📚 GCSE WJEC Chemistry: Last-Minute Revision Notes | GCSE WJEC 化学：考前冲刺笔记
This concise revision guide covers the key topics for the WJEC GCSE Chemistry specification. It is designed to help you refresh the most important concepts, equations, and common exam pitfalls quickly and effectively before the big day.
这份考前冲刺指南覆盖了 WJEC GCSE 化学大纲的核心专题，旨在帮助你在考前快速且有重点地梳理最重要的概念、方程式和常见失分点。
1. Atomic Structure and the Periodic Table | 原子结构与元素周期表
All matter is made of atoms. Each atom has a tiny, dense nucleus containing protons (relative charge +1, relative mass 1) and neutrons (charge 0, mass 1), surrounded by electrons (charge −1, mass ~1/1840) in shells.
所有物质由原子构成。原子有一个极小的致密原子核，内含质子（相对电荷 +1，相对质量 1）和中子（电荷 0，质量 1），核外电子（电荷 −1，质量约为质子的 1/1840）分层排布。
Atomic number (Z) equals the number of protons; mass number (A) equals protons + neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
原子序数（Z）= 质子数；质量数（A）= 质子数 + 中子数。同位素是同种元素中质子数相同但中子数不同的原子。
Elements are arranged in the modern periodic table in order of increasing atomic number. The group number tells you the number of outer-shell electrons, and the period number gives the number of occupied shells. For WJEC, know the patterns: Group 1 alkali metals become more reactive down the group; Group 7 halogens become less reactive down the group; Group 0 noble gases are inert due to full outer shells.
现代元素周期表按原子序数递增排列。族序数对应最外层电子数，周期数对应电子层数。WJEC 考试要求掌握以下规律：第 1 族碱金属从上到下反应性增强；第 7 族卤素从上到下反应性减弱；第 0 族稀有气体因最外层已满而性质稳定。
2. Bonding, Structure and Properties | 化学键、结构与性质
Ionic bonding occurs between metals and non‑metals: electrons are transferred, forming oppositely charged ions held by strong electrostatic forces. Ionic compounds have giant ionic lattices, high melting/boiling points, and conduct electricity when molten or dissolved in water because the ions are free to move.
离子键存在于金属与非金属之间：电子转移形成带相反电荷的离子，通过强静电引力结合。离子化合物具有巨型离子晶格、高熔沸点，在熔融或水溶液中因离子自由移动而导电。
Covalent bonding happens when non‑metal atoms share pairs of electrons to achieve a full outer shell. Simple molecular substances (e.g., H₂O, CO₂, CH₄) have low melting/boiling points due to weak intermolecular forces; giant covalent structures (e.g., diamond, SiO₂) have extremely high melting points because strong covalent bonds extend throughout the structure. Graphite contains layers that can slide, so it conducts electricity along layers and is soft.
共价键是非金属原子通过共用电子对以达到最外层电子稳定结构。简单分子物质（如 H₂O、CO₂、CH₄）因分子间作用力弱而熔沸点低；巨型共价结构（如金刚石、SiO₂）因强大的共价键贯穿整个结构而熔点极高。石墨具有可滑动层，沿层方向可导电，质地软。
Metallic bonding involves a ‘sea’ of delocalised electrons surrounding positive metal ions. This explains why metals are good conductors of heat/electricity, are malleable and ductile. For WJEC, be able to draw dot‑and‑cross diagrams for ionic and covalent compounds and link structure to physical properties.
金属键由离域电子“海洋”与正金属离子构成。这解释了金属的良好导电、导热性以及延展性。WJEC 要求能画出离子和共价化合物的点叉图，并能将结构与物理性质联系起来。
3. Formulae, Equations and Stoichiometry | 化学式、方程式与计量关系
You must be able to write word equations, balanced symbol equations and ionic equations. Use state symbols: (s), (l), (g), (aq). Remember that charges and total number of atoms must balance on both sides.
必须会写文字方程式、配平的符号方程式和离子方程式。使用状态符号：(s) 固体，(l) 液体，(g) 气体，(aq) 溶液。记住两边电荷和原子总数必须守恒。
Relative formula mass (Mᵣ) is the sum of relative atomic masses (Aᵣ) of all atoms in a formula. The mole (mol) is the amount of substance that contains 6.02 × 10²³ particles. Number of moles = mass (g) ÷ Mᵣ.
相对式量（Mᵣ）是化学式中所有原子的相对原子质量（Aᵣ）之和。摩尔（mol）是包含 6.02 × 10²³ 个粒子的物质的量。摩尔数 = 质量 (g) ÷ 相对式量。
Use balanced equations to calculate reacting masses. For WJEC, practise percentage yield and atom economy calculations: % yield = (actual mass/theoretical mass) × 100; atom economy = (Mᵣ of desired product / sum of Mᵣ of all reactants) × 100. Often examined with industrial processes.
利用配平方程式计算反应质量。WJEC 常考百分产率和原子经济性：% 产率 = (实际产量/理论产量) × 100；原子经济性 = (所需产物的 Mᵣ / 所有反应物 Mᵣ 之和) × 100。常结合工业流程考查。
4. Acids, Bases and Salts | 酸、碱与盐
Acids release H⁺ ions in aqueous solution; alkalis release OH⁻ ions. The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 neutral. A neutralisation reaction: H⁺(aq) + OH⁻(aq) → H₂O(l).
酸在水溶液中释放 H⁺ 离子，碱释放 OH⁻ 离子。pH 标度从 0（强酸性）到 14（强碱性），7 为中性。中和反应：H⁺(aq) + OH⁻(aq) → H₂O(l)。
Salts can be made by reacting an acid with a metal, a base (e.g. metal oxide/hydroxide) or a carbonate. Soluble salts are often prepared by titration; insoluble salts by precipitation. Learn the general equations: acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide.
盐可通过酸与金属、碱（如金属氧化物/氢氧化物）或碳酸盐反应制得。可溶盐通常用滴定法制备；不溶盐用沉淀法。记住通式：酸 + 金属 → 盐 + 氢气；酸 + 碱 → 盐 + 水；酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
5. Electrolysis | 电解
Electrolysis uses direct current to drive a non‑spontaneous reaction. The electrolyte is an ionic compound that is molten or dissolved so ions can move. The cathode (negative electrode) attracts cations; reduction occurs here (gain of electrons). The anode (positive electrode) attracts anions; oxidation occurs here (loss of electrons).
电解利用直流电驱动非自发反应。电解质是熔融或溶解状态的离子化合物，使离子能自由移动。阴极（负极）吸引阳离子，发生还原（得电子）。阳极（正极）吸引阴离子，发生氧化（失电子）。
For aqueous solutions, the product at each electrode depends on the reactivity of the ion. In the electrolysis of brine (concentrated NaCl solution), chlorine gas forms at the anode, hydrogen at the cathode, leaving sodium hydroxide. Know half equations: at cathode, 2H⁺ + 2e⁻ → H₂; at anode, 2Cl⁻ → Cl₂ + 2e⁻. Aluminium extraction: Al₂O₃ is dissolved in molten cryolite to lower the operating temperature; Al³⁺ + 3e⁻ → Al at the cathode, 2O²⁻ → O₂ + 4e⁻ at the anode.
在水溶液中，电极产物取决于离子的反应性。电解食盐水（浓 NaCl 溶液）时，阳极产生氯气，阴极产生氢气，溶液中留下氢氧化钠。掌握半方程式：阴极 2H⁺ + 2e⁻ → H₂；阳极 2Cl⁻ → Cl₂ + 2e⁻。铝的冶炼：Al₂O₃ 溶于冰晶石熔体以降低温度；阴极：Al³⁺ + 3e⁻ → Al，阳极：2O²⁻ → O₂ + 4e⁻。
6. Energy Changes: Exothermic and Endothermic | 能量变化与放热/吸热反应
In an exothermic reaction, energy is transferred to the surroundings (temperature rises). Examples: combustion, neutralisation, respiration. In an endothermic reaction, energy is taken in (temperature falls). Examples: thermal decomposition, photosynthesis. Reaction profiles show activation energy (Eₐ) as the minimum energy needed for a reaction to occur.
放热反应向环境释放能量（温度升高），例如燃烧、中和、呼吸作用。吸热反应从环境吸收能量（温度降低），例如热分解、光合作用。反应剖面图中，活化能（Eₐ）是反应发生所需的最低能量。
Energy changes can be calculated using bond energies: ΔH = sum of energy needed to break bonds in reactants − sum of energy released when forming bonds in products. If the bond breaking requires more energy than bond forming releases, the reaction is endothermic; the reverse is exothermic. Practice WJEC‑style numerical questions.
能量变化可用键能计算：ΔH = 断裂反应物化学键吸收的能量 − 形成生成物化学键释放的能量。若断键吸热多于成键放热，反应为吸热；反之为放热。多练习 WJEC 风格的计算题。
7. Rates of Reaction and Equilibrium | 化学反应速率与平衡
Rate of a reaction is the change in amount of reactant or product per unit time. Collision theory states that particles must collide with sufficient energy (≥ Eₐ) and correct orientation for a reaction to occur. Increasing temperature, concentration/pressure, or surface area raises the frequency of successful collisions. A catalyst speeds up a reaction by providing an alternative pathway with lower activation energy, without being used up.
反应速率是单位时间内反应物或产物量的变化。碰撞理论指出，粒子必须发生有效碰撞（能量 ≥ Eₐ 且取向正确）才能反应。升高温度、增加浓度/压强或增大固体表面积均可提高有效碰撞频率。催化剂通过提供更低活化能的替代路径加快反应，自身不被消耗。
For reversible reactions at equilibrium in a closed system, the forward and reverse rates are equal. Le Chatelier’s principle: if a system at equilibrium is subjected to a change in concentration, temperature or pressure, the equilibrium shifts to oppose the change. For the Haber process N₂ + 3H₂ ⇌ 2NH₃, high pressure favours forward (fewer moles), but a compromise is made; iron catalyst increases rate. WJEC often asks for interpretation of equilibrium graphs.
在封闭体系中，可逆反应达到平衡时正逆速率相等。勒夏特列原理：如果改变浓度、温度或压强，平衡会向着减弱这种改变的方向移动。哈伯法：N₂ + 3H₂ ⇌ 2NH₃，加压有利于正向（气体分子数减少），但实际采用折中条件；铁催化剂提高速率。WJEC 常要求解读平衡曲线图。
8. Organic Chemistry: Hydrocarbons and Polymers | 有机化学导论
Crude oil is a mixture of hydrocarbons separated by fractional distillation according to boiling points. Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons. They undergo complete combustion: hydrocarbon + O₂ → CO₂ + H₂O. Incomplete combustion produces CO or C (soot).
原油是烃类的混合物，通过分馏按沸点分离。烷烃（通式 CₙH₂ₙ₊₂）是饱和烃。完全燃烧：烃 + O₂ → CO₂ + H₂O。不完全燃烧产生 CO 或 C（炭黑）。
Alkenes (CₙH₂ₙ) contain a C=C double bond, making them unsaturated. They decolourise bromine water (addition reaction). Cracking breaks long‑chain alkanes into shorter alkanes and alkenes; catalytic cracking uses heat and a catalyst. Alkenes can form addition polymers: e.g., poly(ethene) from ethene monomers. Know how to show repeating unit and identify polymer from monomer.
烯烃（通式 CₙH₂ₙ）含有 C=C 双键，属不饱和烃，能使溴水褪色（加成反应）。裂化将长链烷烃断裂为短链烷烃和烯烃；催化裂化需加热并使用催化剂。烯烃可通过加聚反应生成聚合物，如乙烯单体聚合为聚（乙烯）。要会写出重复单元，能根据聚合物推断单体。
9. Chemical Analysis and Tests | 化学分析与测试
Gas tests: oxygen relights a glowing splint; hydrogen gives a squeaky pop with a lit splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper. These are standard WJEC required practicals.
气体检验：氧气使带火星木条复燃；氢气点燃有“噗”声；二氧化碳使石灰水变浑浊；氯气使湿润石蕊试纸褪色。这些都是 WJEC 规定实验。
Flame tests: Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange‑red, Cu²⁺ blue‑green. For metal ions in solution, add sodium hydroxide: Cu²⁺ gives blue precipitate; Fe²⁺ dirty green; Fe³⁺ brown; Al³⁺ white soluble in excess; Ca²⁺ white insoluble. For anions: carbonates react with acid to produce CO₂; sulfates give white precipitate with BaCl₂ acidified with HCl; halides give specific coloured precipitates with acidified AgNO₃ (Cl⁻ → white, Br⁻ → cream, I⁻ → yellow).
焰色反应：Li⁺ 深红、Na⁺ 黄、K⁺ 淡紫、Ca²⁺ 砖红、Cu²⁺ 蓝绿。溶液中的金属离子加氢氧化钠：Cu²⁺ 蓝色沉淀；Fe²⁺ 暗绿色；Fe³⁺ 棕色；Al³⁺ 白色且溶于过量；Ca²⁺ 白色不溶。阴离子：碳酸盐与酸产生 CO₂；硫酸盐加酸化 BaCl₂ 得白色沉淀；卤离子加酸化 AgNO₃ 生成特征沉淀（Cl⁻ 白，Br⁻ 奶油，I⁻ 黄）。
10. Common Practical Skills and Data Handling | 常见实验技能与数据处理
In WJEC practical‑based questions, always identify independent, dependent and control variables. Be able to select appropriate apparatus for measuring volume (measuring cylinder, pipette, burette), mass (balance) and temperature (thermometer). Know how to perform a titration: rinse burette with solution, use a white tile, swirl the flask, record titre to ±0.05 cm³.
WJEC 实验中要能识别自变量、因变量和控制变量。会选择合适的仪器：量取体积（量筒、移液管、滴定管）、质量（天平）、温度（温度计）。掌握滴定操作：滴定管用溶液润洗、用白色瓷板、摇动锥形瓶、记录滴定管读数至 ±0.05 cm³。
When plotting graphs, draw a line of best fit (straight or smooth curve), never dot‑to‑dot unless specified. Calculate rate from the slope of a graph at a given time. Discuss reasons for anomalous points. For repeatable results, take multiple readings and calculate a mean, ignoring obvious outliers. Always quote results to an appropriate number of significant figures.
作图时画最佳拟合线（直线或平滑曲线），除非特别说明，勿点对点连接。从曲线某时刻的斜率求反应速率。讨论异常点原因。为保证再现性，多次测量取平均值并排除明显异常值。结果的有效数字位数要合理。
Be ready to evaluate a method: suggest improvements to accuracy (e.g. using a pipette instead of a measuring cylinder) or to reduce heat loss in energy experiments by using a lid or insulation.
学会评价实验方法：提出提高准确度的改进（如用移液管代替量筒），或在能量实验中加盖保温以减少热量散失。
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