AQA · GCSE · Chemistry

• Atoms are the building blocks: protons and neutrons in a tiny nucleus, electrons in shells; the outer shell runs the chemistry.
• The periodic table orders elements by atomic number; groups share outer-shell electrons and so share properties.
• Group 0 is unreactive, Group 1 gets more reactive down, Group 7 less; transition metals (chemistry only) behave differently from Group 1.

All substances are made of atoms 原子 — the smallest part of an elemento that can exist. Each element has a chemical symbol (O, Na, …); about 100 elements exist, shown in the periodic table.

Compounds 化合物 form when elements combine chemically in fixed proportions; they are represented by formulae and can only be separated back into elements by chemical reactions. Chemical reactions always form one or more new substances, often with a detectable energy change. Equations: word equations e symbol equations with balanced formulae; (HT) balanced half-equations and ionic equations.

Mixtures 混合物: two or more elements or compounds not chemically combined; each substance keeps its properties. Separated by physical processes only — no new substances:

• filtration 过滤 — insoluble solid from liquid;
• crystallisation 结晶 — dissolved solid from solution;
• simple distillation 蒸馏 简单蒸馏 — liquid from dissolved solid (and solvents);
• fractional distillation 分馏 — liquids with different boiling points;
• chromatography 色谱法 — soluble substances in a mixture.

Development of the atom model: atoms were first thought to be indivisible spheres → the electron's discovery gave the plum pudding model 布丁模型 (ball of positive charge with negative electrons embedded) → the alpha-scattering experiment showed mass and positive charge concentrated in a central nucleus (nuclear model) → Bohr: electrons orbit at specific distances → protons identified in the nucleus → Chadwick: the nêutron 中子. New evidence changes or replaces models.

In an atom protons = electrons, so no overall charge. Atomic number = protons; mass number = protons + neutrons. Atoms of the same element with different neutron numbers are isotopes 同位素. Atom radius ≈ 0.1 nm (1 × 10⁻¹⁰ m); the nucleus is less than 1/10 000 of that. Almost all mass is in the nucleus.

Relative atomic mass (Ar) averages over the isotopes' abundance — calculate it from percentage abundances. Electronic structure: electrons fill the lowest levels first — sodium is 2,8,1; represent the first twenty elements both as numbers and diagrams.

The table orders elements by atomic (proton) number 原子序数, so that elements with similar properties fall in groups (columns). For main-group elements, group position reflects outer-shell electrons 最外层电子 and similar chemical properties; helium has two outer electrons, while the other Group 0 elements have eight. Explain an element's position from its electronic structure; predict probable reactivity from position.

Development: early tables ordered by atomic weight — incomplete, with some elements misplaced. Mendeleev para a esquerda gaps for undiscovered elements and sometimes changed the order; his predicted elements were found, supporting his table. Isótopos later explained why atomic-weight order was sometimes wrong.

Metals and non-metals: metals usually lose electrons to form positive ions; many non-metals gain electrons to form negative ions or share electrons in covalent bonds. Metals sit left and lower; non-metals right and upper. Know the characteristic physical and chemical differences, and link atomic structure to position.

Group 0 — noble gases: unreactive — stable outer-shell arrangements (8 outer electrons; helium 2), so they do not easily form molecules. Boiling points aumentar down the group (with relative atomic mass).

Group 1 — alkali metals: one outer electron. Reactions of Li, Na, K with oxygen, cloro e água (e.g. 2 Na + 2 H₂O → 2 NaOH + H₂). Reactivity increases down the group — the outer electron is lost more easily further from the nucleus.

Group 7 — halogens: seven outer electrons; non-metals, diatomic molecules (Cl₂, Br₂, I₂). Reactivity decreases down the group; melting and boiling points aumentar. A more reactive halogen displaces 置换 a less reactive one from a salt solution (chlorine displaces bromine from potassium bromide).

Compared with Group 1, the transition elements (exemplify with Cr, Mn, Fe, Co, Ni, Cu) have higher melting points, densities, strength and hardness, and are less reactive with oxygen, water and halogens.

Typical properties: ions with different charges (Fe²⁺/Fe³⁺), coloured compounds, and use as catalysts 催化剂 (iron in the Haber process).

• p/n/e table, atomic and mass numbers, isotopes, Ar from abundance, 2,8,x structures for the first 20.
• The atom-model timeline: sphere → plum pudding → nuclear → Bohr → proton → neutron.
• Five separation methods each matched to a mixture; Mendeleev's gaps and the isotope explanation.
• Group 0/1/7 trends with electron explanations; halogen displacement equations.
• (Chem) transition metals vs Group 1: four differences plus ions, colours, catalysts.

• Three bonds hold matter together: ionic (transferred electrons, charged ions), covalent (shared electron pairs) and metallic (delocalised electrons).
• Structure follows bonding: giant ionic lattices, small molecules, polymers, giant covalent structure networks and metal lattices — each predicts melting point and conductivity.
• Carbon shows every trick: diamond, graphite, graphene, fullerenes; nanoparticles (chemistry only) turn surface area into properties.

Three strong chemical bonds, all explained by electrostatic forces:

• Ionic 离子键 — a metal atom transfers outer electrons to a non-metal: metal atoms lose electrons to become íons positivos; non-metal atoms gain them to become negative ions; Groups 1/2 metals and 6/7 non-metals form ions with noble-gas electronic structures. Draw the electron transfer with dot and cross diagrams; deduce ion charges from group number (Group 1 → +1, Group 2 → +2, Group 6 → −2, Group 7 → −1).
• Covalent 共价键 — non-metal atoms share pairs of electrons. Know dot-and-cross diagrams for H₂, Cl₂, O₂, N₂, HCl, H₂O, NH₃, CH₄, and line representations for small molecules, polymer repeating units and giant covalent structures.
• Metallic 金属键 — a giant lattice of metal atoms with delocalised outer electrons 游离电子 free to move through the whole structure; the sharing of these electrons gives strong metallic bonding.

Know the limitations of models: dot-and-cross, ball-and-stick, 2D and 3D diagrams all simplify — no forces shown, fixed bond lengths, giant structures drawn as small fragments. Deduce empirical formulae from lattice models; molecular formulae from molecule diagrams.

Estados da matéria: solid, liquid, gas; melting/freezing at the melting point, boiling/condensing at the boiling point. Particle theory (small solid spheres) explains the changes; the stronger the forces between particles, the higher the melting/boiling points. (HT) The simple model's limits: no forces shown, all spheres, solid particles. State symbols: (s), (l), (g), (aq).

Larger molecules → stronger intermolecular forces → higher melting/boiling points. Alloys 合金 are harder than pure metals because different-sized atoms distort the layers, stopping them sliding.

• Diamante — a giant covalent structure 巨型共价结构 in which each carbon forms four covalent bonds: very hard, very high melting point, does not conduct.
• Grafite — each carbon forms three bonds, layers of hexagonal rings with no covalent bonds between layers (soft, slippery — lubricant); one delocalised electron per atom → conducts electricity like a metal.
• Graphene — a single layer of graphite: one atom thick, strong, conducts — electronics and composites.
• Fullerenes — hollow molecules of carbon hexagons (plus 5- or 7-membered rings); Buckminsterfullerene C₆₀ 富勒烯 is spherical. Carbon nanotubes 碳纳米管 — cylindrical fullerenes with huge length-to-diameter ratios: nanotechnology, electronics, materials.

Nanoscience = structures 1–100 nm (a few hundred atoms). Nanoparticles 纳米颗粒 are smaller than fine particles (PM2.5, 100–2500 nm), which are smaller than coarse particles/dust (PM10, 1 × 10⁻⁵–2.5 × 10⁻⁶ m).

Área superficial : volume: as a cube's side decreases 10×, SA:V increases 10×. Nanoparticles may have different properties from the bulk material because of this high ratio — catalysts (smaller quantities work), medicine delivery, cosmetics, electronics. Concerns: effects inside the body and in the environment are not fully known.

• The three bonds in terms of electrons and electrostatic forces; ion charges from group numbers.
• Dot-and-cross for the eight named molecules; model limitations named.
• The properties table above rebuilt from memory — which forces are overcome on melting.
• Diamond vs graphite vs graphene vs fullerenes, bonding to property.
• (Chem) SA:V factor-of-10 rule; the three particle-size bands and nanoparticle uses and concerns.

• Atoms are neither lost nor made: the balanced equation is a mass ledger, and conservation of mass closes every account.
• The mole converts grams into particle counts; moles turn equations into reacting-mass arithmetic.
• Yield, atom economy, concentration (chem only) and gas volumes (HT) finish the quantitative toolkit.

Conservação da massa 质量守恒: no atoms are lost or made, so the mass of products equals the mass of reactants. Symbol equations are balanceada in atom numbers; know the difference between a multiplier before a formula (numbers of units) and a subscript within it (atoms in the unit).

Relative formula mass (Mr) 相对分子质量 = sum of the relative atomic masses in the numbers shown. In a balanced equation, ΣMr of reactants = ΣMr of products. Percentage by mass of an element = (Ar × number of atoms ÷ Mr) × 100 %.

Mass changes with gases: metal + oxygen → oxide gains mass; thermal decomposition of a carbonate loses the escaped CO₂ — no law is broken once the gas is counted.

Uncertainty: every measurement carries uncertainty; use the range of repeated measurements about the mean as its estimate (and mean ± range/2 in calculations).

O mol 摩尔: the mass of one mole of a substance in grams is numerically its Mr. One mole contains the Avogadro constant 阿伏伽德罗常数 of particles — 6.02 × 10²³ — the same count of stated particles (atoms, molecules, ions) as a mole of any other substance.

Equations as mole ratios: Mg + 2 HCl → MgCl₂ + H₂ reads "1 mol Mg reacts with 2 mol HCl → 1 mol MgCl₂ + 1 mol H₂". Given any one mass, calculate all others: mass → moles → ratio → moles → mass.

Balancing from masses: convert each mass to moles, divide by the smallest, clear fractions to whole numbers.

Limiting reactant 限量反应物: the reactant completely used up limits the product; an excess of the other ensures completion. Calculate the product from the limiting reactant's moles only.

Concentração 浓度: in g/dm³ for all tiers — mass of solute in a given volume; (HT) also mol/dm³: moles = concentration × volume(dm³), rearranged as needed.

Percentage yield 产率 is below 100 % because: the reaction is reversible; product is lost on separation; reactants react in unwanted ways.

Atom economy 原子经济 measures how much of the starting material ends up in the useful product — high atom economy matters for sustainability and cost:

(HT) calculate the theoretical mass from the balanced equation, then the yield.

Concentration in mol/dm³ links moles, mass and volume: moles = C × V; from a titration 滴定, knowing the volumes of both solutions and one concentration gives the other — moles of acid = moles of alkali at neutralisation (respect the ratio in the equation).

A given volume of gas contains the same number of moles at the same temperature and pressure — equal volumes = equal moles. The volumes of reacting gases (and products) follow the equation's ratio directly, e.g. 2 volumes of hydrogen react with 1 volume of oxygen.

• Balance equations; Mr and percentage-by-mass sums; explain apparent mass changes with gases.
• (HT) mole ↔ mass conversions, Avogadro constant, ratio chains, balancing from masses, limiting reactant.
• (Chem/HT) percentage yield with its three reasons; atom economy formula; titration concentration; gas-volume ratios.

• Rate is a quantity per second — read it off tables, graphs and tangents; collision theory 碰撞理论 explains every factor that changes it.
• Reversible reactions settle into dynamic equilibrium 动态平衡; (HT) Le Chatelier predicts how each condition shifts it.
• Industry optimises rate and yield together — compromise conditions.

Calculating rates: rate of reaction 反应速率 = quantity of reactant used ÷ time, or product formed ÷ time — in g/s ou cm³/s (HT also mol/s). Interpret product-vs-time graphs (steeper start, flattening as reactant runs out); draw tangents 切线 and use their gradient as the rate at that instant (HT calculate it).

Factors affecting rate: concentração (solution), pressão (gas), surface area (solid), temperatura, catalisador 催化剂s. RP5: investigate concentration by (a) measuring the volume of gas produced and (b) observing a colour/turbidity change — hypothesis, variables, repeats.

Collision theory: reactions occur only when particles collide with at least the activation energy 活化能. Raising concentration/pressure crowds particles — more frequent collisions; smaller solid pieces raise the surface-area-to-volume ratio — more exposed surface, more frequent collisions; raising temperature gives particles more energy — more collisions AND more collisions that pass the activation energy.

Catalisadores: change the rate but are not used up; each reaction has its own catalyst; enzymes are biological catalysts. A catalyst offers an alternative pathway with lower activation energy — identify it by speeding the reaction yet never appearing in the equation. Its reaction profile keeps the same overall energy change with a lower hump:

Reversible reactions 可逆反应: the products can react back to the reactants — written with the ⇌ arrow. If exothermic one way, endotérmica the other, transferring the same amount of energy.

Equilíbrio: in a closed system, equilibrium is reached when the forward and reverse rates are equal — the concentrations stop changing though both reactions continue (dynamic).

(HT) Le Chatelier's principle 勒夏特列原理: a system at equilibrium responds to counteract any change:

• concentração ↑ of a reactant → more produto forms (and vice versa);
• pressão ↑ → the position moves to the side with fewer gas molecules;
• temperatura ↑ → the position moves in the endothermic direction (↓ for exothermic direction).

A catalyst does not shift the position — it reaches equilibrium faster. Industry picks compromise conditions balancing rate, yield, safety and cost (e.g. the Haber process in topic 10).

• Rate from a table, from a graph's steepness, and (HT) from a tangent gradient.
• Each of the five factors explained by collision theory, naming what happens to collision frequency and energy.
• Catalyst effect on the profile; not used up; enzymes.
• The ⇌ arrow; equilibrium as equal rates; (HT) predict all three condition changes with Le Chatelier; compromise conditions justified.

• A pure substance 纯净物 in chemistry means one element or compound — melting and boiling points prove it; formulation 配方s are deliberate mixtures.
• Chromatography separates by distribution between phases — Rf values identify the spots.
• Gas tests (H₂, O₂, CO₂, Cl₂) and (chemistry only) ion tests — flame colours, hydroxide precipitate 沉淀s, carbonates, halides, sulfates — build the analyst's toolkit.

Pure substance — a single element or compound, not mixed with anything. Pure substances melt and boil at specific, sharp temperatures; mixtures melt over a range — use melting/boiling point data to tell them apart. (In everyday speech "pure" means unadulterated — pure milk — a different sense.)

Formulations — mixtures designed as useful products, each component in carefully measured amounts for a purpose: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods. Identify one from given information.

Cromatografia 色谱法 — a fase estacionária 固定相 (the paper) and a mobile phase 流动相 (the solvent); separation depends on the distribution of each substance between the phases — those more attached to the stationary phase stay lower. Measure:

Different compounds have different Rf values in different solvents — use them to identify; a pure compound gives a single spot in every solvent.

Flame tests (cations): lithium — crimson; sodium — yellow; potassium — lilac; calcium — orange-red; copper — green. In a mixture some colours mask others.

Metal hydroxides (sodium hydroxide solution): Al³⁺, Ca²⁺, Mg²⁺ give white precipitates — but only Al(OH)₃ dissolves in excess NaOH; Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown precipitates.

Carbonatos: add dilute acid — effervescence; the gas turns limewater milky.

Halides: add dilute nitric acid then silver nitrate — AgCl white, AgBr cream, AgI yellow precipitates.

Sulfatos: add dilute hydrochloric acid then barium chloride — a white precipitate of BaSO₄.

(HT) Flame emission spectroscopy 光谱法: instrument draws the line emission spectrum; the positions of the lines identify the metal, their intensity its concentration. Advantages over flame test 焰色试验s: very sensitive, works on mixtures, rapid.

• Pure vs mixture by melting point; name five formulations.
• Rf calculated from a chromatogram; why a pure compound gives one spot.
• The four gas tests with results.
• (Chem) flame colours; hydroxide colours with the aluminium-in-excess trick; carbonate/halide/sulfate reagents and precipitate colours; (HT) spectroscopy advantages.

• The early atmosphere came from volcanic activity — its CO₂ steam-cooled to the oceans; photosynthesis 光合作用 (algae, plants) raised oxygen and buried carbon.
• Greenhouse gases — CO₂ and methane — trap radiation; human activity raises them and the global climate change 全球气候变化s.
• Pollutants from fuels — CO, SO₂, NOₓ, particulates 颗粒物 — each with a source, a chemistry and a consequence.

Theories: the early atmosphere cannot be known for certain — evidence comes from gases trapped in ancient ice and from other planets' atmospheres today. Intense volcanic activity released gases: mostly carbon dioxide, with water vapour, nitrogen and traces of methane and ammonia — little or no oxygen.

The water vapour condensed to form the oceans; CO₂ dissolved in them and was locked into carbonate sediments (shells). Over ~2.7 billion years, algae and plants photosynthesised, absorbing CO₂ and releasing oxygen — oxygen rose, CO₂ fell; much carbon was buried as fossil fuel 化石燃料s. Nitrogen, unreactive, accumulated to dominate today's air: about four-fifths N₂, one-fifth O₂, plus argon and ~0.04 % CO₂.

Greenhouse gases 温室气体 — carbon dioxide, methane and water vapour — absorb and re-emit some of the long-wavelength infrared radiation that the warm Earth radiates to space, keeping the surface warmer than it would otherwise be. The global climate change hypothesis says rising greenhouse-gas levels raise the average temperature.

Human causes: deforestation (less CO₂ absorbed), burning fossil fuels (more CO₂), cattle/rice and landfill (methane). Consequences discussed in the spec: melting ice, sea-level rise, changing rainfall and extreme weather. The scientific consensus rests on peer-reviewed evidence — interpret given data on temperature, CO₂ and methane over time.

Combustion of fuels releases:

• carbon monoxide (CO) — a toxic gas from incomplete combustion 不完全燃烧 in limited oxygen; binds haemoglobin, reducing oxygen transport;
• carbon particulates (soot) — also from incomplete combustion; worsen respiratory problems and cause global dimming;
• sulfur dioxide (SO₂) e oxides of nitrogen (NOₓ) — from impurities in fuel / nitrogen reacting at engine temperatures; cause acid rain 酸雨 (damaging trees, lakes, buildings) — removed from flue gases by neutralisation with alkalis (e.g. calcium oxide);
• NOₓ also forms at high engine temperatures from N₂ + O₂, and contributes to photochemical smog.

Complete combustion — plenty of oxygen — gives CO₂ and water only.

• The evolution sequence with the evidence and its uncertainty; today's composition in fractions.
• Greenhouse mechanism in terms of wavelength; the four human causes; consensus and data interpretation.
• Each pollutant: source, complete vs incomplete combustion, effect, and one mitigation.

• Resources are used, reused and recycled; potable water 饮用水 — filtered and sterilised fresh water, or desalinated sea water — is the first essential.
• Life cycle assessment 生命周期评估s weigh a product's environmental cost stage by stage; reduce–reuse–recycle cuts every stage.
• (Chem) corrosion 腐蚀 prevention, alloys, ceramics/polymers/composite 复合材料s, and the Haber process 哈伯法's compromise chemistry.

Humans use the Earth's resources for warmth, shelter, food and transport; finite resources (ores, fossil fuels) are processed for energy and materials; renewable ones replenish. Sustainable development 可持续发展 meets present needs without compromising future generations.

Potable water — safe to drink: low dissolved salts and microbes — but not pure water (it contains dissolved substances).

In the UK, fresh rainwater is made potable by: choosing a source → filter beds → sterilising (with chlorine, ozone or UV light). Where fresh water is scarce, desalination — distillation ou reverse osmosis — needs large amounts of energy.

Waste water (sewage) treatment: screening and grit removal → sedimentation (sewage sludge + effluent) → anaerobic digestion of the sludge → aerobic biological treatment of the effluent. Potable water is easiest from ground water, harder from waste, hardest (energy-wise) from salt water.

(HT) Alternative metal extraction — copper ores are scarce: phytomining (plants absorb metal compounds; harvest, burn to ash, extract) and bioleaching (bacteria produce leachate solutions of the metal compounds) avoid moving huge amounts of rock.

LCA stages: extracting and processing raw materials; manufacturing and packaging; use and operation; disposal — including transport at each stage. Energy, water, resource use and waste are quantifiable; pollutant effects need value judgements — so LCAs are not purely objective, and selective LCAs can be misused (advertising). Compare plastic vs paper shopping bags.

Reduce, reuse, recycle: metals, glass, building materials, clay ceramics and most plastics come from limited raw materials; recycling cuts mining/quarrying impact and energy — e.g. glass crushed and remelted; scrap steel added to the blast furnace reduces iron-ore extraction. Some products (glass bottles) are reused; others recycled into different products; separation effort depends on the final product's requirements.

Corrosion — destruction of materials by chemical reaction with the environment (e.g. iron + oxygen + water → rust). Prevention: greasing, painting, coating (galvanising with zinc, which gives sacrificial protection 牺牲保护 — it corrodes in place of the iron even when scratched) and alloying (stainless steel). Aluminium resists corrosion by its own protective oxide layer.

Alloys: bronze (copper + tin), brass (copper + zinc); gold jewellery alloyed with silver/copper/zinc — purity in carats (24 = 100 %; 18 = 75 %). Steels — iron + carbon (+ metals): high-carbon steel strong but brittle; low-carbon steel softer, shaped easily; stainless steel (Cr, Ni) hard and corrosion-resistant; aluminium alloys low density (aircraft).

Ceramics, polymers, composites: soda-lime glass (sand + sodium carbonate + limestone), borosilicate glass (sand + boron trioxide — higher melting point); clay ceramics (pottery, bricks — shaped wet clay, then fired). Polymers depend on monomers and conditions — LD and HD poly(ethene) both from ethene. Thermosoftening polymers melt on heating (recyclable chains); thermosetting polymer 热固性聚合物 does not (cross-links). Composites — a matrix/binder surrounding a reinforcement (fibres/fragments); know examples (fibreglass, concrete, carbon-fibre).

Haber process: N₂ + 3 H₂ ⇌ 2 NH₃ — nitrogen from air, hydrogen from natural gas (methane + steam) [or electrolysis of water]. Conditions: iron catalyst, ~450 °C, ~200 atmospheres; the reaction is reversible — cool the mixture, the ammonia liquefies and is removed; unreacted N₂/H₂ are recycled.

(HT) Higher pressure favours ammonia (fewer gas molecules) but is expensive and unsafe; lower temperature favours the exotérmica forward reaction but is slow — so the chosen conditions are a compromise between rate, yield and cost. Apply Le Chatelier to each condition change.

NPK fertilisers — formulations of nitrogen, phosphorus and potassium compounds for plant growth: ammonia → nitric acid (Ostwald); ammonia + nitric acid → ammonium nitrate; potassium chloride/potassium sulfate mined; phosphate rock treated with acid to make superphosphate. Evaluate the industrial production of fertilisers given data (raw materials, energy costs, % yield — e.g. the lab vs industrial ammonium sulfate routes).

• Potable vs pure; the UK treatment sequence; desalination and its cost; sewage-treatment steps in order.
• (HT) phytomining and bioleaching described end to end.
• LCA four stages with the objectivity caveat; reduce-reuse-recycle examples with reasons.
• (Chem) corrosion prevention incl. sacrificial zinc; named alloys with compositions; thermosoftening vs thermosetting; matrix + reinforcement.
• (Chem/HT) Haber conditions with the compromise explained by equilibrium; the NPK compounds and their origins.