A pure substance has a fixed composition and definite properties throughout. Types: (1) Elements – made of one type of atom; cannot be broken down further by chemical reactions; e.g., gold, oxygen, iron. (2) Compounds – two or more elements chemically combined in a fixed ratio; properties different from constituent elements; e.g., water (H₂O), salt (NaCl). A mixture contains two or more substances mixed in any proportion; no chemical bonding; properties of individual components retained; can be separated by physical methods.

Solution: homogeneous mixture; solute + solvent; particles < 1 nm; do not scatter light; do not settle; e.g., salt in water. Suspension: heterogeneous mixture; particles > 100 nm; visible to naked eye; settle on standing; scatter light; e.g., chalk in water. Colloid: heterogeneous but appears homogeneous; particles 1–100 nm; do not settle; scatter light (Tyndall Effect); e.g., milk, fog, blood. Tyndall Effect: scattering of light by colloidal particles, making the path of light visible.

Methods to separate mixtures: (1) Evaporation – separate dissolved solid from liquid; e.g., salt from water. (2) Filtration – separate insoluble solid from liquid using filter paper. (3) Distillation – separate miscible liquids with different boiling points; e.g., acetone and water. (4) Fractional distillation – for liquids with close boiling points; e.g., crude oil. (5) Chromatography – separate dissolved coloured substances based on solubility; e.g., dyes in ink. (6) Centrifugation – spinning at high speed to separate denser particles; e.g., cream from milk. (7) Crystallisation – obtain pure crystals of solid from solution.

Physical change: no new substance formed, reversible, no change in chemical composition; e.g., melting ice, boiling water, cutting paper, dissolving sugar. Chemical change: new substance(s) formed with different properties, usually irreversible, energy change involved; e.g., burning of wood, rusting of iron, cooking food, formation of curd. Signs of chemical change: change in colour, smell, temperature, evolution of gas, formation of precipitate.

A mixture is a combination of two or more substances in which each substance retains its own chemical properties. Mixtures have no fixed composition and can be separated by physical methods. Types: (1) Homogeneous mixture — uniform composition throughout; cannot distinguish components; e.g., salt solution, air, vinegar. (2) Heterogeneous mixture — non-uniform composition; components can be seen; e.g., soil, blood, sand in water. Difference from compound: in a compound, elements are chemically combined in a fixed ratio and cannot be separated by physical means; mixtures are physically combined and can be separated.

A solution is a homogeneous mixture of two or more substances. Components: (1) Solvent — present in larger quantity; dissolves the solute; e.g., water in salt water. (2) Solute — present in smaller quantity; gets dissolved; e.g., salt in salt water. Properties: uniform composition, transparent, particles smaller than 1 nm, particles not visible, do not scatter light (no Tyndall effect), cannot be separated by filtration or centrifugation. Concentration: amount of solute in a given amount of solution. Saturated solution — cannot dissolve more solute at given temperature. Unsaturated — can dissolve more. Supersaturated — contains more solute than normally possible (unstable). Solubility increases with temperature for most solids but decreases for gases.

Suspension: heterogeneous mixture where solute particles are larger than 100 nm; particles visible to naked eye; settle on standing; can be separated by filtration; scatter light (Tyndall effect); e.g., chalk in water, muddy water, paints. Colloid (Colloidal solution): intermediate between solution and suspension; particle size 1–100 nm; particles not visible to naked eye; do not settle on standing; cannot be separated by filtration; scatter light strongly (Tyndall effect); e.g., milk (fat in water), fog (water in air), smoke (carbon in air), butter (water in fat), blood (proteins in plasma), starch solution. Tyndall Effect: scattering of light by colloidal particles when a beam of light passes through a colloid; makes the path of light visible. Used to distinguish colloids from true solutions.

(1) Evaporation — used to separate a dissolved solid from a liquid; liquid is heated until it vaporizes, leaving behind the solid; e.g., obtaining salt from seawater. Not suitable for heat-sensitive solutes. (2) Distillation — used to separate miscible liquids with different boiling points (difference > 25°C); liquid mixture is heated; component with lower boiling point vaporizes first, is condensed and collected; e.g., separating acetone from water; also used to purify water. Fractional distillation for similar boiling points (e.g., crude oil refining, air separation). (3) Crystallisation — purer form of evaporation; solution is heated to saturation and then cooled slowly; pure crystals form on cooling; e.g., purification of copper sulphate, alum; used to get pure salt from impure salt.

(4) Filtration — separates an insoluble solid from a liquid; mixture poured through filter paper; solid (residue) stays on filter; liquid (filtrate) passes through; e.g., separating sand from water. (5) Centrifugation — used when particles are too small to filter; mixture spun at high speed; denser particles settle at bottom; e.g., separating cream from milk, separating cells from blood plasma, in washing machines. (6) Chromatography — separates components of a mixture based on different rates of movement through a stationary phase carried by a mobile phase; paper chromatography: components of ink/dye mixture separate as solvent moves up paper; used to separate dyes, pigments, detect adulteration in food; component ratio = Rf value (distance by component / distance by solvent). (7) Sublimation — separating a sublimable solid (directly converts from solid to gas) from non-sublimable impurities; e.g., separating ammonium chloride from salt or sand.

(8) Separating funnel — used to separate two immiscible liquids (liquids that do not mix) based on difference in density; denser liquid settles at bottom and is drained first; e.g., oil and water, kerosene and water. (9) Magnetic separation — used to separate magnetic material from non-magnetic mixture; a magnet is moved over the mixture; magnetic particles get attracted; e.g., separating iron filings from sulphur powder, used in mining. (10) Sedimentation and decantation — heavier particles settle at bottom (sedimentation); the clear liquid on top is poured off gently (decantation); e.g., separating sand from water. Combined with centrifugation for blood analysis. Summary: choice of separation technique depends on the nature of the mixture (solid-liquid, liquid-liquid, solid-solid), solubility, density, magnetic properties, and boiling points of components.

Physical change: only physical properties change; no new substance is formed; change is usually reversible; chemical composition unchanged; e.g., cutting paper, melting ice, dissolving salt in water, stretching rubber, magnetizing iron. Chemical change (chemical reaction): new substance(s) formed; chemical composition changes; usually irreversible; energy absorbed or released; change in colour, smell, gas produced, precipitate formed, or temperature change; e.g., burning wood, rusting of iron, cooking food, digestion, photosynthesis. Key test: if original substance can be recovered by simple physical means → physical change. If not → chemical change. Note: dissolving sugar in water is a physical change (sugar can be recovered by evaporation). Burning of sugar is a chemical change (produces CO₂ and H₂O — cannot be reversed).

Importance of mixtures: (1) Air is a mixture of N₂ (78%), O₂ (21%), Ar (0.93%), CO₂ (0.04%) and water vapour — essential for all living organisms. (2) Seawater — solution containing dissolved salts, gases and minerals — source of table salt, bromine, magnesium. (3) Alloys — homogeneous mixtures of metals; e.g., brass (Cu + Zn), bronze (Cu + Sn), steel (Fe + C), solder (Pb + Sn); have better properties than pure metals. (4) Petroleum — complex mixture of hydrocarbons; separated by fractional distillation into LPG, petrol, diesel, kerosene, lubricating oil, tar. (5) Blood — colloidal/suspension mixture vital for transport. (6) Medicines — often mixtures of active ingredients and excipients. (7) Food — most foods are mixtures (e.g., milk, fruit juice). Industrial separation of air (liquid air fractional distillation) gives N₂ (fertilizers), O₂ (hospitals, welding), Ar (light bulbs), Ne, He (discharge tubes).

Concentration is the amount of solute present in a given amount of solution. Mass by mass percentage = (mass of solute ÷ mass of solution) × 100. Mass by volume percentage = (mass of solute ÷ volume of solution) × 100. Volume by volume percentage = (volume of solute ÷ volume of solution) × 100. Example: 10 g of salt dissolved in 90 g of water gives 100 g of solution, so the mass percentage is 10%. A solution that cannot dissolve any more solute at a given temperature is saturated; one that can dissolve more is unsaturated. Solubility is the amount of solute that dissolves in 100 g of solvent to form a saturated solution at a given temperature. For most solids solubility increases with temperature; for gases it decreases.

Colloids are classified by the dispersed phase and the dispersion medium. Aerosol: liquid or solid in gas — fog, clouds, smoke. Foam: gas in liquid — shaving cream, whipped cream. Emulsion: liquid in liquid — milk, face cream. Sol: solid in liquid — milk of magnesia, muddy water (coarse), ink. Gel: liquid in solid — jelly, cheese, butter. Solid sol: solid in solid — coloured glass, gemstones. Remember: milk is an emulsion (fat droplets in water), fog is an aerosol (water droplets in air).

Tyndall effect: the scattering of a beam of light by particles in a colloid or fine suspension, which makes the path of the beam visible. It occurs because colloidal particles (1 nm to 1000 nm) are big enough to scatter light. True solutions do not show it as their particles are too small. Examples: a beam of sunlight through a dense forest canopy, light from a torch in a dusty or smoky room. Brownian motion: the continuous zig-zag random motion of colloidal particles caused by collisions with the molecules of the dispersion medium. It keeps the particles from settling, so colloids are stable.

Pure substance: one kind of particle (element or compound). Mixture: two or more substances, variable composition, components keep their properties. Homogeneous mixture (solution): uniform composition, particles under 1 nm, no Tyndall effect, does not settle. Heterogeneous mixture: non-uniform. Suspension: particles over 1000 nm, visible, settle on standing, can be filtered, scatter light. Colloid: particles 1–1000 nm, do not settle, cannot be filtered normally, show Tyndall effect. Separation methods: evaporation (dissolved solid from liquid), filtration (insoluble solid), centrifugation (cream from milk), separating funnel (immiscible liquids), distillation (miscible liquids, bp difference over 25 K), fractional distillation (bp difference under 25 K), sublimation (ammonium chloride, camphor, naphthalene), crystallisation (pure crystals from solution), chromatography (dyes, pigments).