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 $\downarrow$m.p., $\uparrow$b.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$_2$O$_3$, 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, $\downarrow$m.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$^\circ$C – hydrogenation; Br$_2$ test – decolourises; polymerisation).

• Cracking: large alkanes $\rightarrow$ smaller alkanes + alkenes + $\text{H}_2$ (600$^\circ$C, Al$_2$O$_3$/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