Chapter 2 · Class 9 · Chemistry · CBSE/NCERT
Is Matter Around Us Pure?
From the salt produced in Gujarat's coastal pans to the fog over Delhi on a winter morning, chemistry of mixtures surrounds us. This chapter covers pure substances, elements, compounds, solutions, suspensions, colloids, concentration, and all major separation methods.
Pure SubstancesElementsCompoundsMixturesSolutionsColloidsSeparation
Complete Chapter Roadmap — 15 Topics
01Pure Substances02Elements03Compounds04Mixtures05Homogeneous Mixtures06Heterogeneous Mixtures07Solutions08Suspensions09Colloids and Tyndall Effect10Concentration of Solution11Separation Methods — Overview12Evaporation and Crystallisation13Distillation and Fractional Distillation14Centrifugation, Chromatography, Sublimation15Chapter Summary
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Pure Substances
Fixed composition and uniform properties throughout
Pure Substance vs Mixture
A pure substance has fixed composition and fixed properties. A mixture can have variable composition and the properties may vary accordingly.
A pure substance is one that contains only one kind of constituent particles and has a fixed, definite composition. Its properties are uniform throughout the sample.
Distilled water is a pure substance — every sample of pure water has the same boiling point (100 °C), the same freezing point (0 °C), and the same density. These properties do not vary because the composition is fixed.
In everyday life, very few substances are truly pure. Tap water, milk, sea water, and the air we breathe are all mixtures. Pure substances must be obtained through chemical or industrial processes.
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Elements
The simplest pure substances — made of only one kind of atom
An element is a pure substance made of only one type of atom. Elements cannot be broken down into simpler substances by chemical means.
There are 118 known elements. Examples: iron (Fe) used in bridges and buildings, gold (Au) in jewellery shops in Chandni Chowk, copper (Cu) in electrical wires, oxygen (O) in the air, carbon (C) in graphite pencil leads and diamonds.
Elements are broadly classified as metals (iron, copper, aluminium, gold), non-metals (oxygen, hydrogen, nitrogen, carbon, sulphur), and metalloids (silicon, germanium — with properties of both).
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Compounds
Elements chemically combined in fixed ratios
Compound vs Mixture
Compound: fixed ratio, new properties, separated only by chemical methods.
Mixture: variable ratio, components retain their properties, separated by physical methods.
Mixture: variable ratio, components retain their properties, separated by physical methods.
A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. The compound has properties entirely different from those of its constituent elements.
Water (H₂O) is a compound of hydrogen and oxygen. Hydrogen is a flammable gas; oxygen supports burning. Yet water is used to put out fires. The combination creates a substance with entirely new properties.
Common salt (NaCl) is a compound of sodium (a reactive metal) and chlorine (a poisonous gas). Yet their compound is the table salt sprinkled on Indian food. The components of a compound can be separated only by chemical methods, not by physical ones.
Quick Check
Is air a compound or a mixture?
Answer: Air is a mixture. Its composition is not fixed (it varies with location and altitude), its components retain individual properties, and it can be separated by physical methods (e.g., fractional distillation of liquid air).
Answer: Air is a mixture. Its composition is not fixed (it varies with location and altitude), its components retain individual properties, and it can be separated by physical methods (e.g., fractional distillation of liquid air).
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Mixtures
Two or more substances physically combined — variable composition
A mixture is formed when two or more substances are physically combined without any chemical change. The components of a mixture retain their individual properties and can be separated by physical methods.
Air is a mixture of nitrogen, oxygen, argon, carbon dioxide, and other gases. Sea water is a mixture of water and many dissolved salts. Dal (lentil soup) is a complex mixture of water, lentil particles, spices, and other ingredients.
Unlike compounds, mixtures do not have a fixed composition. You can make a dilute or concentrated nimbu pani simply by adding more or less sugar — the proportions are variable.
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Homogeneous Mixtures
Uniform composition throughout — no visible boundary between components
A homogeneous mixture has a uniform composition throughout. You cannot see a boundary between the components, and any portion of the mixture has the same composition as any other portion.
Salt solution: stir a spoonful of common salt into a glass of water until it dissolves completely. Every sip from this glass will taste equally salty — the salt and water are uniformly mixed.
Other examples: sugar in water (sharbat), alcohol in water, alloys such as brass (copper + zinc) and bronze (copper + tin). Alloys look uniform throughout and are widely used in Indian coinage and utensils.
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Heterogeneous Mixtures
Non-uniform composition — boundaries between components are visible
A heterogeneous mixture does not have uniform composition. Different parts of the mixture have different compositions, and the components are often visible as distinct regions.
Muddy Ganga water is heterogeneous — you can see suspended soil, sand, and other particles. A mixture of rice and dal before cooking is heterogeneous — you can clearly see the separate components.
Oil and water do not mix — they form two distinct layers. This is a classic heterogeneous liquid mixture. Salad dressing separates into an oil layer and a vinegar-water layer if not shaken.
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Solutions
True homogeneous mixtures with very small solute particles
Properties of a True Solution
✓ Homogeneous | ✓ Particle size < 1 nm | ✓ Stable (does not settle) | ✓ Cannot be separated by filtration | ✗ No Tyndall effect
A solution is a homogeneous mixture of a solute dissolved in a solvent. The solute particles are smaller than 1 nm in diameter, too small to scatter light or settle on standing.
The most familiar solvent is water. Salt dissolved in water, sugar dissolved in water, and glucose in water are all aqueous solutions. The solute and solvent can be any state — air is a gaseous solution, brass is a solid solution.
Solutions cannot be separated by filtration because the solute particles pass through even the finest filter paper. They do not show the Tyndall effect.
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Suspensions
Heterogeneous mixtures with large, visible particles that settle
Properties of a Suspension
✓ Heterogeneous | ✓ Particle size > 100 nm | ✓ Particles settle on standing | ✓ Can be separated by filtration | ✓ Scatters light
A suspension is a heterogeneous mixture in which the solute particles are larger than 100 nm. These particles are visible to the naked eye or at least under a microscope and settle down on standing.
Chalk powder stirred in water is a suspension — the particles are visible, the mixture looks cloudy, and on standing the chalk settles to the bottom. Muddy river water is another everyday Indian example.
Suspensions can be separated by filtration. The particles are large enough to be retained on filter paper.
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Colloids and the Tyndall Effect
Intermediate particle size — stable, scatters light
Tyndall Effect
Colloids scatter a beam of light (Tyndall effect). Solutions do not. This is the simplest test to distinguish a colloid from a true solution.
A colloid is a mixture with particle sizes between 1 nm and 100 nm — intermediate between a solution and a suspension. Colloidal particles are too small to settle on standing, but they are large enough to scatter light.
Milk is a colloid — tiny fat droplets are dispersed in water. Curd, face creams, butter, fog, smoke, and cloud are all colloids. The blue colour of the sky is partly due to scattering of light by colloidal-sized dust and gas particles in the atmosphere.
The Tyndall effect is the scattering of a beam of light by colloidal particles. When a beam of sunlight enters a dusty room through a crack, you can see the beam because the dust particles scatter the light. Solutions do not show this effect because their particles are too small.
Real-Life Example
Fog on a Delhi winter morning
On a foggy winter morning in Delhi, the headlights of vehicles are visible as beams in the air. This is the Tyndall effect — the tiny water droplets in the fog scatter the light of the headlamps, making the beam visible. Clear water does not do this.
How to distinguish a solution from a colloid in the lab
Pass a beam of light through both samples in a dark room.
The colloid (e.g., milk) will scatter the light beam — you will see the beam path clearly inside the liquid. This is the Tyndall effect.
The solution (e.g., salt water) will not scatter the light — the beam is invisible inside the liquid.
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Concentration of a Solution
How much solute is dissolved in a given amount of solvent or solution
Formula
Concentration tells us how much solute is dissolved in a given amount of solution or solvent. A concentrated nimbu pani has more sugar and lemon dissolved in the same volume of water than a dilute one.
Mass percent is the simplest way to express concentration. It tells what percentage of the total mass of the solution is due to the solute.
Solved Example
Finding mass percent
15 g of sugar is dissolved in 85 g of water. Find the mass percent of sugar.
Mass of solution = 15 + 85 = 100 g.
Mass of solution = 15 + 85 = 100 g.
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Separation Methods — Overview
Choose the method based on the property difference between components
Method → Property Used
Filtration: soluble vs insoluble.
Evaporation: solid solute from volatile solvent.
Distillation: different boiling points.
Centrifugation: different densities.
Sublimation: one component sublimes.
Chromatography: different solubilities in moving solvent.
Magnetic separation: one component is magnetic.
Evaporation: solid solute from volatile solvent.
Distillation: different boiling points.
Centrifugation: different densities.
Sublimation: one component sublimes.
Chromatography: different solubilities in moving solvent.
Magnetic separation: one component is magnetic.
The right separation method depends on what property differs between the components. Understanding this reason makes all separation methods easy to remember and apply.
Key questions to ask: Is one component insoluble? Is one magnetic? Do they have different boiling points? Different densities? Can one sublime? This logical approach avoids rote memorisation.
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Evaporation and Crystallisation
Getting salt or crystals from a solution
Evaporation is used when the solid solute is needed and the solvent can simply be evaporated away. In salt pans along the coasts of Gujarat and Tamil Nadu, sea water is allowed to evaporate under the sun, leaving behind common salt.
Crystallisation is used when purer solid crystals are needed. The solution is heated and then cooled slowly. As the temperature drops, pure crystals of the solute form first. Impurities remain in the solution.
Crystallisation gives purer crystals than simple evaporation and is preferred in laboratories and the pharmaceutical industry to obtain pure compounds.
Indian Salt Production
Salt pans in coastal India
Salt pans in coastal areas of Gujarat (Rann of Kutch) and Tamil Nadu use simple evaporation — the sun and wind evaporate water from shallow pans of sea water, leaving behind a thick crust of salt that is then collected. This is one of the oldest chemical separation methods used in India.
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Distillation and Fractional Distillation
Separating liquids with different boiling points
Distillation is used when a liquid must be separated from a dissolved solid or from another liquid with a significantly different boiling point. The mixture is heated; the component with the lower boiling point vaporises first, is collected as vapour, and then condensed back to liquid.
Simple distillation is used to get pure water from sea water or to recover a solvent from a solution.
Fractional distillation is used when two or more liquids have close but different boiling points. A fractionating column is used. This is the basis of petroleum refining at Indian refineries like those in Jamnagar, Panipat, and Koyali, where crude oil is separated into petrol, diesel, kerosene, and other fractions.
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Centrifugation, Chromatography, Sublimation
Specialised methods for specific separation needs
Centrifugation separates components of different densities by spinning the mixture at high speed. The denser component moves outward and settles at the bottom of the tube. Used in dairy plants to separate cream from milk and in diagnostic labs to separate blood cells from plasma.
Chromatography separates components based on their different speeds of travel through an absorbing material with a moving solvent. Paper chromatography separates the different coloured dyes present in black ink or food colouring — useful in forensic science.
Sublimation separates a sublimable component from one that does not sublime. Ammonium chloride can be separated from common salt by sublimation because ammonium chloride sublimes on heating while NaCl does not.
Quick Check
Which method is used to separate cream from milk?
Answer: Centrifugation. Spinning the mixture causes the denser milk to settle outward while the lighter cream remains at the centre.
Answer: Centrifugation. Spinning the mixture causes the denser milk to settle outward while the lighter cream remains at the centre.
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Complete Chapter Summary
Key ideas and methods to revise before exams
Pure substance: fixed composition. Element: one type of atom. Compound: elements in fixed ratio, new properties.
Mixture: variable composition, physical combination. Homogeneous: uniform. Heterogeneous: non-uniform.
Solution: particle < 1 nm, stable. Suspension: particle > 100 nm, settles. Colloid: 1–100 nm, Tyndall effect.
Tyndall effect: light scattered by colloidal particles. Solutions do not show it.
Mass percent = (mass of solute / mass of solution) × 100.
Choose separation method based on the property difference: insolubility → filtration; density → centrifugation; boiling point → distillation; sublimation → ammonium chloride from salt.
Is Matter Around Us Pure: revision sheet
Close the notes and explain each idea in your own words. Then check the formulas and attempt the examples before opening their solutions.
Recall checklist
- Chapter Intro
- Pure Substances, Elements, and Compounds
- Mixtures and Their Types
- Solutions, Suspensions, and Colloids
- Concentration and Separation Methods
- How to Choose the Right Separation Method
For each topic: state the rule, explain when it applies, give an example and identify one mistake to avoid. Return to any topic you cannot explain without looking.
Formula recap (1)
Concentration and Separation Methods
Explain your method
Write the given information, choose a method, show the steps, then check units and whether the answer makes sense.
Why is air called a mixture and not a compound?
Check worked solution 1
Air contains several gases physically mixed in variable proportions, and the components retain their own properties.
What is the main difference between an element and a compound?
Check worked solution 2
An element contains only one kind of atom, whereas a compound contains atoms of different elements chemically combined in a fixed ratio.
Turn mistakes into revision
Record the question, the step that went wrong and the corrected reason. Try it again tomorrow without the solution, then revisit it after a week. A correct guess still needs an explanation.