Exploring Mixtures and their Separation

Chapter at a glance

• Mixtures are classified as homogeneous (uniform composition throughout, also called solutions) or heterogeneous (non-uniform composition).
• Solutions are prepared by dissolving a solute in a solvent; concentration expresses the amount of solute in a given quantity of solvent or solution.
• Concentration of solutions is expressed as mass by mass percentage (% m/m), mass by volume percentage (% m/v) or volume by volume percentage (% v/v).
• Solubility of solids in liquids generally increases with temperature while that of gases decreases; a saturated solution cannot dissolve more solute at a given temperature.
• Homogeneous mixtures are separated by crystallisation (difference in solubility with temperature), distillation (difference in boiling points ≥ 25 °C) or paper chromatography (difference in rates of movement on paper).
• Heterogeneous mixtures are separated using a separating funnel (immiscible liquids of different densities), sublimation (direct solid-to-vapour transition) or centrifugation/coagulation (suspensions).
• Suspensions have visible particles (> 1000 nm) that settle; colloids (1–1000 nm) do not settle but show the Tyndall effect; solutions (< 1 nm) are transparent and show no Tyndall effect.
• The Tyndall effect (scattering of light by particles) distinguishes colloids and suspensions from solutions.

Definitions and laws

• “A mixture of sugar and water has a uniform composition throughout … Such a mixture is called a homogeneous mixture or a solution.”
• “A stirred mixture of sand and water is not uniform … Such a mixture is called a heterogeneous mixture.”
• “The amount of solute dissolved in a given amount of solvent or solution is termed as the concentration of the solution.”
• “The maximum amount of solute that dissolves in a fixed quantity of the solvent (100 mL or 100 g) is called its solubility at a given temperature.”
• “A solution that cannot dissolve any more solute at that temperature is called a saturated solution.”
• “A crystal is a solid that is made up of particles arranged in a regular geometric pattern.”
• “The transition of a solid directly into a vapour (below its melting point) without passing through the liquid state is called sublimation.”
• “The scattering of light by particles is known as the Tyndall effect.”
• “The solute-like component or the dispersed particles, in a colloid form the dispersed phase and the component in which the dispersed phase is suspended is known as the dispersion medium.”

Important diagrams and activities

• Fig. 5.1 & 5.2: Homogeneous vs heterogeneous mixture – shows uniform vs visibly non-uniform appearance.
• Fig. 5.3 / Activity 5.1: Laser beam through salt-water, chalk-water and milk-water – demonstrates visibility of light path (Tyndall effect) and filtration behaviour.
• Fig. 5.6 / Activity 5.2: Solubility curves of compounds A and B – illustrates effect of temperature on solubility.
• Fig. 5.8 / Activity 5.3: Steps of crystallisation (preparing saturated solution, hot filtration, slow cooling) – shows formation of pure crystals from a saturated solution.
• Fig. 5.11 & 5.12: Distillation process and set-up – shows vaporisation of lower-boiling liquid, condensation and collection of distillate.
• Fig. 5.15 / Activity 5.5: Paper chromatography of black ink – demonstrates separation of mixture components by differential movement on paper.
• Fig. 5.16 / Activity 5.6: Separating funnel with mustard oil and water – shows separation of two immiscible liquids by density.
• Fig. 5.17 / Activity 5.7: Sublimation of camphor–sand mixture – shows direct solid-to-vapour transition and deposition on cold surface.
• Fig. 5.19: Centrifugation – illustrates separation of heavier particles by centrifugal force.
• Fig. 5.24: Tyndall effect in a sports stadium – shows scattering of light beam by colloidal/suspended particles.

Common misconceptions and exam pitfalls

• Confusing homogeneous appearance of colloids with true solutions; always test with Tyndall effect or filtration.
• Assuming evaporation and crystallisation are identical; crystallisation relies on solubility change with temperature and yields pure crystals while evaporation may leave impurities.
• Using the wrong percentage expression (% m/m vs % m/v vs % v/v) in numericals; check whether mass or volume of solution/solute is given.
• Thinking distillation works for any two liquids; the text specifies a minimum ~25 °C boiling-point difference.
• Believing all heterogeneous mixtures can be separated by filtration; suspensions with very fine particles require coagulation or centrifugation.
• Forgetting that temperature must be stated when reporting solubility.

Formula sheet