Combined Science – Condensed Revision Notes

Experimental Design & Apparatus

  • Time – stopwatch;

  • Temperature – thermometer;

  • Mass – electronic balance;

  • Volume – burette/pipette (high accuracy), measuring cylinder, gas syringe.

  • Burette: \pm0.05\,\text{cm}^3 variable volumes;

  • Pipette: fixed volumes.

  • Gas collection:

    • gas syringe (universal);

    • displacement of water (insoluble gases);

    • downward delivery (gas M_r>30, denser than air);

    • upward delivery (lighter than air).

Separation & Purification

  • Use of solvent + filtration (solid-solid).

  • Filtration – removes insoluble solid from liquid.

  • Evaporation to dryness – for heat-stable solutes;

  • Crystallisation – heat-sensitive solutes (cool saturated solution).

  • Simple distillation – obtain solvent;

  • Fractional distillation – separate miscible liquids with different b.p.

  • Chromatography – separation by solubility;

  • pure substance = one spot;

  • start line in pencil;

  • Pure substance: fixed m.p./b.p.;

  • impurities \downarrowm.p., \uparrowb.p., range.

Kinetic Particle Theory

  • Solid: fixed shape/volume, particles closely & orderly, vibrate.

  • Liquid: fixed volume, not shape, particles close but disorderly, slide.

  • Gas: no fixed shape/volume, compressible, particles far apart, rapid.

  • Heating \uparrow kinetic energy, \downarrow attractive forces;

  • plateaus on heating/cooling curves = change of state (constant T).

Atomic Structure & Isotopes

  • Proton: charge +1, mass 1;

  • Neutron: 0, 1;

  • Electron: -1, \frac1{1840}.

  • Proton (atomic) number Z;

  • Nucleon (mass) number A.

  • Ions: loss = cation, gain = anion;

  • achieve noble gas configuration.

  • Isotopes: same Z, different neutrons;

  • same chemistry, different physical properties.

Chemical Bonding & Structures

  • Ionic: metal + non-metal, e⁻ transfer, giant lattice, high m.p./b.p., conducts when molten/aqueous (mobile ions).

  • Covalent: non-metals share e⁻, simple molecules – low m.p./b.p., non-conductors.

  • Metallic: lattice of positive ions in sea of electrons – malleable, good conductor.

  • Alloys: mixture of metal + element; different atom sizes hinder layer sliding \rightarrow harder/stronger.

Elements, Compounds & Mixtures

  • Element – single type of atom;

  • Compound – fixed ratio chemically bonded;

  • Mixture – physical combination, variable ratio, separated by physical means.

Chemical Calculations

  • M_r = sum of A_r.

  • Mole triangle: \text{mass}=\text{moles}\times \text{M}_r

  • Concentration & gas volume not required here.

Acids, Bases & Neutralisation

  • Acid: \text{H}^+ in aq;

  • Alkali: \text{OH}^- in aq.

  • Reactions:

    • Metal + acid \rightarrow salt + hydrogen.

    • Acid + carbonate \rightarrow salt + water + carbon dioxide.

    • Acid + base/alkali \rightarrow salt + water (hydrogen ion + hydroxide ion \rightarrow water).

    • Alkali + ammonium salt \rightarrow salt + ammonia + water.

  • pH: 0–14; low pH = more \text{H}^+;

  • universal indicator: red (acid) \rightarrow green (7) \rightarrow violet (alkali).

  • Soil too acidic: add \text{CaO} or \text{Ca(OH)}_2 (liming).

  • Oxides: basic (metal), acidic (non-metal), amphoteric (Al_2O_3, ZnO, PbO), neutral (CO, NO).

Qualitative Gas Tests

  • \text{H}_2 – lighted splint ‘pop’.

  • \text{O}_2 – glowing splint relights.

  • \text{CO}_2 – turns limewater milky.

  • \text{NH}_3 – damp red litmus \rightarrow blue.

Periodic Table & Groups

  • Arranged by increasing Z. Period = shells;

  • Group = valence electrons \rightarrow similar chemistry.

  • Across period: metallic \rightarrow non-metallic.

  • Group 1 (Li, Na, K): soft, low d, \downarrowm.p. down group, reactivity \uparrow;

  • react with water \rightarrow hydroxide + \text{H}_2.

  • Group 17 (Cl_2, Br_2, I_2): coloured diatomic gases/liquid/solid;

  • m.p./b.p. \uparrow down group, reactivity \downarrow;

  • more reactive halogen displaces less reactive from halide.

  • Group 18: noble gases – inert, monatomic, full outer shell.

Reactivity Series & Corrosion

  • Order (most \rightarrow least): K Na Ca Mg Al (C) Zn Fe Pb (H) Cu Ag Au.

  • Only metals above H react with dilute acids (Pb sluggish due insoluble salt layer).

  • Extraction: above C – electrolysis;

  • below C – reduction with carbon;

  • Au native.

  • Rusting of Fe needs \text{O}_2 and \text{H}_2 ext{O};

  • prevented by barrier (paint, grease) or sacrificial metal (Zn/Mg).

Organic Chemistry Essentials

  • Homologous series: same functional group, general formula, gradation of physical props, \Delta \text{CH}_2.

  • Alkanes (saturated) C_nH_{2n+2}: mainly unreactive;

  • combustion;

  • substitution with Cl_2 (UV).

  • Alkenes (unsaturated) C_nH_{2n}: more reactive;

  • combustion, addition (H_2/Ni 200^\circC – hydrogenation; Br_2 test – decolourises; polymerisation).

  • Cracking: large alkanes \rightarrow smaller alkanes + alkenes + \text{H}_2 (600^\circC, Al_2O_3/SiO_2);

  • meets demand for petrol & alkenes.

  • Poly(ethene): addition polymer of ethene;

  • uses – plastic bags, bottles, clingfilm.

  • Plastic disposal issues: non-biodegradable (landfill), toxic fumes on burning;

  • recycle by melting into pellets (physical) or cracking to fuels (chemical).

Fuels & Biofuels

  • Crude oil – mixture of hydrocarbons;

  • fractional distillation gives: gas, petrol, naphtha, kerosene, diesel, lubes, bitumen.

  • Natural gas = mainly methane.

  • Bioethanol from sugarcane fermentation: renewable; \text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O};

  • \text{CO}_2 uptake during growth offsets emissions.

Air Quality & Pollutants

  • Dry air: 78\%\,\text{N}_2, 21\%\,\text{O}_2, \approx1\% others (Ar, \text{CO}_2).

  • Pollutants & sources:

    • Incomplete combustion of fuels causes carbon monoxide (\text{CO}), which is toxic because it binds to hemoglobin.

    • Sulfur dioxide (\text{SO}_2) is caused by volcanoes and burning sulfur fuels, leading to acid rain.

    • Lightning and high engine temperatures cause nitrogen oxides (NO/NO_2), which contribute to acid rain