C, so 31 g saturates 50 g water.

Activities in the Chapter

5.1 Three mixtures tested with a laser and filter paper.

5.2 Reading solubility curves of compounds A and B.

5.3 Growing copper sulfate crystals: saturate with a drop of dilute sulfuric acid (added by teacher) in a water bath, filter hot, cool slowly, rinse and dry. Copper sulfate is toxic — don't touch with bare hands.

5.4 Describing salt manufacture from seawater.

5.5 Paper chromatography of black sketch-pen ink (also green food colour with 2% m/v salt solution, spinach pigments).

5.6 Separating mustard oil and water with a separating funnel.

5.7 Sublimation of camphor from sand.

5.8 Making a cardboard-and-thread mini centrifuge.

5.9 Comparing solutions, suspensions and colloids.

Key Terms

Homogeneous mixture (समांगी मिश्रण)

Heterogeneous mixture (विषमांगी मिश्रण)

Solute / Solvent (विलेय / विलायक)

Concentration (सांद्रता)

Solubility (विलेयता)

Saturated solution (संतृप्त विलयन)

Crystallization (क्रिस्टलीकरण)

Distillation (आसवन)

Chromatography (वर्णलेखन)

Sublimation (ऊर्ध्वपातन)

Centrifugation (अपकेंद्रण)

Coagulation (स्कंदन)

Suspension (निलंबन)

Colloid (कोलॉइड)

Alloy (मिश्रधातु)

Tyndall effect (टिंडल प्रभाव)

Real-life Connections

ORS, 0.9% saline, and 5% glucose drips all depend on the right concentration.

Blood donation: donated blood is centrifuged into plasma, platelets, WBCs and RBCs and stored in blood banks.

Sewage treatment uses sedimentation, coagulation and filtration; waste segregation separates dry (recyclable) and wet (compostable) waste; researchers recover lithium from old batteries.

Hazy skies in smoky, dusty cities are the Tyndall effect.

Common Misconceptions

"Evaporation and crystallization are the same." — Crystallization cools a saturated solution to get pure crystals; evaporation simply removes the solvent.

"Distillation works for any two liquids." — The boiling points must differ by at least about 25 °C; otherwise use fractional distillation.

"In chromatography the solvent should cover the spot." — The solvent level must be below the spot, or the ink dissolves away into the solvent.

"Milk is a solution because it looks uniform." — Milk is a colloid; it shows the Tyndall effect.

"Solutions scatter light." — Solution particles (< 1 nm) are too small; solutions do not show the Tyndall effect.

Revise, Reflect, Refine — Hints

Choose a method by the property that differs: boiling point → distillation; solubility with temperature → crystallization; density/immiscibility → separating funnel; sublimation → camphor/naphthalene; particle size → filtration or centrifugation.

For sand + salt + naphthalene: sublime naphthalene, dissolve salt in water and filter sand, then evaporate/crystallize salt.

Liquids with boiling points 60 °C and 90 °C differ by 30 °C → simple distillation works.

💡 Key Learning Points

✓Classify mixtures as homogeneous/heterogeneous and as solutions, colloids or suspensions using particle size, settling, filtration and the Tyndall effect.
✓Express concentration as % m/m, % m/v and % v/v and solve simple concentration problems.
✓Explain solubility, saturated solutions and how temperature affects solubility of solids and gases.
✓Choose and describe separation methods — crystallization, distillation, chromatography, separating funnel, sublimation, centrifugation, coagulation — based on the property that differs.
✓Connect separation science to ORS, blood donation, sewage treatment, perfumes (Deg-Bhapka) and crude-oil refining.

👨‍🏫 Teaching Tips

→Run Activity 5.1 as a demonstration with a laser pointer (teacher-held, never into eyes) through salt water, chalk water and diluted milk.
→Bring real packaged products (milk powder, glucose bottle, vinegar) and let students read and compare the % m/m, % m/v and % v/v labels.
→Grow copper sulfate (or common salt) crystals and compare slow vs fast cooling as a 'Think as a Scientist' investigation.
→Do black sketch-pen chromatography on filter paper strips in beakers — cheap, quick and colourful.
→Use the phugadi/kikli spinning game to introduce centrifugation before showing the paperfuge.
→Make paneer with lemon juice as a kitchen example of coagulation.
→End with a 'which method?' card game: each card names a mixture and students pick the method and the property it uses.

📋 Assessment Questions

Can the student classify given mixtures as solution, colloid or suspension and justify using particle size and the Tyndall effect?
Can the student calculate % m/m, % m/v and % v/v, including mass of solution = solute + solvent?
Can the student read a solubility table/curve and predict the mass that crystallizes on cooling?
Can the student select a separation method for a given mixture and explain the property it relies on?
Can the student describe the correct set-up for distillation, chromatography and a separating funnel?

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