Class 9 Science – Exploring Mixtures and their Separation

📝 Section 1 — Classroom Notes

Look around you right now. Your air is a mixture of gases. Your orange juice is a mixture of water, sugar, and fruit compounds. The soil outside is a mixture of minerals, water, and organic matter. In fact, almost everything we encounter in daily life is a mixture — a combination of two or more pure substances that are not chemically joined together and can be separated.

Have you ever wondered how doctors separate blood cells from plasma, or how chemists get pure salt from seawater, or how perfume makers collect the scent from flowers? Each method uses a clever trick based on the physical differences between substances — their boiling points, densities, particle sizes, or how they dissolve. Understanding these differences gives us powerful tools to purify and separate materials in industry, medicine, and everyday life.

In this chapter, we explore the three types of mixtures — solutions, colloids, and suspensions — learn how to calculate concentration, and discover fascinating separation techniques like distillation, chromatography, sublimation, and centrifugation. By the end, you will understand why salt water tastes salty even without visible salt crystals, and how a scientist can separate the colours in a single black ink with just paper and water!

🌳 Topic 1: Types of Mixtures

A mixture contains two or more substances mixed together but not chemically joined.
Homogeneous mixture – looks the same throughout (uniform). Example: sugar dissolved in tea.
Heterogeneous mixture – looks different in different parts (non-uniform). Example: vegetable soup with visible pieces.
Homogeneous mixtures are also called solutions.
The substance that dissolves is the solute; the substance that does the dissolving is the solvent.

⚖ Topic 2: Types of Mixtures – Three Categories

Solution – particles < 1 nm (too tiny to see). Example: salt water, vinegar.
Colloid – particles 1–1000 nm (not visible, do not settle). Example: milk, fog, butter.
Suspension – particles > 1000 nm (visible, settle on standing). Example: muddy water, chalk in water.

📈 Topic 3: Concentration of a Solution

Concentration tells us how much solute is in a given amount of solution.

A. Mass by Mass Percentage (% m/m)

Used for solid-in-solid or solid-in-liquid mixtures (e.g., packaged food labels).
Formula: (Mass of solute ÷ Mass of solution) × 100

B. Mass by Volume Percentage (% m/v)

Used in medicines and laboratories.
Formula: (Mass of solute in grams ÷ Volume of solution in mL) × 100

C. Volume by Volume Percentage (% v/v)

Used when two liquids are mixed (perfumes, cosmetics).
Formula: (Volume of solute ÷ Volume of solution) × 100

❄ Topic 4: Solubility

Solubility – the maximum amount of solute that dissolves in 100 g (or 100 mL) of solvent at a given temperature.
A saturated solution cannot dissolve any more solute at that temperature.
For most solids in liquids: solubility increases as temperature rises.
For gases in liquids: solubility decreases as temperature rises (fizzy drinks go flat when warm).

💦 Topic 5: Crystallization

Crystallization – forming pure solid crystals from a saturated solution by cooling it slowly.
When a hot saturated solution cools, the extra solute cannot stay dissolved and forms crystals.
Used to purify impure solids (removes impurities that stay dissolved).
Examples of natural crystals: rock salt, snowflakes, quartz.
Cooling slowly gives larger, purer crystals. Cooling quickly gives small, rough crystals.

💧 Topic 6: Distillation

Distillation – separating two miscible liquids by heating, vaporising the lower-boiling liquid, then cooling to collect it.
Works when boiling points differ by at least 25 °C.
Also separates a dissolved solid from its liquid solvent (recover the solvent).
Fractional distillation – used when boiling points are close (< 25 °C apart), e.g., separating crude oil products at petroleum refineries.

🎨 Topic 7: Paper Chromatography

Chromatography – separating the components of a mixture based on how fast they move through paper with a solvent.
The word comes from Greek: chroma (colour) + graphein (to write).
Each component travels at a different speed → they separate into bands of colour.
Used to separate dyes, inks, plant pigments, and identify unknown substances.

🍼 Topic 8: Separating Immiscible Liquids

Immiscible liquids do not mix (e.g., oil and water) and form separate layers.
A separating funnel is used – the denser liquid drains out first through the stopcock.
The less dense liquid (e.g., oil) stays on top; the denser liquid (e.g., water) sinks below.

💥 Topic 9: Sublimation

Sublimation – a solid turns directly into vapour without melting into liquid first.
Deposition – vapour turns directly back into solid (reverse of sublimation).
Used when one substance in a mixture sublimes and the other does not.
Examples of substances that sublime: camphor, naphthalene, dry ice (solid CO₂).

🔄 Topic 10: Centrifugation and Coagulation

Centrifugation – spinning a mixture at high speed so heavier particles move outward and settle, while lighter liquid stays on top.
Used to separate blood into plasma and red blood cells; also used in dairies.
Coagulation – adding a substance (coagulant) that causes tiny suspended particles to clump together, making them heavy enough to settle.
Example: adding alum (fitkari) to muddy water causes mud particles to clump and settle.
Making paneer from milk is also coagulation (lemon juice causes milk proteins to clump).

💡 Topic 11: Tyndall Effect

Tyndall Effect – the scattering of a beam of light by the particles in a colloid or suspension, making the path of light visible.
Does not occur in a true solution (particles too small to scatter light).
Examples: light beam visible in a foggy room; floodlights in a stadium; sunlight through leaves in a dense forest.
Named after scientist John Tyndall.

🔨 Section 3 — Worked Examples

📈 Example 1: Mass by Mass Percentage

Question: 18 g of coffee powder is dissolved in 162 g of hot water. Calculate the mass by mass percentage of coffee in the solution.

Identify: Mass of solute (coffee) = 18 g, Mass of solvent (water) = 162 g

Total mass of solution = 18 + 162 = 180 g

Apply formula: % m/m = (Mass of solute ÷ Mass of solution) × 100

% m/m = (18 ÷ 180) × 100 = 10% m/m

✅ Answer: The coffee solution is 10% m/m

📈 Example 2: Mass by Volume Percentage

Question: A student dissolves 12 g of salt in enough water to make 300 mL of solution. Calculate the mass by volume percentage.

Mass of solute (salt) = 12 g, Volume of solution = 300 mL

% m/v = (Mass of solute ÷ Volume of solution) × 100

% m/v = (12 ÷ 300) × 100 = 4% m/v

✅ Answer: The salt solution is 4% m/v (4 g of salt per 100 mL of solution)

📈 Example 3: Volume by Volume Percentage

Question: A flavouring syrup (25 mL) is added to water to make 500 mL of a drink. Calculate the % v/v concentration.

Volume of solute (syrup) = 25 mL, Total volume of solution = 500 mL

% v/v = (Volume of solute ÷ Volume of solution) × 100

% v/v = (25 ÷ 500) × 100 = 5% v/v

✅ Answer: The drink is 5% v/v flavouring syrup

🍶 Example 4: Choosing the Right Separation Method

Question: You have a mixture of sand, table salt, and naphthalene (mothballs). How would you separate all three?

Sublimation first – heat gently: naphthalene sublimes and deposits on the cool surface above. Sand and salt remain.

Add water – salt dissolves in water; sand does not. Filter: sand stays on filter paper, salt solution passes through.

Evaporate / crystallize – heat the salt solution; water evaporates (or cool slowly to get pure salt crystals).

✅ Result: naphthalene (sublimation) + sand (filtration) + salt (crystallization/evaporation)

📋 Section 4 — Cheat Sheet

✅ Quick Formulas

% m/m = (Mass of solute ÷ Mass of solution) × 100

% m/v = (Mass of solute in g ÷ Volume of solution in mL) × 100

% v/v = (Volume of solute ÷ Volume of solution) × 100

📄 Separation Methods at a Glance

Method	What it separates	Key condition / tool
Crystallization	Solid from its solution	Cool a hot saturated solution slowly
Distillation	Two miscible liquids (or solid + liquid)	Boiling points differ by ≥ 25 °C; condenser cools vapour
Fractional distillation	Liquids with close boiling points	Fractionating column; used in petroleum refining
Paper chromatography	Dissolved pigments/dyes	Different rates of movement on paper with solvent
Separating funnel	Two immiscible liquids	Different densities; denser sinks to bottom
Sublimation	A substance that sublimes from one that does not	Heat gently; e.g., camphor + sand
Centrifugation	Heavier solid particles from liquid	High-speed spinning; e.g., blood components
Coagulation	Tiny suspended particles	Coagulant added (e.g., alum for muddy water)
Filtration	Insoluble solid from liquid	Filter paper or cloth; suspension-sized particles

🔭 Solution / Colloid / Suspension Comparison

Property	Solution	Colloid	Suspension
Particle size	< 1 nm	1 – 1000 nm	> 1000 nm
Appearance	Clear, transparent	May look clear or slightly milky	Cloudy / opaque
Particles visible?	No	No (too small)	Yes (naked eye)
Settles on standing?	No	No	Yes
Tyndall Effect?	No	Yes	Yes
Filtered by paper?	No (passes through)	No (passes through)	Yes
Examples	Salt water, vinegar	Milk, blood, fog, butter	Muddy water, chalk in water

💡 Key Vocabulary

Solute
The substance that dissolves

Solvent
The substance doing the dissolving

Saturated solution
Cannot dissolve any more solute at that temperature

Crystal
Solid with particles in a regular geometric pattern

Sublimation
Solid → Vapour (no liquid stage)

Tyndall Effect
Scattering of light by colloid/suspension particles

🏠 Section 5 — Take-Home Handout

Name: ___________________________ Date: ___________ Class: ________

Part A — Fill in the Blanks

A mixture that looks the same throughout is called a _________________ mixture.
The substance that dissolves in a solution is called the _________________.
% m/m = (Mass of solute ÷ Mass of _________________) × 100.
Distillation works when the boiling points of two liquids differ by at least _________________ °C.
In sublimation, a solid changes directly into _________________ without melting.
The scattering of light by a colloid is called the _________________ effect.
Adding alum to muddy water causes the mud particles to clump together — this process is called _________________.
A solution that cannot dissolve any more solute is called a _________________ solution.

Part B — True or False

Colloid particles settle to the bottom when left undisturbed. True / False
Distillation can separate two liquids with the same boiling point. True / False
A true solution shows the Tyndall effect. True / False
In paper chromatography, different components travel at different speeds. True / False
In a separating funnel, the less dense liquid forms the upper layer. True / False

Part C — Solve It

Problem 1

A gardener dissolves 24 g of fertiliser in 576 g of water. Calculate the mass by mass percentage (% m/m) of the fertiliser solution.

Problem 2

A nurse prepares a 6% m/v antiseptic solution. She needs 250 mL of it. How many grams of antiseptic must she use?

Part D — Match the Method

Mixture	Best Method
Oil and water	A. Distillation
Camphor and sand	B. Separating funnel
Ethanol and water (boiling points 78 °C and 100 °C)	C. Sublimation
Red blood cells and plasma	D. Paper chromatography
Pigments in a leaf extract	E. Centrifugation

✅ Detailed Solutions — Section 5 Take-Home Handout

Part A — Fill in the Blanks (Answers)

A mixture that looks the same throughout is called a homogeneous mixture (also called a solution).
The substance that dissolves in a solution is called the solute. (The liquid that does the dissolving is the solvent.)
% m/m = (Mass of solute ÷ Mass of solution) × 100. (Note: solution = solute + solvent.)
Distillation works when the boiling points of two liquids differ by at least 25 °C.
In sublimation, a solid changes directly into vapour (gas) without melting into liquid first. Example: camphor, dry ice.
The scattering of light by a colloid is called the Tyndall effect.
Adding alum causes mud particles to clump together — this process is called coagulation (or flocculation).
A solution that cannot dissolve any more solute is called a saturated solution.

Part B — True or False (Answers)

False. Colloid particles (size 1–1000 nm) do NOT settle. Only suspension particles (>1000 nm) settle on standing.
False. Distillation cannot separate liquids with the same boiling point — they would boil together as one.
False. A true solution does NOT show the Tyndall effect. Only colloids and suspensions scatter light visibly.
True. In paper chromatography, different components have different affinities for the paper and the solvent, so they travel at different speeds and separate.
True. In a separating funnel, the less dense (lighter) liquid floats on top, forming the upper layer. The denser liquid sinks to the bottom and drains first.

Part C — Solve (Step-by-Step)

Problem 1: 24 g fertiliser in 576 g water. Find % m/m.

1Formula: % m/m = (Mass of solute ÷ Mass of solution) × 100

2Mass of solution = mass of solute + mass of solvent = 24 + 576 = 600 g

3% m/m = (24 ÷ 600) × 100 = 0.04 × 100 = 4%

✔ Answer: The fertiliser solution is 4% m/m.

Problem 2: 6% m/v antiseptic solution, 250 mL needed. How many grams?

1Formula: % m/v = (Mass of solute in g ÷ Volume of solution in mL) × 100

2Rearrange: Mass of solute = (% m/v × Volume) ÷ 100

3= (6 × 250) ÷ 100 = 1500 ÷ 100 = 15 g

✔ Answer: The nurse needs 15 grams of antiseptic.

Part D — Match the Method (Answers)

Oil and water → B. Separating funnel (oil floats on water, different densities, immiscible)
Camphor and sand → C. Sublimation (camphor turns directly to vapour, sand stays behind)
Ethanol and water (78°C and 100°C) → A. Distillation (boiling points differ by 22°C — close to 25°C, use fractional distillation)
Red blood cells and plasma → E. Centrifugation (spinning separates denser cells from liquid plasma)
Pigments in a leaf extract → D. Paper chromatography (different pigments travel at different speeds on paper)

🃏 Section 6 — Revision Flashcards

Use these for quick oral revision or pair quizzing.

Q: What is a homogeneous mixture?
A mixture that looks the same throughout (uniform composition) is called a homogeneous mixture or solution.

Q: What is the Tyndall effect?
Scattering of a beam of light by particles in a colloid or suspension, making the light path visible.

Q: What is the difference between colloid and suspension?
Colloid particles (1–1000 nm) do NOT settle; suspension particles (>1000 nm) DO settle on standing.

Q: Why does solubility decrease when a gas is heated?
Gas molecules gain energy and escape from the liquid faster; that is why fizzy drinks go flat in hot weather.

Q: When is distillation preferred over evaporation?
When we want to recover the solvent (liquid) too, not just the solute — distillation collects both.

Q: What happens in crystallization?
A hot saturated solution is cooled slowly; extra dissolved solid comes out as pure crystals.

Q: What substance is a coagulant in water purification?
Alum (fitkari) — it causes fine mud particles to clump together and settle by gravity.

Q: What is sublimation?
A solid changing directly into vapour without passing through the liquid state. Example: camphor, dry ice.

Q: What is fractional distillation used for?
Separating liquids whose boiling points are close together (less than 25 °C apart); e.g., crude oil refining.

Q: How does a separating funnel work?
Two immiscible liquids form layers by density; the denser (heavier) liquid sinks and drains through the stopcock first.

⚖ Class 9 Science – Exploring Mixtures and their SeparationOriginal Visual Revision Notes Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.

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