Elements, Compounds, Mixtures & Their Separation

Classification of Matter

• All matter ➜ either a pure substance or a mixture.
  • Pure substance: fixed composition & identical properties throughout.
  • Elements (one kind of atom)
  • Compounds (two or more elements chemically combined in a fixed mass ratio)
  • Mixtures: two or more elements/compounds physically intermingled in any ratio.
  • Homogeneous (true solutions; uniform properties)
  • Heterogeneous (colloids, suspensions; non-uniform properties)

Elements

• Definition: pure substances consisting of only one type of atom, either free or bonded to atoms of the same kind.
• Atomicity (number of atoms per molecule)
  • Mono-atomic: He, Ne, Ar
  • Di-atomic: $H<em>2, O</em>2, N<em>2, Cl</em>2$
  • Tri-atomic: $O_3$ (ozone)
  • Poly-atomic: $P<em>4, S</em>8, B_{12}$
• Classification
  • Metals: lustrous, malleable, ductile, high density & m.p/b.p, good conductors (e.g. Cu)
  • Non-metals: dull, brittle, low density & m.p/b.p, poor conductors (e.g. S, Br)
  • Metalloids: intermediate (Si, As, Sb, B)
  • Noble gases: inert mono-atomic gases (He, Ne, Ar…)
• Chemical symbols
  • One‐letter: $C, S, K$ etc.
  • Two-letter: $Na$ (sodium), $Po$ (polonium) etc.
  • Latin-derived: $K$ (kalium), $Fe$ (ferrum)
• Historical note: 118 elements recognised; IUPAC approves names/symbols.

Compounds

• Definition: pure, homogeneous substances formed when atoms of different elements chemically combine in a fixed ratio by mass.
• Characteristics
  • Fixed composition (law of constant proportion).
  • Properties differ completely from constituent elements (e.g. $H<em>2$ combustible, $O</em>2$ supports combustion, but $H_2O$ neither).
  • Represented by chemical formula; e.g. water = $H_2O$ (H:O :: 2:1 atoms).
  • Cannot be separated by physical means; require chemical reactions (electrolysis, reduction, etc.).
  • Formation involves energy changes (heat/light absorbed or evolved).
  • Combustion of candle: $Hydrocarbon + O<em>2 \rightarrow CO</em>2 + H_2O + \text{heat + light}$
  • Photosynthesis: $6CO<em>2 + 6H</em>2O \xrightarrow{light} C<em>6H</em>{12}O<em>6 + 6O</em>2$ (endothermic)
• Examples of formation
  • $2H<em>2 + O</em>2 \rightarrow 2H_2O$
  • $Fe + S \xrightarrow{\Delta} FeS$

Mixtures

• Definition: physical combination of two or more substances in any proportion retaining individual properties.
• Characteristics
  • Variable composition; no fixed formula.
  • Components retain original properties (magnet attracts Fe from Fe–S mixture).
  • Often heterogeneous but can be homogeneous (e.g. air).
  • Components separable by physical methods.
  • No significant energy change on mixing.
• Types (by components)
  • Element + Element (Fe + S)
  • Element + Compound (S + NaCl)
  • Compound + Compound (NaCl + H₂O)
• Special solid–gas mixtures: bread & pumice stone (air dispersed in solid matrix), smoke (solid + gas)

Importance of Pure Substances

• Medicines: impurities alter efficacy & may harm health.
• Food & spices: adulteration causes nutritional loss & toxicity.
• Drinking water: impurities spread diseases (typhoid, cholera, dysentery).
• Research laboratories: purity necessary for reproducible results.

Separation Techniques (Overview Table)

• Solid + Solid
  • Hand-picking (size/colour difference)
  • Magnet (one component magnetic)
  • Sublimation (one component sublimes; e.g. iodine + salt)
  • Solvent extraction + filtration (one dissolves; e.g. salt + sulphur)
• Liquid + Liquid
  • Separating funnel (immiscible; density difference; e.g. oil + water)
  • Fractional distillation (miscible; different b.p.; e.g. alcohol + water)
• Solid + Liquid
  • Filtration (insoluble & light; e.g. sawdust + water)
  • Sedimentation & decantation (insoluble & heavy; e.g. sand + water)
  • Evaporation (recover soluble solid only; e.g. salt from seawater)
  • Distillation (recover both solute & solvent; e.g. salt + water)

Detailed Separation Methods

Filtration

• Principle: porous barrier allows liquid to pass, retains insoluble solid.
• Procedure
  • Fold filter paper into cone, fit in funnel sealed to walls (no air gap).
  • Pour mixture; collect filtrate; residue washed, dried.
• Domestic & industrial use: water purification, pharma manufacture.
• Traditional household filter: gravel➜coarse sand➜fine sand layers.

Sedimentation & Decantation

• Sedimentation: heavy insoluble particles settle under gravity forming sediment; clear liquid = supernatant.
• Decantation: carefully pour off supernatant, leaving sediment.
• Employed in municipal water treatment before filtration.

Evaporation & Crystallisation

• Used for homogeneous solid-liquid mixtures where only solid is needed.
• Evaporation: heat solution until solvent vapour leaves; solid residue remains.
• Crystallisation: gentle evaporation until supersaturated; cool to form crystals (e.g. copper sulphate).
• Large-scale: solar evaporation ponds for table salt.

Distillation (Simple)

• Separates a volatile liquid from non-volatile solute; recovers both.
• Apparatus: round-bottom flask ➜ condenser (Liebig) ➜ receiver.
• Example: desalination – obtain pure water; salt left behind.
• Process: heating ➜ vapourisation ➜ condensation ➜ collection.

Fractional Distillation

• Separates miscible liquids with different boiling points.
• Fractionating column provides successive condensation–vaporisation cycles (reflux) = many theoretical plates.
• Examples
  • $C<em>2H</em>5OH$ (b.p 80 °C) from water (100 °C)
  • Crude oil ➜ refinery gases, petrol, naphtha, kerosene, diesel, fuel oil, lubricating oil, bitumen (temperature gradient 25–>350\,^{\circ}C)

Separating Funnel

• For two immiscible liquids of different densities.
• Steps: pour mixture, allow layers to form, open stop-cock to drain heavier liquid, recover lighter layer.
• Example: oil-water separation.

Sublimation

• Certain solids transition directly $\text{solid}\rightarrow\text{vapour}$ below their melting point.
• Subliming solids: ammonium chloride, iodine, camphor, napthalene, solid $CO_2$.
• Separation of $NH_4Cl$ & NaCl
  • Heat mixture in evaporating dish; invert dry funnel with cotton plug;
  • $NH_4Cl$ sublimes, condenses on funnel walls; NaCl remains as residue.

Chromatography

• Principle: differential adsorption & capillary movement on stationary phase.
• Types introduced: paper chromatography, chalk (column) chromatography.
• Ink experiment
  1. Punch hole in filter paper; insert cotton wick.
  2. Place ink drop near hole; dry.
  3. Lay paper on water surface; allow solvent front to move ~30 min.
  4. Coloured bands appear at differing distances (Rf values differ).
• Advantages
  • Requires only micrograms of sample.
  • Components remain intact for further analysis.
  • Excellent for mixtures with similar physical/chemical properties (amino acids, plant pigments, blood).

Complex Mixture Example

Mixture: ammonium chloride + common salt + sulphur

1. Add carbon disulphide (CS₂); sulphur dissolves.
2. Filter ➜ filtrate = sulphur solution; evaporate to recover sulphur crystals.
3. Residue (NH₄Cl + NaCl): heat ➜ sublimation separates $NH<em>4Cl$ (sublimate) & NaCl (residue). Flow summary: $\text{NH}</em>4Cl + NaCl + S \xrightarrow{CS<em>2} \text{solution of }S + (NH</em>4Cl+NaCl)<em>{res} \xrightarrow{sublimation} NH</em>4Cl + NaCl$

List of Important Solvents & Solutes

• Carbon disulphide: dissolves sulphur, phosphorus.
• Turpentine oil: paraffin wax, paint solvent.
• Benzene: rubber.
• Ethyl alcohol: iodine, shellac.
• Water: potassium nitrate, copper sulphate.
• Acetone: nail polish.
• Petrol: grease, chlorophyll, oils.
• Methylated spirit / oxalic acid: rust removal.

Laboratory Apparatus (Visual references)

• Funnel (filtration)
• Clamp stand & burette (titrations, support)
• Separating funnel (immiscible liquids)
• Conical & round-bottom flasks
• Evaporating dish, watch glass
• Mortar & pestle (grinding)
• Dropper pipette (small volume addition)
• Test-tube holder, boiling tube, burner

Energy Changes in Formation

• Mixture formation ≈ no significant enthalpy change.
• Compound formation
  • Exothermic: $C + O<em>2 \rightarrow CO</em>2 + 394\,kJ$ releases heat.
  • Endothermic: $6CO<em>2 + 6H</em>2O \xrightarrow{light} C<em>6H</em>{12}O<em>6 + 6O</em>2$ absorbs solar energy.

Periodic Perspective & Historical Context

• Ancient concept: four elements (earth, water, air, fire).
• Dalton (1803): Atomic Theory introduced scientific basis.
• Mendeleev (1869): Periodic Law & first periodic table predicted undiscovered elements.
• Modern Periodic Table: 118 elements, arranged by increasing atomic number, grouped as
  • Alkali metals, alkaline earth metals, transition metals, basic metals, metalloids, non-metals, halogens, noble gases, lanthanides, actinides.

Key Equations & Symbols (Quick Reference)

• Iron–sulphur mixture (physical): $Fe + S \text{ (no heat)} \rightarrow Fe/S\,\text{mixture}$ (magnet \rightarrow Fe)
• Iron sulphide compound: $Fe + S \xrightarrow{\Delta} FeS$
• Electrolysis of water: $2H<em>2O \xrightarrow{electric\,current} 2H</em>2 + O_2$
• Reduction of iron oxide: $Fe<em>2O</em>3 + 3C \xrightarrow{\Delta} 2Fe + 3CO$

Practical & Ethical Implications

• Food adulteration endangers health ➜ legislations & quality control laboratories rely on separation tests.
• Pharmaceutical industry employs high-precision chromatography & distillation to ensure dosage accuracy.
• Environmental monitoring: chromatography tracks pollutants; distillation aids desalination; sedimentation used in sewage treatment.

Study Tips & Connections

• Always link property differences to choice of separation.
• Memorise typical boiling points: $C<em>2H</em>5OH$ 80 °C, $H_2O$ 100 °C ➜ rationalise fractional distillation order.
• Recall list of sublimating solids for quick identification questions.
• Visualise apparatus setups; sketch filtration cone, Liebig condenser, fractionating column internals.
• Compare energy diagrams of mixture vs compound formation to reinforce thermodynamic distinctions.