A mixture is a physical combination of two or more substances in any proportion. CBSE Class 9 introduces mixtures; Class 11 elaborates. NEET rarely tests this directly, but the concepts underlie chromatography and distillation.
Core Concepts
Homogeneous and heterogeneous
Homogeneous — uniform composition throughout (salt water, air). Heterogeneous — visible distinct components (salad, muddy water). The distinction depends on scale of observation.
At the molecular level, a homogeneous mixture has solute particles smaller than 1 nm — they are individual molecules or ions dispersed uniformly. A heterogeneous mixture has particles large enough to see or settle. Between these two extremes lies a third category — colloids.
Types of mixtures
Solutions (solvent + solute, one phase). Suspensions (large particles settle). Colloids (intermediate). Alloys (mixtures of metals). Most of chemistry deals with solutions.
| Type | Particle Size | Visibility | Settles? | Filters? | Example |
|---|---|---|---|---|---|
| Solution | <1 nm | Transparent | No | No | Salt water, sugar water |
| Colloid | 1-1000 nm | Translucent (Tyndall effect) | No | Not by ordinary filter | Milk, fog, ink |
| Suspension | >1000 nm | Opaque | Yes | Yes | Muddy water, flour in water |
The Tyndall effect is the best way to distinguish a colloid from a true solution. Shine a beam of light through the liquid — if the path of light is visible (scattered), it is a colloid. True solutions do not scatter light.
Separation by physical methods
Sieving (different sizes). Filtration (solid from liquid). Sedimentation and decantation (gravity separation). Distillation (volatile liquids from mixtures). Crystallisation (dissolved solid from solution). Chromatography (components with different adsorption).
The choice of method depends on the property difference between the components:
| Property Difference | Method | Example |
|---|---|---|
| Particle size | Sieving, filtration | Sand from gravel, precipitate from liquid |
| Density | Sedimentation, centrifugation | Blood components, muddy water |
| Boiling point | Distillation | Alcohol from water, petroleum refining |
| Solubility | Crystallisation, extraction | Salt from sand, caffeine from tea |
| Adsorption | Chromatography | Pigments from leaves, dyes from ink |
| Magnetism | Magnetic separation | Iron filings from sand |
| Sublimation | Heating | Camphor from salt, ammonium chloride |
Distillation
Simple distillation separates liquid from dissolved solid. Fractional distillation separates liquids with similar boiling points (crude oil refining, alcohol from water).
Simple distillation works when the boiling points differ by more than 25°C. The liquid with the lower boiling point vaporises first, is condensed, and collected.
Fractional distillation uses a fractionating column packed with glass beads or plates. The column creates a temperature gradient — cooler at the top, hotter at the bottom. Vapours condense and re-evaporate multiple times as they rise, achieving better separation. This is how petroleum refineries separate crude oil into petrol, diesel, kerosene, and other fractions.
| Fraction | Boiling Range | Carbon Atoms | Use |
|---|---|---|---|
| LPG | <40°C | C1-C4 | Cooking gas |
| Petrol | 40-200°C | C5-C12 | Vehicle fuel |
| Kerosene | 175-325°C | C12-C16 | Jet fuel, lamps |
| Diesel | 250-400°C | C15-C25 | Heavy vehicles |
| Lubricating oil | >350°C | C20-C35 | Machine lubrication |
| Bitumen | Residue | C35+ | Road surfacing |
Chromatography
Components of a mixture travel at different rates over a stationary phase. Paper, thin layer (TLC), column, gas (GC), liquid (HPLC). Fundamental analytical tool.
The principle behind all chromatography is differential partitioning — different components have different affinities for the stationary phase vs the mobile phase. A component that sticks more to the stationary phase moves slower; one that dissolves more in the mobile phase moves faster.
Paper chromatography is the simplest form. A spot of the mixture is placed on filter paper. The paper is dipped in a solvent (mobile phase). The solvent rises by capillary action, carrying different components to different heights. Each component’s position is measured as an value.
is characteristic for a given compound in a given solvent system. It is always between 0 and 1. A component with did not move (fully adsorbed); moved with the solvent (no adsorption).
Worked Examples
Use a magnet for iron. Dissolve salt in water, filter out sand. Evaporate water to recover salt. Three different separation principles — magnetism, filtration, evaporation.
Ethanol boils at 78°C, water at 100°C. Heating the mixture vaporises ethanol first (mostly), which is condensed and collected. Fractional distillation gives concentrations up to about 95% ethanol.
In a paper chromatography experiment, the solvent front moved 10 cm from the origin. Component A moved 7 cm and component B moved 3 cm.
Component A has higher affinity for the mobile phase (it moved further). Component B has higher affinity for the stationary phase (paper).
Ethanol and water form an azeotrope at 95.6% ethanol. At this composition, both components vaporise together and distillation cannot separate them further. To get absolute (100%) ethanol, you need a drying agent like CaO or molecular sieves.
Heat the mixture. NH4Cl sublimes (converts directly to gas) and is collected on a cold surface. NaCl remains behind as it does not sublime. This works because of the sublimation property of ammonium chloride.
Common Mistakes
Confusing solution and suspension. Solutions are clear; suspensions are cloudy and settle.
Saying filtration can separate dissolved salt from water. It cannot — you need evaporation or distillation.
Writing that homogeneous mixtures are pure substances. They are still mixtures.
Confusing the Tyndall effect with fluorescence. The Tyndall effect is scattering of light by colloidal particles. Fluorescence is emission of light by certain molecules after absorbing UV light.
Saying chromatography separates by boiling point. Chromatography separates by differential adsorption or partitioning, not boiling point. Distillation separates by boiling point.
Exam Weightage and Revision
Mixtures and separation techniques carry 3-5 marks in CBSE Class 9 exams. NEET does not test mixtures as a standalone topic but uses chromatography and distillation concepts in organic chemistry. JEE may ask a numerical on Raoult’s law (ideal solutions) in the solutions chapter.
| Question Type | Exam | Typical Question |
|---|---|---|
| Identify separation method | CBSE Class 9 | How to separate sand and salt? |
| Distinguish colloid and solution | CBSE Class 9 | What is the Tyndall effect? |
| Fractional distillation | CBSE, NEET (indirect) | Why is a fractionating column needed? |
| Chromatography principle | CBSE Class 11 | What does Rf value represent? |
| Petroleum fractions | CBSE Class 8 | Name fractions of crude oil |
The CBSE favourite question: “You are given a mixture of sand, salt, and iron filings. Describe how you would separate them.” Answer step by step: magnet for iron, dissolve in water + filter for sand, evaporate for salt. Full marks require all three steps with reasoning.
Practice Questions
Q1. Classify the following as homogeneous or heterogeneous: (a) Air (b) Brass (c) Oil and water (d) Vinegar (e) Blood
(a) Air — homogeneous (uniform mixture of gases). (b) Brass — homogeneous (alloy of Cu and Zn, uniform at atomic level). (c) Oil and water — heterogeneous (two visible layers, immiscible). (d) Vinegar — homogeneous (acetic acid dissolved in water). (e) Blood — heterogeneous (plasma is liquid, RBCs/WBCs are suspended particles — it is a colloid).
Q2. Why does fractional distillation use a fractionating column while simple distillation does not?
Fractional distillation is used for liquids with close boiling points. The fractionating column provides a large surface area where vapours condense and re-evaporate repeatedly. Each condensation-evaporation cycle enriches the vapour in the lower-boiling component. Without the column, the separation would be incomplete because both liquids vaporise together to some extent.
Q3. In a chromatography experiment, two dyes A and B have Rf values of 0.8 and 0.2 respectively. Which dye is more strongly adsorbed on the stationary phase?
Dye B (). A lower means the component moved less distance, indicating stronger attraction to the stationary phase (it spent more time stuck to the paper). Dye A moved further, meaning it prefers the mobile phase.
Q4. How would you separate a mixture of kerosene and water?
Use a separating funnel. Kerosene and water are immiscible (they form two layers because kerosene is non-polar and water is polar). Pour the mixture into a separating funnel, let it stand until the layers separate. Water (denser) settles at the bottom. Open the stopcock to drain the water layer first, then collect the kerosene layer.
FAQs
Is air a mixture or a compound?
Air is a mixture — it has variable composition (humidity varies, pollution levels differ). A compound has fixed composition. Air contains N2 (~78%), O2 (~21%), Ar (~0.9%), CO2 (~0.04%), and trace gases. The components can be separated by fractional distillation of liquid air.
Why is crystallisation better than evaporation for recovering sugar from solution?
Evaporation at high temperature can decompose sugar (caramelisation). Crystallisation uses slow, controlled cooling to form pure crystals while impurities remain in solution. The resulting crystals are purer.
What is the difference between evaporation and distillation?
In evaporation, the vapour is lost to the atmosphere — you recover only the solid. In distillation, the vapour is condensed and collected — you recover the liquid too. Use distillation when you want both components.
Match method to target — sieving for sizes, filtration for solids, distillation for liquids, chromatography for fine mixtures.
Mixtures and their separations are basic lab skills. Every analytical technique you will ever use builds on these concepts.