📚 IGCSE AQA Science: Chemical Reactions – Key Points Explained | IGCSE AQA 科学：化学反应 考点精讲
Chemical reactions are at the heart of chemistry. In the IGCSE AQA syllabus, you must be able to identify signs of a reaction, write balanced equations, classify reaction types, and understand energy changes. This article breaks down every essential concept, from collision theory to redox, helping you revise effectively and score higher.
化学反应是化学的核心。在 IGCSE AQA 课程中，你需要能够识别反应的迹象、书写配平方程式、对反应类型进行分类，并理解能量变化。本文拆解了从碰撞理论到氧化还原的每一个重要概念，帮助你高效复习，拿下高分。
1. Recognising Chemical Reactions | 识别化学反应
A chemical reaction occurs when one or more substances change into new substances. You can spot a reaction by observing colour changes, formation of a precipitate, gas production (bubbles or odour), or an energy change such as heating or cooling.
当一种或多种物质转变为新物质时，就发生了化学反应。你可以通过观察颜色变化、沉淀生成、气体产生（气泡或气味）或者能量变化（如放热或吸热）来发现反应。
The only foolproof way to confirm a chemical change is to show that a new substance has been produced. Changes of state (melting, boiling) are physical, not chemical, because the substance keeps its identity.
确认化学变化的唯一可靠方法是证明生成了新物质。状态变化（熔化、沸腾）是物理变化而非化学变化，因为物质本身并未改变。
2. Word and Symbol Equations | 文字方程式与符号方程式
Word equations show the names of reactants and products, e.g. ‘magnesium + oxygen → magnesium oxide’. They tell you what reacts but not the number of particles involved.
文字方程式显示反应物和生成物的名称，如”镁 + 氧气 → 氧化镁”。它告诉你什么物质参与了反应，但不涉及微粒数量。
Symbol equations use chemical formulas and must be balanced because atoms are conserved. For the same reaction: 2Mg + O₂ → 2MgO. Balancing ensures the same number of each type of atom on both sides.
符号方程式使用化学式，并且必须配平，因为原子是守恒的。同一反应写为：2Mg + O₂ → 2MgO。配平能保证方程式两侧每种原子的数目相等。
State symbols (s), (l), (g), (aq) show solid, liquid, gas, and aqueous solution. Always add them to a balanced symbol equation for full marks.
状态符号 (s)、(l)、(g)、(aq) 分别表示固体、液体、气体和水溶液。记得在配平后的符号方程式中加上它们以拿到满分。
3. Conservation of Mass | 质量守恒
In a chemical reaction, no atoms are created or destroyed, so total mass stays the same. This is the Law of Conservation of Mass. If the mass appears to change in an open container, it is usually because a gas has entered or escaped.
在化学反应中，原子不会凭空产生或消失，因此总质量保持不变。这就是质量守恒定律。如果在敞口容器中质量看似变化了，通常是因为有气体进入或逸出。
Always check that the total mass of reactants equals the total mass of products in a balanced equation. In an experiment, a reaction producing a gas will lose mass unless the system is sealed.
在配平方程式中，务必检查反应物的总质量是否等于生成物的总质量。在实验中，生成气体的反应若未在密闭系统中进行，质量会减少。
4. Collision Theory and Rates | 碰撞理论与反应速率
Particles must collide with sufficient energy (activation energy) and correct orientation for a reaction to happen. The rate depends on frequency of successful collisions.
粒子必须发生碰撞，并且具有足够的能量（活化能）和正确的取向，反应才能发生。反应速率取决于有效碰撞的频率。
Factors that increase rate: concentration or pressure (more particles in a space), temperature (particles move faster, more collisions with energy ≥ Ea), surface area (more solid exposed), and catalysts (lower activation energy).
提高速率的因素：浓度或压强（空间内粒子更多）、温度（粒子运动加快，更多碰撞能量 ≥ 活化能）、表面积（裸露的固体更多）以及催化剂（降低活化能）。
5. Exothermic and Endothermic Reactions | 放热与吸热反应
An exothermic reaction transfers energy to the surroundings, usually as heat, so the temperature rises. Combustion, neutralisation, and oxidation are exothermic.
放热反应将能量传递到周围环境，通常表现为放热，因此温度升高。燃烧、中和、氧化反应都是放热的。
An endothermic reaction takes in energy from the surroundings, causing a temperature drop. Thermal decomposition and photosynthesis are endothermic. Reaction profile diagrams show reactants and products energy levels with an activation energy hump.
吸热反应从环境中吸收能量，导致温度下降。热分解和光合作用都是吸热的。反应剖面图展示反应物和生成物的能级，并带有一个活化能的峰值。
Energy change overall: ΔH negative for exothermic, ΔH positive for endothermic. Bond breaking absorbs energy; bond making releases energy.
总能量变化：放热反应的 ΔH 为负，吸热反应的 ΔH 为正。断键吸收能量；成键释放能量。
6. Neutralisation and Salts | 中和反应与盐
Neutralisation is a reaction between an acid and a base to form a salt and water. The general ionic equation: H⁺ + OH⁻ → H₂O.
中和是酸和碱反应生成盐和水的过程。其离子方程式通式为：H⁺ + OH⁻ → H₂O。
Bases include metal oxides and metal hydroxides. If the base is an alkali (soluble), the salt forms in solution. Naming salts: from the acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate) and the metal or cation.
碱包括金属氧化物和金属氢氧化物。如果碱是可溶的（即碱性溶液），盐就在溶液中生成。命名盐：由酸根（盐酸 → 氯化物、硫酸 → 硫酸盐、硝酸 → 硝酸盐）结合金属或阳离子决定。
Making a pure dry salt involves neutralising with an appropriate method: titration for soluble salts, or adding excess solid base, filtering, and crystallising.
制备纯净干燥的盐要选用合适的方法：可溶盐用滴定法，或用过量固体碱反应后过滤、蒸发结晶。
7. Oxidation and Reduction (Redox) | 氧化与还原（氧还反应）
Oxidation originally meant gaining oxygen; reduction meant losing oxygen. Now we use electron transfer: Oxidation Is Loss of electrons (OIL), Reduction Is Gain of electrons (RIG).
氧化最初指得氧；还原指失氧。如今我们使用电子转移来定义：氧化是失去电子（OIL），还原是得到电子（RIG）。
A redox reaction always has both processes occurring together. The reactant that is oxidised gives away electrons; the reactant that is reduced accepts them. Oxidising agents cause oxidation and are themselves reduced.
氧还反应必定同时发生这两个过程。被氧化的反应物失去电子；被还原的反应物得到电子。氧化剂引发氧化，自身被还原。
Examples: displacement reactions (zinc + copper sulfate solution: Zn + Cu²⁺ → Zn²⁺ + Cu) where zinc is oxidised and copper(II) ions are reduced.
例子：置换反应（锌 + 硫酸铜溶液：Zn + Cu²⁺ → Zn²⁺ + Cu），锌被氧化，铜(II)离子被还原。
8. Thermal Decomposition | 热分解反应
Thermal decomposition is when a substance breaks down into simpler substances when heated. It is always endothermic. A common example is the breakdown of metal carbonates, e.g. copper(II) carbonate → copper(II) oxide + carbon dioxide.
热分解是指物质在受热时分解为更简单的物质。它总是吸热的。一个常见例子是金属碳酸盐的分解，如碳酸铜(II) → 氧化铜(II) + 二氧化碳。
The pattern: metal carbonate → metal oxide + CO₂ (except for Group 1 carbonates, which are stable under normal Bunsen heating). Testing for CO₂ uses limewater turning milky.
规律：金属碳酸盐 → 金属氧化物 + CO₂（第 1 族碳酸盐在普通本生灯加热下稳定，例外）。检验 CO₂ 可用石灰水，会变浑浊。
9. Displacement Reactions | 置换反应
A more reactive element displaces a less reactive element from its compound. This applies to metals (reactivity series) and halogens. For metals, a higher metal pushes out a lower one in a salt solution.
较活泼的元素能将较不活泼的元素从其化合物中置换出来。这适用于金属（活动性顺序）和卤素。对金属来说，位于活动性顺序上方的金属能从盐溶液中置换出下方的金属。
The reactivity series: K, Na, Ca, Mg, Al, (C), Zn, Fe, (H), Cu, Ag, Au. Carbon and hydrogen are included as non‑metals for comparison: carbon can reduce metal oxides below it.
活动性顺序：K、Na、Ca、Mg、Al、(C)、Zn、Fe、(H)、Cu、Ag、Au。碳和氢作为非金属列入以供比较：碳能还原其下方的金属氧化物。
Halogen displacement: a more reactive halogen (e.g. chlorine) displaces a less reactive one (e.g. bromide or iodide) from its salt solution. The colour change shows the displaced halogen.
卤素置换：较活泼的卤素（如氯）能从卤化物溶液中置换出较不活泼的（如溴化物或碘化物）。颜色变化可指示被置换出的卤素。
10. Precipitation Reactions | 沉淀反应
A precipitate is an insoluble solid formed when two solutions react. Precipitation is a useful method to identify ions in solution because specific precipitates have characteristic colours.
沉淀是两种溶液反应时生成的不溶性固体。沉淀反应是鉴定溶液中离子的实用方法，因为特定沉淀有其特征颜色。
Key precipitation tests: chloride ions with silver nitrate (white precipitate, soluble in dilute NH₃); bromide (cream) and iodide (yellow) tested similarly. Sulfate ions with barium chloride + HCl (white precipitate of barium sulfate). Carbonates produce CO₂ with acid, not precipitation.
重要沉淀检验：氯离子与硝酸银反应（白色沉淀，溶于稀氨水）；溴离子（奶油色）、碘离子（黄色）检验类似。硫酸根离子用氯化钡加盐酸（白色硫酸钡沉淀）。碳酸根遇酸放出 CO₂，不用沉淀。
Ionic equations for precipitation show only the ions that form the precipitate, e.g. Ag⁺ + Cl⁻ → AgCl(s). Spectator ions are left out.
沉淀反应的离子方程式只写出形成沉淀的离子，如 Ag⁺ + Cl⁻ → AgCl(s)。旁观离子不写入。
11. Reversible Reactions and Equilibrium | 可逆反应与平衡
Some reactions go in both directions: reactants form products while products revert to reactants. This is indicated by the ⇌ symbol. In a closed system, a dynamic equilibrium is reached when forward and backward rates are equal, and amounts of each substance remain constant.
有些反应可双向进行：反应物生成产物，同时产物又变回反应物。用 ⇌ 符号表示。在密闭系统中，当正逆反应速率相等，各物质的量保持恒定时，即达到动态平衡。
Le Chatelier’s Principle: if a system at equilibrium is disturbed, the position shifts to oppose the change. Increasing temperature favours the endothermic direction; increasing pressure favours the side with fewer gas moles (AQA only qualitatively).
勒夏特列原理：如果一个平衡体系受到干扰，平衡位置会移动以抵消这种变化。升高温度向吸热方向移动；增大压强向气体分子数较少的方向移动（AQA 只要求定性理解）。
A catalyst speeds up both forward and backward reactions equally, so equilibrium is reached faster but the position is unchanged.
催化剂同等程度地加快正逆反应速率，因此能更快达到平衡，但平衡位置不变。
12. Chemical Cells and Fuel Cells | 化学电池与燃料电池
A simple cell uses two different metals in an electrolyte. The greater the difference in reactivity, the larger the voltage. Hydrogen‑oxygen fuel cells (e.g. alkaline) produce electricity with only water as waste: 2H₂ + O₂ → 2H₂O.
简单电池使用两种不同金属浸入电解质中。金属活泼性差异越大，电压越高。氢氧燃料电池（如碱性）发电，唯一的废物是水：2H₂ + O₂ → 2H₂O。
In a hydrogen fuel cell, hydrogen is oxidised at the anode: H₂ + 2OH⁻ → 2H₂O + 2e⁻, and oxygen is reduced at the cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻. Overall reaction is clean; fuel cells are more efficient than combustion engines.
在氢燃料电池中，氢在负极被氧化：H₂ + 2OH⁻ → 2H₂O + 2e⁻，氧在正极被还原：O₂ + 2H₂O + 4e⁻ → 4OH⁻。总反应清洁无污染；燃料电池效率高于内燃机。
Advantages: no greenhouse gas or pollutants produced at point of use; higher energy conversion efficiency. Challenges include storing and producing hydrogen sustainably.
优点：使用时不排放温室气体或污染物；能量转化效率更高。挑战包括可持续地储存和生产氢气。
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