Chapter 1 — Exploring Mixtures and Their Separation

Section 1.1 – 1.2

Pure Substances and Mixtures

To a scientist, "pure" does not mean "free from adulteration" — it means the substance is made of only one kind of particle. On the basis of composition, all matter is either a pure substance or a mixture.

🗂️ What is a Mixture?

A mixture is a combination of two or more pure substances in which each substance keeps its own chemical properties. For example, "pure" milk is actually a mixture of fats, proteins, carbohydrates, mineral salts, vitamins and water.

Classification of matter by chemical composition

Homogeneous Mixture

Has the same composition throughout — only one phase is visible. Also called a solution.

Examples: salt solution, copper sulphate solution, air, vinegar, and alloys like brass & bronze.

Heterogeneous Mixture

Different constituents are not mixed uniformly; particles can often be seen separately.

Examples: sand + salt, iron filings + sulphur, soil, chalk powder in water.

❖ Why alloys are still called mixtures

Alloys (brass = copper + zinc; bronze = copper + tin; stainless steel = iron + chromium + carbon) are homogeneous, yet they are mixtures because their composition can vary and each metal keeps its own chemical properties. Unlike compounds, they have no fixed ratio.

The constituents of a mixture may be present in any proportion.
A mixture retains the properties of its individual constituents.
Components can be separated by physical means (evaporation, filtration, distillation).
No significant energy is evolved or absorbed when a mixture forms.

Activity Comparison

Solution vs Suspension vs Colloid

Mix salt, wheat flour and milk in three beakers of water. Watching whether particles are visible, whether they settle, whether light passes, and whether filtration leaves a residue lets us tell the three apart.

Path of light is invisible in a solution, but visible in a colloid and (a fine) suspension — the Tyndall effect

Property	Solution	Suspension	Colloid
Nature	Homogeneous	Heterogeneous	Heterogeneous
Particle size	< 1 nm	> 1000 nm	1 nm – 1000 nm
Particles visible?	No (not even under microscope)	Yes (naked eye)	Under powerful microscope
Settle on standing?	No (stable)	Yes (unstable)	No (stable)
Pass through filter paper?	Yes	No (residue left)	Yes
Tyndall effect?	No	Yes	Yes

Section 1.3

Solutions: Solute and Solvent

A solution is a homogeneous mixture of two or more substances. It has two main parts — the solute and the solvent.

Solute (minor component, dissolved) + Solvent (major component) → Solution

Solute

salt

The substance present in smaller amount; it gets dissolved.

Solvent

water

The substance present in larger amount; it does the dissolving. Water is the "universal solvent".

True solution

< 1 nm

Solute particles so fine they can't be seen even under a powerful microscope.

Solutions can exist in all three states

Type	Example	Solvent	Solute
Solid – Solid	Brass (70% Cu, 30% Zn)	Copper	Zinc
Solid – Liquid	Tincture of iodine	Ethyl alcohol	Iodine
Liquid – Liquid	Vinegar	Water	Acetic acid
Liquid – Gas	Aerated (soda) water	Water	Carbon dioxide
Gas – Gas	Air	Nitrogen	Oxygen + other gases

💡 Did You Know?

Two solid reactants like lead acetate and potassium iodide barely react when their crystals touch — but mix their solutions and a bright yellow precipitate of lead iodide forms in a fraction of a second. In solution, particles are broken to the molecular level, so they make very close contact. This is why medicines and saline-glucose are given in solution form.

Section 1.4

Concentration of a Solution

Concentration is the amount of solute present in a given quantity of solution. It is most commonly expressed as a percentage — by mass, by volume, or by mass-by-volume.

Mass percentage (w/w)

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

Mass of solution = mass of solute + mass of solvent. The answer is a pure number with no units.

Volume percentage (v/v) & mass-by-volume (w/v)

Vol % = (Vol of solute ÷ Vol of solution) × 100
Mass/Vol % = (Mass of solute in g ÷ Vol of solution in mL) × 100

⚠️

Common slip: percentage concentration refers to solute per 100 g (or mL) of solution, NOT per 100 g of solvent. A 20% salt solution = 20 g salt + 80 g water, giving 100 g of solution.

Worked Example25 g of sugar in 175 g of water

Mass of solute = 25 g; mass of solvent = 175 g.

Mass of solution = 25 + 175 = 200 g.

Mass % = (25 ÷ 200) × 100 = 12.5%

Concentration = 12.5% by mass

Section 1.5 – 1.6

Saturation & Solubility

As you keep adding solute, a solution eventually cannot dissolve any more. How much it can hold — and how that changes with temperature — is described by solubility and the solubility curve.

Unsaturated

Can still dissolve more solute at that temperature.

Saturated

Has dissolved as much as it can at that temperature; extra solute won't dissolve.

Supersaturated

Holds more than a saturated solution can normally hold (e.g. sugar syrups).

❖ Solubility

The maximum mass of solute (in grams) that dissolves in 100 g of solvent at a given temperature. Example: solubility of potassium nitrate is 21 g/100 g water at 10°C.

Factors affecting solubility

Change	Solubility of solids in liquids	Solubility of gases in liquids
Increase temperature	Generally increases	Decreases (gas escapes)
Increase pressure	No effect	Increases

Solubility curves: KNO₃ rises steeply with temperature, NaCl barely changes, and Ca(OH)₂ actually decreases

💡 Did You Know?

Open a soda bottle and it fizzes — the sudden drop in pressure lowers the solubility of the dissolved CO₂ gas, so it escapes. The same reason heated water releases its dissolved air as tiny bubbles before boiling.

Section 1.7 – 1.8

Suspensions, Colloids & the Tyndall Effect

A suspension has large particles that settle and can be filtered. A colloid sits in between a solution and a suspension — its particles never settle, pass through filter paper, yet are big enough to scatter light.

Tyndall effect — scattering of a light beam by colloidal particles, discovered by John Tyndall

🧼 Classification of colloids

Colloids are named by the physical states of the dispersed phase (the spread-out particles) and the dispersion medium (what they are spread in).

Type	Dispersed phase	Medium	Examples
Sol	Solid	Liquid	Mud, soap solution
Emulsion	Liquid	Liquid	Milk, face cream
Foam	Gas	Liquid	Shaving cream
Aerosol	Liquid / Solid	Gas	Fog, clouds / smoke
Gel	Liquid	Solid	Cheese, butter
Solid sol	Solid	Solid	Coloured gemstones
Solid foam	Gas	Solid	Sponge, pumice, rubber

💡 Did You Know?

Distant hills look blue because the colloidal mist of air and tiny water droplets scatters light — the very same Tyndall effect that makes a torch beam visible in fog or in a dusty sunbeam through a window.

Section 1.9

Separating Two Solids

Which method we choose depends on a difference in a physical property — solubility, magnetism, or the ability to sublime.

Suitable solvent

If one solid dissolves and the other doesn't — e.g. salt + sand: dissolve in water, filter off sand, then evaporate to recover salt.

Using a magnet

If one solid is magnetic — e.g. iron filings + sulphur: a magnet attracts the iron, leaving sulphur behind.

Sublimation

If one solid sublimes on heating — e.g. ammonium chloride + salt, or camphor/iodine/naphthalene mixtures.

Sublimation: ammonium chloride turns to vapour and re-deposits as white solid on the cool funnel; salt stays in the dish

Section 1.10

Separating a Solid from a Liquid

Seven techniques, each suited to a different situation.

Method	When to use it	Example
Filtration	Insoluble solid in a liquid	Sand from water; chalk from water
Coagulation	Very fine / colloidal particles won't settle — add a coagulant (alum)	Clearing muddy water
Crystallisation	Get a pure soluble solid back as crystals from its hot saturated solution	Pure copper sulphate from impure sample
Centrifugation	Tiny particles that pass through filter paper	Cream from milk; blood components
Chromatography	Separate dissolved coloured constituents	Dyes in ink
Evaporation	Recover a soluble non-volatile solid (liquid is not needed)	Salt from salt solution
Distillation	Recover both the liquid and the solid	Pure water + CuSO₄ from its solution

Filtration: residue stays on the paper, filtrate passes through

Paper chromatography: ink's dyes rise at different rates and separate into colour bands

💡 Did You Know?

A spinning dancer feels an outward pull — the same idea behind a centrifuge. Spinning a sample at high speed throws the heavier particles outward, separating cream from milk or the cells from blood plasma.

Section 1.11

Separating Two Liquids

Immiscible liquids (which form separate layers) are separated by a separating funnel; miscible liquids (which mix completely) are separated by fractional distillation.

Immiscible liquids: open the tap to drain the heavier lower layer first

Miscible liquids (b.p. differ by < 25°C): the column gives repeated distillation so the more volatile liquid distils first

💡 Did You Know?

Fractional distillation separates the components of crude oil (petrol, kerosene, diesel, wax) and even cools air into liquid to draw off pure nitrogen (b.p. −195°C) and oxygen (b.p. −183°C).

Solved Examples

Worked Sample Problems

Model answers in the exact CBSE style — follow the reasoning, then try the practice bank.

Example 1Why is air regarded as a mixture?

Air is a mixture because (i) its composition varies from place to place; (ii) its gases can be separated by physical means (fractional distillation of liquid air); (iii) no energy change occurs when its gases mix; and (iv) each gas keeps its own properties (oxygen supports combustion, CO₂ turns limewater milky).

Example 2How much water must be added to 50 g of glucose to get a 12% solution?

12% means 12 g glucose per 100 g solution, so mass of solution = (mass of solute × 100) ÷ 12 = (50 × 100) ÷ 12 = 416.67 g.

Water to add = solution − solute = 416.67 − 50 = 366.67 g.

Add 366.67 g of water.

Example 3Separate: sand, iron filings, ammonium chloride and sodium chloride

Magnet removes the iron filings.

Sublimation removes ammonium chloride (it sublimes; the rest don't).

Add water — salt dissolves, sand doesn't. Filter off the sand.

Evaporate / crystallise the filtrate to recover sodium chloride.

Glossary

Key Terms

Quick definitions for fast revision before the exam.

Pure substance: Matter made of only one kind of particle, with a definite set of properties.
Mixture: Two or more substances combined in any proportion without chemical change; each keeps its properties.
Homogeneous / Heterogeneous: Uniform composition throughout / non-uniform composition.
Solution: A homogeneous mixture of two or more substances (solute + solvent).
Solute / Solvent: The dissolved minor component / the dissolving major component.
Concentration: Amount of solute in a given quantity of solution (often a percentage).
Saturated / Unsaturated / Supersaturated: Cannot dissolve more / can dissolve more / holds more than a saturated solution.
Solubility: Maximum mass of solute that dissolves in 100 g of solvent at a given temperature.
Suspension: Heterogeneous mixture of insoluble particles (> 1000 nm) that settle on standing.
Colloid: Heterogeneous mixture with particle size 1–1000 nm that never settle and show the Tyndall effect.
Tyndall effect: Scattering of a light beam by colloidal particles, making its path visible.
Sublimation: A solid changing directly to vapour on heating and back to solid on cooling.
Crystallisation: Obtaining pure crystals of a solid from its hot saturated solution on cooling.
Centrifugation: Separating fine particles by spinning a mixture at high speed.
Chromatography: Separating dissolved constituents by their different rates of movement over an adsorbent.
Distillation / Fractional distillation: Separating liquids using differences in boiling point (the latter for b.p. differences under 25°C).

Exam Strategy

Top Exam Tips

High-yield reminders that catch most students out.

Percentage = per 100 g of SOLUTION

Not per 100 g of solvent. Always compute mass of solution = solute + solvent first.

Tyndall effect tells colloid from solution

Beam visible → colloid (or fine suspension). Beam invisible → true solution.

Particle-size ladder

Solution < 1 nm < colloid < 1000 nm < suspension. Memorise this order.

Match method to property

Magnetism → magnet; sublimes → sublimation; soluble → solvent + filtration; need both parts → distillation.

Crystallisation beats evaporation

For pure crystals, crystallise — evaporating to dryness can decompose the solid or trap impurities.

Distillation vs fractional

Simple distillation if b.p. differ by > 25°C; fractional (with a column) if the difference is smaller.

Alloys are mixtures

Homogeneous, but variable composition + each metal keeps its properties → mixture, not compound.

Gases: heat lowers solubility, pressure raises it

Opposite of solids. Remember the fizz from an opened soda bottle.

Self-Evaluation

Practice Question Bank — 150 Questions

CBSE-pattern questions across all five sections with toggle answers. Section A: 50 MCQ & Assertion-Reason (1 mark) · Section B: 30 Very Short Answer (2 marks) · Section C: 30 Short Answer (3 marks) · Section D: 20 Long Answer (5 marks) · Section E: 20 Case-Based (4 marks).