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Exploring Mixtures and their Separation

NCERT Class 9 · Science Based on NCERT Class 9 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the classification of mixtures into homogeneous and heterogeneous types, including solutions, suspensions, and colloids. It delves into various separation techniques such as crystallization, distillation, paper chromatography, centrifugation, coagulation, and sublimation, emphasizing their real-life applications in industries, medicine, and daily life. The big idea is that understanding the properties of mixtures—like particle size, solubility, and boiling point—enables us to choose the right method to separate components for practical use.
Key Concepts (28)
Homogeneous mixture
A mixture that has a uniform composition throughout, also called a solution.
e.g. Sugar dissolved in water is equally sweet in every sip.
Heterogeneous mixture
A mixture that is not uniform; components are visible and may settle over time.
e.g. Sand and water mixture where sand particles settle at the bottom.
Solution
A homogeneous mixture of a solute dissolved in a solvent.
e.g. Vinegar (acetic acid in water).
Solute
The substance that gets dissolved in a solvent to form a solution.
e.g. Sugar in sugar-water solution.
Solvent
The substance that dissolves the solute to form a solution.
e.g. Water in sugar-water solution.
Concentration of a solution
The amount of solute dissolved in a given amount of solvent or solution.
e.g. ORS has specific amounts of salt and sugar in water.
Mass by mass percentage (% m/m or % w/w)
The number of grams of solute present in 100 grams of the solution.
e.g. 10 g salt in 90 g water gives 10% m/m solution.
Mass by volume percentage (% m/v or % w/v)
The number of grams of solute present in 100 millilitres of the solution.
e.g. 5 g glucose in 100 mL solution gives 5% m/v glucose solution.
Volume by volume percentage (% v/v)
The number of millilitres of solute present in 100 millilitres of the solution.
e.g. 1 mL pesticide in 100 mL spray gives 1% v/v solution.
Solubility
The maximum amount of solute that dissolves in a fixed quantity of solvent (100 mL or 100 g) at a given temperature.
e.g. Solubility of potassium nitrate increases with temperature.
Saturated solution
A solution that cannot dissolve any more solute at that temperature.
e.g. A solution with excess undissolved solute at the bottom.
Crystallization
The process of forming pure solid crystals from a saturated solution by cooling or evaporation.
e.g. Obtaining copper sulfate crystals from a hot saturated solution.
Distillation
A process to separate a homogeneous mixture of two miscible liquids with a boiling point difference of at least 25°C by heating and condensing vapours.
e.g. Separating acetone (boiling point 56°C) from water (boiling point 100°C).
Fractional distillation
The process of separating components of a mixture with relatively small differences (less than 25°C) in their boiling points.
e.g. Separating crude petroleum into petrol, kerosene, diesel, etc.
Paper chromatography
A method of separating components of a mixture based on differences in their interactions with the solvent and paper.
e.g. Separating different pigments from black ink or spinach leaves.
Immiscible liquids
Liquids that do not mix and form separate layers when combined.
e.g. Oil and water.
Separating funnel
An apparatus used to separate two immiscible liquids based on their different densities.
e.g. Separating mustard oil from water.
Sublimation
The process in which a solid changes directly into vapour without passing through the liquid state.
e.g. Camphor or naphthalene turning into vapour on heating.
Deposition
The process in which vapour changes directly into solid without becoming liquid.
e.g. Camphor vapours depositing on the inner wall of a funnel.
Alloy
A homogeneous mixture of two or more metals, or a metal and a non-metal.
e.g. Brass (80% copper and 20% zinc).
Suspension
A heterogeneous mixture in which solid particles do not dissolve but remain suspended throughout the bulk of the medium; particles are visible to the naked eye and settle over time.
e.g. Muddy water or sand in water.
Centrifugation
A process that uses rapid spinning to separate heavier particles from lighter ones in a mixture.
e.g. Separating red blood cells from plasma in blood samples.
Coagulation
The process of adding a coagulant to make fine suspended particles clump together and settle.
e.g. Adding alum (fitkari) to muddy water to settle impurities.
Colloid
A mixture with particle sizes between 1 nm and 1000 nm, uniformly dispersed, does not settle, and shows Tyndall effect.
e.g. Milk, blood, tomato sauce.
Tyndall effect
The scattering of light by particles in a colloid or suspension, making the path of light visible.
e.g. Laser beam visible through milk-water mixture but not through salt solution.
Dispersed phase
The solute-like component or the dispersed particles in a colloid.
e.g. Oil droplets in milk (oil-in-water emulsion).
Dispersion medium
The component in which the dispersed phase is suspended in a colloid.
e.g. Water in milk (oil-in-water emulsion).
Emulsion
A colloid in which both the dispersed phase and dispersion medium are liquids.
e.g. Milk (oil-in-water), butter (water-in-oil).
Formulas & Equations (3)
Mass by mass percentage = (Mass of solute / Mass of solution) × 100
Reads as: Mass by mass percentage equals mass of solute divided by mass of solution, multiplied by 100
Variables: Mass of solute (g), Mass of solution = Mass of solute + Mass of solvent (g)
Use when: When the concentration needs to be expressed in terms of mass of solute per 100 g of solution, e.g., in packaged foods or spice mixtures.
⚠ Students often use mass of solvent instead of mass of solution in the denominator.
Mass by volume percentage = (Mass of solute / Volume of solution) × 100
Reads as: Mass by volume percentage equals mass of solute divided by volume of solution, multiplied by 100
Variables: Mass of solute (g), Volume of solution (mL)
Use when: When measuring volume of liquid is easier than weighing, e.g., in medicines like 5% glucose solution or 0.9% saline drip.
⚠ Students forget to use volume in mL and use litres without conversion.
Volume by volume percentage = (Volume of solute / Volume of solution) × 100
Reads as: Volume by volume percentage equals volume of solute divided by volume of solution, multiplied by 100
Variables: Volume of solute (mL), Volume of solution (mL)
Use when: When two miscible liquids are mixed, e.g., in perfumes, cosmetics, vinegar (5% v/v acetic acid).
⚠ Students mistakenly use mass instead of volume for both solute and solution.
How It Works
Crystallization
  1. Prepare a saturated solution of the substance by dissolving it in a solvent at a higher temperature.
  2. Filter the hot solution to remove insoluble impurities.
  3. Allow the filtrate to cool slowly without disturbance.
  4. As the solution cools, pure crystals form due to decreased solubility.
  5. Filter the crystals, rinse with cold water, and dry them.
💡 Cooling must be slow to form large, well-shaped crystals; rapid cooling gives small, poorly formed crystals.
Distillation
  1. Place the mixture in a distillation flask and heat it.
  2. The liquid with the lower boiling point vaporises first.
  3. Vapours pass through a condenser where they are cooled by circulating water.
  4. The condensed liquid (distillate) is collected in a separate vessel.
  5. The higher-boiling liquid or solid remains in the distillation flask.
💡 The boiling point difference must be at least 25°C for effective separation; the condenser water enters at the bottom and exits at the top.
Paper chromatography
  1. Draw a pencil line 2 cm from the bottom of a chromatographic paper strip.
  2. Place a spot of the mixture (e.g., black ink) on the pencil line.
  3. Dip the lower end of the paper into a solvent (e.g., water) such that the solvent level is below the spot.
  4. Allow the solvent to rise up the paper by capillary action.
  5. Different components move at different rates, separating into distinct spots.
💡 The solvent level must be below the sample spot; otherwise, the sample will dissolve directly into the solvent instead of separating.
Separation using a separating funnel
  1. Pour the mixture of two immiscible liquids into the separating funnel.
  2. Let it stand undisturbed until two distinct layers form.
  3. Open the stopcock to drain the lower layer into a container.
  4. Close the stopcock when the interface is reached.
  5. Collect the upper layer separately by opening the stopcock again.
💡 The denser liquid forms the lower layer; the stopcock is used to control drainage.
Sublimation
  1. Place the mixture (e.g., camphor and sand) in a china dish.
  2. Cover the dish with an inverted glass funnel plugged with cotton at the nozzle.
  3. Heat the dish gently.
  4. The sublimable substance (camphor) turns into vapour directly.
  5. Vapours condense on the cool inner wall of the funnel as solid deposits.
  6. The non-sublimable component (sand) remains in the dish.
💡 Sublimation occurs below the melting point of the solid; the cotton plug prevents vapours from escaping.
Centrifugation
  1. Place the mixture in tubes and load them into a centrifuge machine.
  2. Spin the tubes at high speed.
  3. Centrifugal force pushes heavier particles outward and downward.
  4. Heavier particles settle at the bottom of the tube.
  5. The lighter liquid remains at the top and can be decanted.
💡 The paperfuge is a hand-powered, low-cost alternative for remote areas.
Coagulation
  1. Add a coagulant (e.g., alum) to the mixture containing fine suspended particles.
  2. The coagulant causes the particles to clump together into larger aggregates.
  3. The larger clumps settle by gravity (sedimentation).
  4. The clear water can be separated by decantation or filtration.
💡 Coagulation is different from filtration; it makes tiny particles clump so they can settle or be filtered easily.
Exam Traps — Don't Make These Mistakes
⚠ Confusing mass of solution with mass of solvent
✗ Mass by mass percentage = (Mass of solute / Mass of solvent) × 100
→ The denominator should be the total mass of the solution (solute + solvent), not just the solvent.
✓ Mass by mass percentage = (Mass of solute / Mass of solution) × 100, where Mass of solution = Mass of solute + Mass of solvent.
⚠ Thinking evaporation and crystallization are the same
✗ Evaporation and crystallization are the same process.
→ Evaporation removes solvent to leave behind solute (often as powder), while crystallization forms well-defined crystals from a saturated solution by controlled cooling or slow evaporation.
✓ Evaporation is used to recover solute by removing solvent, but crystallization yields pure, geometrically shaped crystals and can separate two solids with different solubilities.
⚠ Assuming all mixtures with visible particles are suspensions
✗ Milk is a suspension because it looks cloudy.
→ Milk is a colloid; its particles (1-1000 nm) are not visible to the naked eye and do not settle, unlike suspensions where particles are >1000 nm and settle over time.
✓ Milk is a colloid because it shows Tyndall effect, particles do not settle, and cannot be separated by filtration.
⚠ Misplacing the solvent level in paper chromatography
✗ The solvent level should be above the sample spot.
→ If the solvent level is above the spot, the sample will dissolve directly into the solvent instead of being carried up the paper, ruining the separation.
✓ The solvent level must be below the sample spot so that the solvent rises by capillary action and carries the components up the paper.
⚠ Confusing distillation with fractional distillation
✗ Distillation can separate liquids with any boiling point difference.
→ Simple distillation requires a minimum difference of 25°C; for smaller differences, fractional distillation is needed.
✓ Distillation is used when boiling point difference is at least 25°C; fractional distillation is used for smaller differences (e.g., petroleum refining).
Compare & Contrast
Particle size
Solution
Colloid
Solution particles are less than 1 nm in diameter and are not visible; colloid particles are between 1 nm and 1000 nm and show Tyndall effect.
Stability and settling
Suspension
Colloid
Suspension particles are larger than 1000 nm, visible to naked eye, and settle over time; colloid particles do not settle and remain uniformly dispersed.
Process and outcome
Evaporation
Crystallization
Evaporation removes solvent to leave behind solute as a powder or residue; crystallization forms pure, geometrically shaped crystals from a saturated solution by slow cooling.
Boiling point difference
Distillation
Fractional distillation
Distillation separates liquids with at least 25°C difference in boiling points; fractional distillation separates liquids with smaller differences (less than 25°C) using a fractionating column.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks) on concentration or separation, 1 Long Answer (5 marks) on comparing separation methods or solving concentration problems
Most asked: Classification of mixtures (homogeneous vs heterogeneous) · Concentration of solutions (all three percentage methods) · Tyndall effect and its identification in colloids and suspensions · Separation techniques: crystallization, distillation, sublimation, centrifugation, chromatography
Don't skip: Numerical problems on mass by mass, mass by volume, and volume by volume percentage; Distinguishing between solution, suspension, and colloid with properties and examples; Diagram-based questions on distillation or sublimation setup; Application-based questions: 'How would you separate a mixture of sand, salt, and naphthalene?'
One-Line Revision (read aloud before the exam)
  1. Homogeneous mixtures have uniform composition; heterogeneous mixtures do not.
  2. A solution is a homogeneous mixture of solute and solvent.
  3. Mass by mass percentage = (Mass of solute / Mass of solution) × 100.
  4. Mass by volume percentage = (Mass of solute / Volume of solution in mL) × 100.
  5. Volume by volume percentage = (Volume of solute / Volume of solution) × 100.
  6. Solubility is the maximum amount of solute that dissolves in 100 g or 100 mL of solvent at a given temperature.
  7. A saturated solution cannot dissolve more solute at that temperature.
  8. Crystallization separates pure solids from saturated solutions by cooling.
  9. Distillation separates miscible liquids with boiling point difference ≥ 25°C.
  10. Fractional distillation separates liquids with small boiling point differences.
  11. Paper chromatography separates components based on their movement rates on paper.
  12. Immiscible liquids are separated using a separating funnel based on density difference.
  13. Sublimation is the direct change of a solid to vapour without melting.
  14. Centrifugation uses high-speed spinning to separate heavier particles from lighter ones.
  15. Coagulation uses a coagulant like alum to clump fine particles for settling.
  16. Colloids have particle size 1-1000 nm and show Tyndall effect.
  17. Suspensions have particles >1000 nm, visible to naked eye, and settle over time.
  18. Tyndall effect is the scattering of light by particles in a colloid or suspension.
  19. Milk is a colloid (emulsion) and shows Tyndall effect.
  20. Blood is a colloid; its components can be separated by centrifugation.

MCQs — 10 Questions with Answers

Q1. Which of the following is a homogeneous mixture?

  • A. Sugar dissolved in water ✓
  • B. Sand suspended in water
  • C. Oil and water together
  • D. Chalk powder in water

Answer: A — Sugar in water forms a homogeneous mixture with uniform composition throughout, while sand, chalk, and oil remain visibly separated.

Q2. If 8 g of salt is dissolved in 92 g of water, what is the mass by mass percentage of the solution?

  • A. 8% ✓
  • B. 92%
  • C. 8.7%
  • D. 50%

Answer: A — Total solution mass = 8 + 92 = 100 g; % m/m = (8/100) × 100 = 8%.

Q3. What does the Tyndall effect indicate about a mixture?

  • A. The mixture is a true solution
  • B. The mixture contains colloidal or suspended particles ✓
  • C. The mixture is completely transparent
  • D. The solute has completely dissolved

Answer: B — Tyndall effect (light scattering) only occurs when particles are large enough to scatter light, characteristic of colloids and suspensions, not true solutions.

Q4. Why is it essential to use the correct proportion of salt and sugar when preparing ORS?

  • A. To make the solution taste better
  • B. To ensure the solution effectively rehydrates the body ✓
  • C. To reduce the cost of preparation
  • D. To increase the shelf life of the solution

Answer: B — ORS works only when salt and sugar are in specific proportions; incorrect ratios make it ineffective for treating dehydration.

Q5. A laser pointer beam passes through four containers. The beam is visible only in two containers. Which two are likely heterogeneous or colloidal mixtures?

  • A. Container 1 (salt solution) and Container 2 (sugar solution)
  • B. Container 3 (milk diluted in water) and Container 4 (chalk powder in water) ✓
  • C. Container 1 (vinegar) and Container 2 (aerated drink)
  • D. Container 3 (pure water) and Container 4 (salt solution)

Answer: B — The laser beam becomes visible (Tyndall effect) only when particles are large enough to scatter light; milk (colloid) and chalk (suspension) show this, while solutions do not.

Q6. Which statement about solutions is NOT correct?

  • A. Solutions are always homogeneous mixtures
  • B. In a solution, the solute is always visible to the naked eye ✓
  • C. Solutions have uniform composition throughout
  • D. The solute in a solution dissolves completely in the solvent

Answer: B — Solutions are homogeneous with invisible solutes; saying the solute is always visible contradicts the definition of a true solution.

Q7. If 6 g of glucose is dissolved in water to prepare 150 mL of solution, what is the mass by volume percentage?

  • A. 4% ✓
  • B. 6%
  • C. 2.5%
  • D. 25%

Answer: A — % m/v = (6 g ÷ 150 mL) × 100 = 4%; this differs from % m/m and requires careful unit handling.

Q8. Ramesh observes that when he mixes oil and water vigorously, they separate after a few minutes. What type of mixture is this, and why does separation occur?

  • A. Homogeneous mixture; particles are too small to see
  • B. Heterogeneous mixture; oil and water have different polarities and do not mix ✓
  • C. Colloidal mixture; particles undergo Brownian motion
  • D. Solution; the solute eventually precipitates

Answer: B — Oil and water form a heterogeneous mixture because their different chemical polarities prevent them from mixing uniformly, causing visible separation.

Q9. A farmer needs to spray pesticide on crops. The label says use 5% m/v pesticide solution. What does this mean?

  • A. 5 grams of pesticide in 100 grams of water
  • B. 5 grams of pesticide in 100 mL of total solution ✓
  • C. 5 mL of pesticide in 100 mL of solution
  • D. 5 grams of pesticide mixed with 95 grams of water

Answer: B — % m/v = mass (grams) per volume (mL); 5% m/v means 5 grams solute in 100 mL solution, essential for correct pesticide strength and crop safety.

Q10. Why do suspended particles in muddy water settle to the bottom over time, while particles in milk remain dispersed?

  • A. Muddy water is a suspension with larger particles; milk is a colloid with smaller particles that resist settling due to random molecular motion ✓
  • B. Water is lighter than milk, so particles in milk float
  • C. Mud particles are denser than milk particles
  • D. Muddy water has higher salt concentration that causes settling

Answer: A — Suspensions have larger particles that settle by gravity; colloids like milk have smaller particles suspended by Brownian motion, preventing visible settling.

Flashcards

What is a homogeneous mixture? Give one example.

A mixture with uniform composition throughout where particles are invisible, like sugar dissolved in water or vinegar.

Define heterogeneous mixture with an example.

A mixture with non-uniform composition where particles are visible and may settle, such as sand in water.

What is the Tyndall effect and what does it indicate?

Scattering of light when a laser beam passes through a mixture, indicating the presence of colloidal or suspended particles.

What is the solute and solvent in a salt solution?

Salt is the solute (substance dissolved) and water is the solvent (substance that dissolves the solute).

Define concentration of a solution in simple terms.

The amount of solute dissolved in a given amount of solvent or solution, determining how strong or weak the solution is.

What is mass by mass percentage (% m/m)? When is it used?

It expresses how many grams of solute are in 100 grams of total solution; commonly used for packaged foods and dry mixtures.

What is mass by volume percentage (% m/v)? Give one example.

It expresses how many grams of solute are in 100 mL of solution; used in medicines like 5% glucose solution.

Why is the right proportion important when preparing ORS?

Incorrect proportions of salt and sugar make it ineffective for rehydration; too little won't work and too much can be harmful.

Why do suspended particles settle in muddy water but not in milk?

Muddy water is a suspension with larger particles that settle by gravity, while milk is a colloidal suspension where particles remain dispersed.

What is the formula for mass by volume percentage?

% m/v = (Mass of solute ÷ Volume of solution) × 100, where volume is measured in millilitres.

Important Board Questions

Define homogeneous and heterogeneous mixtures. Give one example of each. [2 marks]

State that homogeneous has uniform composition with invisible particles (e.g., sugar solution), while heterogeneous has non-uniform composition with visible particles (e.g., sand in water); both must be clearly named.

Explain with reasoning why the Tyndall effect is observed in milk but not in a salt solution, even though both appear to be mixtures. [3 marks]

Identify that milk is a colloidal mixture with particles large enough to scatter light, causing Tyndall effect; salt solution is a true solution with dissolved ions too small to scatter light, so no Tyndall effect occurs.

A doctor prescribes a 2% m/v glucose solution for a patient. Explain what this concentration means, calculate how much glucose would be needed to prepare 500 mL of this solution, and justify why precise concentration is critical in medical applications. [5 marks]

Define 2% m/v as 2 grams solute per 100 mL solution; calculate using proportion (2/100 × 500 = 10 g); justify that incorrect concentration in medicine can lead to ineffective treatment or harmful side effects, emphasizing the importance of precision in healthcare.

Assertion-Reason Questions for Exploring Mixtures and their Separation

Q1. Assertion (A):

A homogeneous mixture always remains visually uniform no matter how long it is left undisturbed.

Reason (R):

Homogeneous mixtures have the same composition throughout because solute particles dissolve completely into the solvent at the molecular level.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the assertion is true. The molecular-level dissolution ensures uniform composition, which is why homogeneous mixtures remain visually uniform indefinitely.

Q2. Assertion (A):

Heterogeneous mixtures like chalk powder in water can be effectively separated by filtration.

Reason (R):

Heterogeneous mixtures contain particles that are visible and suspended, which get trapped in filter paper during filtration.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true and the reason correctly explains why filtration works for heterogeneous mixtures. The larger particle size in suspensions allows them to be caught by filter paper while the liquid passes through.

Q3. Assertion (A):

Mass by volume percentage (%m/v) is preferred over mass by mass percentage (%m/m) when preparing medicines and solutions in laboratories.

Reason (R):

Using %m/v is preferred because the solute in medicines is always a liquid, making volume measurement more convenient.

Answer: C — A is true but R is false — The assertion is true—the chapter states %m/v is used in medicines and laboratories. However, the reason is false. It's not because solutes are always liquids; rather, it's because measuring the volume of the final liquid solution is practically easier than weighing both solute and solvent in laboratory settings. Many medicine solutes are solids (like glucose) but are still expressed in %m/v.

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Case-Based Questions (CBQ) for Exploring Mixtures and their Separation

Case Study:

A water treatment plant tests samples of water from different sources. Sample A is clear and transparent with a uniform taste throughout. Sample B appears cloudy with visible sediment that settles at the bottom when left undisturbed. Sample C shows a visible path when a laser beam is directed through it. The plant supervisor notes that Sample A needs only a simple chlorination process, while Samples B and C require filtration before chemical treatment. Based on these observations, the supervisor must classify each sample to determine the appropriate purification method.

Q1. What type of mixture is Sample A most likely to be?

  • A. A heterogeneous mixture with large suspended particles
  • B. A homogeneous solution that is dissolved at the molecular level ✓
  • C. A colloid that shows the Tyndall effect
  • D. A suspension that will settle over time

Answer: B

Q2. Why does Sample B require filtration as the first step in treatment?

  • A. Because it is a homogeneous mixture with dissolved impurities
  • B. Because it is a heterogeneous mixture with visible sediment that can be separated mechanically ✓
  • C. Because all water samples must be filtered before chemical treatment
  • D. Because filtration makes the water transparent

Answer: B

Q3. What does the visible laser path in Sample C indicate about the nature of particles in this water?

  • A. The water contains large dissolved molecules like salt
  • B. The water is a true solution with only molecular-level particles
  • C. The water contains colloidal particles that scatter light (Tyndall effect) ✓
  • D. The water is pure and requires no treatment

Answer: C

Q4. Between Samples B and C, which would be easier to separate using simple filtration and why?

  • A. Sample B, because it has larger visible suspended particles that get trapped in filter paper ✓
  • B. Sample C, because its colloidal particles are easier to filter
  • C. Both equally, because they both show visible particles
  • D. Neither, because both are homogeneous mixtures

Answer: A

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