📚 Acids and Bases Theory: IGCSE WJEC Chemistry Revision | 酸碱理论：IGCSE WJEC 化学考点精讲
Understanding acids and bases is fundamental to chemistry. This article will guide you through the key theories, properties and reactions you need to master for the IGCSE WJEC Chemistry specification.
理解酸和碱是化学的基础。本文将带你掌握 IGCSE WJEC 化学考纲中关于酸碱的关键理论、性质及反应。
1. What Are Acids and Bases? | 什么是酸和碱？
Acids are substances that taste sour, turn blue litmus red and have a pH less than 7. Bases are substances that taste bitter, feel soapy, turn red litmus blue and have a pH greater than 7. However, these definitions are based on observable properties and do not fully explain their behaviour at the molecular level.
酸是有酸味、能使蓝色石蕊试纸变红且 pH 小于 7 的物质。碱是有苦味、触感滑腻、能使红色石蕊试纸变蓝且 pH 大于 7 的物质。然而，这类定义仅基于可观察的性质，未能从分子层面完整解释其行为。
In modern chemistry, we classify substances into acids, bases and alkalis. An alkali is a soluble base that releases hydroxide ions (OH⁻) in water.
在现代化学中，我们将物质分为酸、碱和碱溶液。碱溶液是指能在水中释放氢氧根离子（OH⁻）的可溶性碱。
2. The Arrhenius Theory | 阿伦尼乌斯酸碱理论
According to Svante Arrhenius, an acid is a substance that dissociates in water to produce hydrogen ions (H⁺). A base is a substance that dissociates in water to produce hydroxide ions (OH⁻).
根据阿伦尼乌斯理论，酸是在水中解离产生氢离子（H⁺）的物质，碱是在水中解离产生氢氧根离子（OH⁻）的物质。
) ionises in water: HCl → H⁺ + Cl⁻. Sodium hydroxide (NaOH) dissociates: NaOH → Na⁺ + OH⁻.
例如，盐酸（HCl）在水中电离：HCl → H⁺ + Cl⁻；氢氧化钠（NaOH）解离：NaOH → Na⁺ + OH⁻。
This theory is simple but limited to aqueous solutions and cannot explain the basicity of substances like ammonia (NH₃) that do not contain OH⁻.
该理论简单但局限于水溶液，无法解释氨气（NH₃）等不含 OH⁻ 的物质的碱性。
3. The Brønsted–Lowry Theory | 布朗斯特–劳里酸碱理论
The Brønsted–Lowry theory defines an acid as a proton (H⁺) donor and a base as a proton acceptor. This broadens the definition to include reactions in non-aqueous solvents and gases.
布朗斯特–劳里理论将酸定义为质子（H⁺）供体，将碱定义为质子受体。这一定义拓展至非水溶剂和气相反应。
Example: HCl + H₂O → H₃O⁺ + Cl⁻. Here, HCl donates a proton to water, acting as a Brønsted–Lowry acid. Water accepts a proton, acting as a base.
例如：HCl + H₂O → H₃O⁺ + Cl⁻。在此反应中，HCl 向水提供质子，作为布朗斯特–劳里酸；水接受质子，作为碱。
Ammonia (NH₃) can act as a base by accepting a proton: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. This explains its basicity even though it lacks OH⁻ initially.
氨气（NH₃）可通过接受质子表现出碱性：NH₃ + H₂O ⇌ NH₄⁺ + OH⁻。这解释了它初始不含 OH⁻ 却显碱性的原因。
4. Conjugate Acid-Base Pairs | 共轭酸碱对
In a Brønsted–Lowry reaction, an acid donates a proton and forms its conjugate base. A base accepts a proton and forms its conjugate acid. Every acid–base reaction involves two conjugate pairs.
在布朗斯特–劳里反应中，酸给出质子后形成其共轭碱，碱接受质子后形成其共轭酸。每个酸碱反应都包含两对共轭对。
For the reaction: HA + B ⇌ A⁻ + BH⁺, the conjugate pairs are HA/A⁻ and B/BH⁺. For instance, in the reaction of HCl with water, HCl/Cl⁻ and H₂O/H₃O⁺ are the conjugate pairs.
对于反应：HA + B ⇌ A⁻ + BH⁺，共轭对为 HA/A⁻ 和 B/BH⁺。例如 HCl 与水的反应中，共轭对为 HCl/Cl⁻ 和 H₂O/H₃O⁺。
The strength of the acid or base determines how far the equilibrium lies. Strong acids have weak conjugate bases, and strong bases have weak conjugate acids.
酸或碱的强度决定了平衡的位置。强酸的共轭碱是弱碱，强碱的共轭酸是弱酸。
5. Strong and Weak Acids & Bases | 强酸弱酸与强碱弱碱
A strong acid completely dissociates in water, giving a high concentration of H⁺ ions. Examples: HCl, H₂SO₄, HNO₃. A weak acid only partially dissociates, e.g., ethanoic acid (CH₃COOH) ⇌ CH₃COO⁻ + H⁺.
强酸在水中完全电离，产生高浓度 H⁺。例如 HCl、H₂SO₄、HNO₃。弱酸仅部分电离，如乙酸（CH₃COOH）⇌ CH₃COO⁻ + H⁺。
Similarly, a strong base fully dissociates to give OH⁻ ions (e.g., NaOH, KOH). A weak base such as ammonia reacts partially with water to produce OH⁻.
类似地，强碱完全电离产生 OH⁻（如 NaOH、KOH）。弱碱如氨与水仅部分反应产生 OH⁻。
Conductivity and pH measurements can distinguish strong from weak electrolytes. At the same concentration, strong acids have lower pH values than weak ones.
电导率和 pH 测定可以区分强电解质和弱电解质。相同的物质的量浓度下，强酸的 pH 低于弱酸。
6. The pH Scale and Its Measurement | pH 标度及其测量
The pH scale ranges from 0 to 14 and measures the acidity or alkalinity of a solution. It is defined as:
pH = −log₁₀[H⁺]
7, alkaline solutions pH > 7. Each unit change in pH represents a tenfold change in [H⁺].
pH 标度范围为 0 至 14，衡量溶液的酸碱性。定义为：pH = −log₁₀[H⁺]。酸性溶液 pH < 7，中性 pH = 7，碱性 pH > 7。pH 值每变化 1 个单位，[H⁺] 改变 10 倍。
Indicators change colour over a specific pH range. Common indicators include litmus, phenolphthalein, methyl orange and universal indicator.
指示剂在特定 pH 范围内变色。常用指示剂有石蕊、酚酞、甲基橙和通用指示剂。
| Indicator | Colour in acid | Colour in neutral | Colour in alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
7. Neutralisation Reactions | 中和反应
alt and water. For example: HCl + NaOH → NaCl + H₂O. The ionic equation is H⁺ + OH⁻ → H₂O.
中和是酸与碱反应生成盐和水的过程。例如：HCl + NaOH → NaCl + H₂O。离子方程式为 H⁺ + OH⁻ → H₂O。
O₄ → CuSO₄ + H₂O. Carbonates also react with acids to give salt, water and carbon dioxide.
金属氧化物和金属氢氧化物能中和酸。例如：CuO + H₂SO₄ → CuSO₄ + H₂O。碳酸盐与酸反应则生成盐、水和二氧化碳。
All neutralisation reactions are exothermic. The heat released can be measured and used in thermometric titrations.
所有中和反应都是放热的。释放的热量可以测量并用于热量滴定。
8. Acid-Base Titration | 酸碱滴定
Titration is a quantitative technique used to determine the concentration of an unknown acid or base by neutralising it with a standard solution of known concentration.
滴定是一种定量技术，利用已知浓度的标准溶液中和未知浓度的酸或碱，从而测定其浓度。
The key equation for titration calculations is:
c₁V₁/n₁ = c₂V₂/n₂
on (mol/dm³), V = volume (dm³), n = number of moles of H⁺ or OH⁻ involved in the reaction.
滴定计算的关键公式为：c₁V₁/n₁ = c₂V₂/n₂，其中 c 为浓度（mol/dm³），V 为体积（dm³），n 为反应中涉及的 H⁺ 或 OH⁻ 的摩尔数。
Common indicators are chosen to match the equivalence point pH. For a strong acid–strong base titration, phenolphthalein or methyl orange works well.
通常根据等当点的 pH 选择合适的指示剂。强酸强碱滴定可选用酚酞或甲基橙。
- Rinse burette with the solution to be used.
- Fill the burette and note the initial reading.
- Pipette a known volume of the other solution into a conical flask.
- Add a few drops of indicator.
- Titrate until the colour change indicates the endpoint.
- Repeat to obtain concordant titres (within 0.10 cm³).
- 用待装液润洗滴定管。
- 装液并记录初始读数。
- 用移液管量取一定体积的另一溶液至锥形瓶中。
- 加入几滴指示剂。
- 滴定至指示剂变色即终点。
- 重复实验至获得吻合的滴定体积（差值在 0.10 cm³ 以内）。
9. Salts: Preparation and Uses | 盐的制备与用途
A salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion. Salts can be prepared by various methods depending on solubility.
盐是酸中的氢离子被金属离子或铵根离子取代后形成的离子化合物。根据溶解度，盐有多种制备方法。
| Salt type | Typical preparation method |
|---|---|
| Soluble salt (except Group 1, ammonium) | Acid + insoluble base/metal/carbonate, filtration, crystallisation |
| Soluble Group 1 or ammonium salt | Titration, then evaporation |
| Insoluble salt | Precipitation (mixing two soluble salt solutions), filtration, washing, drying |
Salts have many applications: sodium chloride (table salt), potassium nitrate (fertiliser), calcium sulfate (plaster), silver bromide (photography).
盐用途广泛：氯化钠（食盐）、硝酸钾（肥料）、硫酸钙（石膏）、溴化银（摄影）。
10. Classification of Oxides | 氧化物的分类
Oxides can be classified as acidic, basic, amphoteric or neutral based on their acid–base properties. This topic is essential for understanding environmental and industrial chemistry.
根据酸碱性质，氧化物可分为酸性氧化物、碱性氧化物、两性氧化物和中性氧化物。理解它们对于环境化学和工业化学至关重要。
- Basic oxides: metal oxides that react with acids, e.g., MgO, CuO, Na₂O.
- Acidic oxides: non-metal oxides that react with bases, e.g., SO₂, CO₂, P₄O₁₀.
- Amphoteric oxides: react with both acids and bases, e.g., Al₂O₃, ZnO.
- Neutral oxides: do not react with acids or bases, e.g., CO, NO, N₂O.
- 碱性氧化物：金属氧化物，与酸反应，如 MgO、CuO、Na₂O。
- 酸性氧化物：非金属氧化物，与碱反应，如 SO₂、CO₂、P₄O₁₀。
- 两性氧化物：既与酸又与碱反应，如 Al₂O₃、ZnO。
- 中性氧化物：既不与酸也不与碱反应，如 CO、NO、N₂O。
solutions. For example, SO₂ + H₂O → H₂SO₃ (sulfurous acid). They contribute to acid rain.
酸性氧化物溶于水形成酸性溶液。例如 SO₂ + H₂O → H₂SO₃（亚硫酸）。它们会导致酸雨。
11. Everyday Applications of Acids and Bases | 酸和碱的日常应用
Acids are used in car batteries (H₂SO₄), food preservation (citric acid, ethanoic acid), and cleaning products (HCl in toilet cleaners). Bases are used in soap making (NaOH), agriculture (CaO to neutralise acidic soil), and antacids (Mg(OH)₂).
酸用于汽车电池（H₂SO₄）、食品保存（柠檬酸、乙酸）以及清洁产品（洁厕剂中的 HCl）。碱用于制皂（NaOH）、农业（CaO 中和酸性土壤）和抗酸胃药（Mg(OH)₂）。
Understanding the correct use of acids and bases prevents accidents and enables sustainable practices, such as treating industrial waste before discharge.
正确认识和使用酸碱有助于防止事故，并实现可持续做法，如在排放前处理工业废水。
12. Exam Tips for IGCSE WJEC Chemistry | IGCSE WJEC 化学考试技巧
Link definitions to chemical equations: show proton transfer or H⁺/OH⁻ donation clearly. Label conjugate pairs in the equation.
将定义与化学方程式结合：清晰表示质子转移或 H⁺/OH⁻ 的给出，并在方程式中标注共轭对。
When comparing strong vs weak acids, always mention degree of dissociation, not concentration. Use equilibrium arrows for weak acids.
比较强酸与弱酸时，务必提及电离程度而非浓度。弱酸使用可逆箭头。
For titration calculations, convert volumes to dm³ (divide by 1000) and apply the mole ratio from the balanced equation. Show all working steps.
滴定计算中，将体积转换为 dm³（除以 1000），并运用配平方程式中的摩尔比。写出所有计算步骤。
Use precise language: 'dissociate' for strong acids, 'partially ionise' for weak ones, and 'neutralisation' for acid–base reactions forming water.
使用精准术语：强酸用“dissociate（完全解离）”，弱酸用“partially ionise（部分电离）”，酸与碱生成水的反应使用“neutralisation（中和）”。
Expect questions on the interpretation of pH curves, choice of indicator, and the preparation of specific salts. Practise drawing labelled diagrams of titration apparatus.
需准备 pH 曲线的解读、指示剂的选择以及特定盐的制备等问题。练习绘制滴定装置示意图并标注名称。
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