Pure Substances, Mixtures and their Types

Introduction: From Sugarcane to Crystals of Sugar

Picture a sugarcane farmer in rural India. She crushes thick green sugarcane stalks in a heavy iron press and a sweet, greenish-brown juice flows out. That juice is far from pure sugar — it carries dissolved sugars, plant fibres, mineral salts, water, organic acids and tiny bits of leaf. To turn this thick juice into the white, sparkling sugar you keep in your kitchen jar, the juice is boiled, filtered, treated with lime, evaporated and finally allowed to crystallise. Step by step, every unwanted ingredient is removed until only one substance remains: sucrose (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)).

This everyday journey — from a complicated juice to a single pure substance — sums up the central question of this chapter. What is a pure substance? What is a mixture? How can the two be told apart, and how can we tear a mixture into the individual substances that built it up?

5.1 What Is a Pure Substance?

In ordinary speech, “pure milk” means milk that has not been watered down. But in chemistry, the word pure has a much stricter meaning. A pure substance is made up of only one kind of particle. Every drop of distilled water contains only \(\text{H}_2\text{O}\) molecules. Every grain of common salt contains only sodium ions and chloride ions stacked together as \(\text{NaCl}\). There is nothing else hiding in between.

Because a pure substance contains only one kind of particle, it has fixed properties: a sharp melting point, a sharp boiling point, a definite density and a single colour. If you boil a flask of pure water at sea level, the temperature rises smoothly to 100°C and stays there until all the water has turned to steam. A mixture rarely behaves so neatly.

Pure Substance vs Mixture — the basic split

5.2 Mixtures — Homogeneous and Heterogeneous

A mixture is made by simply putting two or more substances together in any ratio. The substances do not change chemically — they keep their identities and can, in principle, be separated again. Sugar dissolved in water is still sugar; the water is still water. We can boil one off and recover the other.

Mixtures come in two flavours depending on how uniformly the components are spread.

Homogeneous mixtures

A homogeneous mixture looks completely uniform. No matter where you sample it, you get the same composition. Salt water, vinegar, brass, soft drinks and the air around you are all homogeneous mixtures.

Heterogeneous mixtures

A heterogeneous mixture has visibly different parts. Sand stirred into water, oil floating on water, iron filings sprinkled into sulphur powder, soil and a fruit salad — in each case you can pick out separate components or see boundaries between them.

5.3 Solutions — The Most Common Homogeneous Mixtures

A solution is a homogeneous mixture in which one substance is completely dissolved in another. The component present in larger amount is the solvent, and the one present in smaller amount is the solute. In a glass of sugar water, water is the solvent and sugar is the solute. Solutions can be solid (alloys like brass), liquid (lemonade) or gaseous (air).

Properties of a true solution

• It is perfectly clear and transparent — you can read print through it.
• The particle size is extremely small (less than 1 nm), so the solute does not settle on standing.
• The solute particles cannot be seen even under a powerful microscope.
• It cannot be separated by ordinary filter paper — the particles slip straight through.
• A beam of light passing through a true solution is not visible — it does not show the Tyndall effect.

5.4 Suspensions

A suspension is a heterogeneous mixture in which solid particles are dispersed but do not dissolve. The particles are large — bigger than 100 nm — so they are visible to the eye, scatter light strongly, settle on standing, and can be filtered out with a piece of filter paper. Muddy river water, chalk stirred into water and the cloudy mixture inside a bottle of milky paint are familiar suspensions.

5.5 Colloids and the Tyndall Effect

Between the very fine solution and the coarse suspension lies a third class — the colloid. Colloidal particles are 1 nm to 100 nm in size. They look uniform like a solution, but a beam of light passing through is clearly visible because the particles are large enough to scatter the light. This scattering is the Tyndall effect.

You see the Tyndall effect when a torch beam cuts through morning mist, when sunlight pierces through gaps in a forest canopy, or when a film projector beam cuts through smoky air. The medium that carries the particles is called the dispersion medium, and the dispersed substance is called the dispersed phase.

Examples of colloids in everyday life

5.6 Comparison Table — Solution, Suspension, Colloid

Competency-Based Questions L4 Analyse

Assertion & Reason Questions L5 Evaluate