IGCSE WJEC Science: Acids and Bases Key Points | IGCSE WJEC 科学：酸与碱考点精讲

📚 IGCSE WJEC Science: Acids and Bases Key Points | IGCSE WJEC 科学：酸与碱考点精讲

Acids and bases are fundamental substances in chemistry that surround us in everyday life, from the citric acid in lemons to the sodium hydroxide in cleaning products. Mastering the concepts of acids and bases is essential for IGCSE WJEC Science, as they appear in both theoretical explanations and practical investigations. This article will guide you through the key definitions, reactions, and applications, ensuring you are fully prepared for your examination.

酸和碱是化学中随处可见的基本物质，从柠檬中的柠檬酸到清洁用品中的氢氧化钠。掌握酸与碱的概念对 IGCSE WJEC 科学考试至关重要，因为它们在理论解释和实验探究中都经常出现。本文将带你全面梳理关键定义、反应及应用，助你从容应对考试。

1. What are Acids and Bases? | 什么是酸和碱？

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl⁻ ions. In the WJEC specification, the Arrhenius definition is primarily used, where an acid is a proton donor. A base is a substance that can neutralise an acid, often by providing hydroxide ions (OH⁻) in water. Alkalis are soluble bases that release OH⁻ ions, such as sodium hydroxide (NaOH).

酸是一种溶于水后释放氢离子 (H⁺) 的物质。例如，盐酸 (HCl) 在水中解离出 H⁺ 和 Cl⁻ 离子。WJEC 考试大纲主要采用阿伦尼乌斯定义，即酸是质子的提供者。碱是一种能中和酸的物质，通常在水溶液中提供氢氧根离子 (OH⁻)。可溶性的碱称为碱液，例如氢氧化钠 (NaOH)，能释放 OH⁻ 离子。

The general equation for the dissociation of an acid in water is: HA → H⁺ + A⁻. For bases, the dissociation typically looks like: BOH → B⁺ + OH⁻. It is important to remember that hydrogen ions in water are often represented as H₃O⁺ (hydronium ions), but H⁺ is commonly accepted in IGCSE answers.

酸在水中的解离通式为：HA → H⁺ + A⁻。碱的解离通常为：BOH → B⁺ + OH⁻。需要记住，水中的氢离子常以 H₃O⁺（水合氢离子）形式存在，但在 IGCSE 的答案中，简写为 H⁺ 也是被接受的。

2. The pH Scale | pH 标度

The pH scale is a numerical measure of how acidic or basic a solution is, ranging from 0 to 14. A pH less than 7 indicates an acidic solution, a pH of 7 is neutral (pure water), and a pH greater than 7 indicates a basic (alkaline) solution. The scale is logarithmic, meaning each whole pH value change represents a tenfold change in hydrogen ion concentration.

pH 标度是一个衡量溶液酸碱性的数值指标，范围在 0 到 14 之间。pH 值小于 7 表示酸性溶液，pH 等于 7 为中性（纯水），pH 大于 7 则为碱性溶液。该标度是对数形式，即 pH 值每变化 1 个单位，氢离子浓度就相差 10 倍。

For instance, a solution with pH 3 has ten times more H⁺ ions than one with pH 4. The relationship is given by: pH = -log₁₀[H⁺]. Although you do not need to perform logarithmic calculations in the exam, you should know that lower pH means higher H⁺ concentration and stronger acidity.

例如，pH 为 3 的溶液的 H⁺ 离子浓度是 pH 为 4 的溶液的 10 倍。关系式为：pH = -log₁₀[H⁺]。虽然考试中不需要进行对数计算，但你必须明白 pH 值越低，H⁺ 浓度越高，酸性越强。

pH Range	Description	Example
0-2	Strongly acidic	Stomach acid (HCl)
3-5	Weakly acidic	Vinegar, lemon juice
6-8	Neutral or near neutral	Pure water, saliva
9-11	Weakly alkaline	Soap, washing soda
12-14	Strongly alkaline	Oven cleaner (NaOH)

3. Indicators | 指示剂

Indicators are substances that change colour depending on the pH of the solution. They are used to determine whether a substance is acidic, neutral, or alkaline. Common indicators for WJEC IGCSE include litmus, phenolphthalein, and methyl orange. Each has a specific colour change at different pH ranges.

指示剂是能根据溶液的 pH 值改变颜色的物质，用于判断物质是酸性、中性还是碱性。WJEC IGCSE 中常见的指示剂包括石蕊、酚酞和甲基橙。每种指示剂在特定的 pH 范围内都有其特定的颜色变化。

Litmus paper: red in acidic solutions (pH < 5), blue in alkaline solutions (pH > 8), and purple in neutral. It is a simple qualitative indicator.

石蕊试纸：酸性溶液中变红 (pH < 5)，碱性溶液中变蓝 (pH > 8)，中性时为紫色。是一种简单的定性指示剂。
Phenolphthalein: colourless in acidic and neutral solutions, turns pink in alkaline conditions (pH > 8.3). It is often used in titrations.

酚酞：在酸性和中性溶液中无色，在碱性条件下 (pH > 8.3) 变为粉红色。常用于滴定实验。
Methyl orange: red in acidic solutions (pH < 3.1), yellow in neutral and alkaline solutions (pH > 4.4). It is useful for strong acid–weak base titrations.

甲基橙：酸性溶液中呈红色 (pH < 3.1)，中性和碱性溶液中呈黄色 (pH > 4.4)。适用于强酸–弱碱滴定。

Universal indicator is a mixture of several indicators which shows a gradual range of colours across the pH scale, from red (strongly acidic) through green (neutral) to purple (strongly alkaline). It provides a more accurate pH reading than single indicators.

通用指示剂是多种指示剂的混合物，能在整个 pH 标度上显示渐变的颜色，从红色（强酸）经绿色（中性）到紫色（强碱）。它比单一指示剂能提供更精确的 pH 读数。

4. Reactions of Acids with Metals | 酸与金属的反应

Acids react with certain metals to produce a salt and hydrogen gas. Not all metals react; only those more reactive than hydrogen in the reactivity series, such as magnesium, zinc, and iron, will displace hydrogen from acids. Unreactive metals like copper and silver do not react with dilute acids.

酸能与某些金属反应，生成盐和氢气。并非所有金属都能反应，只有金属活动性顺序表中位于氢之前的金属（如镁、锌、铁）才能从酸中置换出氢。不活泼金属如铜和银不与稀酸反应。

The general word equation is: metal + acid → salt + hydrogen. For example, magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas: Mg + 2HCl → MgCl₂ + H₂. The ionic equation shows the metal atoms losing electrons: Mg + 2H⁺ → Mg²⁺ + H₂. The test for hydrogen gas is a ‘squeaky pop’ with a lighted splint.

通用的文字方程式为：金属 + 酸 → 盐 + 氢气。例如，镁与盐酸反应生成氯化镁和氢气：Mg + 2HCl → MgCl₂ + H₂。离子方程式显示了金属原子失去电子：Mg + 2H⁺ → Mg²⁺ + H₂。检验氢气的方法是用点燃的木条靠近，会发出“噗”的爆鸣声。

General equation: Metal + Acid → Salt + Hydrogen

Metal	Acid	Salt produced
Zinc	Sulfuric acid	Zinc sulfate
Iron	Hydrochloric acid	Iron(II) chloride
Magnesium	Nitric acid	Magnesium nitrate

5. Reactions of Acids with Carbonates | 酸与碳酸盐的反应

Acids react with metal carbonates and hydrogencarbonates to produce a salt, water, and carbon dioxide gas. This is a common reaction used to prepare carbon dioxide in the laboratory. The fizzing or effervescence observed is due to the release of CO₂ gas.

酸与金属碳酸盐及碳酸氢盐反应，生成盐、水和二氧化碳气体。这是实验室制备二氧化碳的常见反应。观察到的冒泡或泡腾现象就是由于释放了 CO₂ 气体。

General word equation: carbonate + acid → salt + water + carbon dioxide. For instance, calcium carbonate (limestone) reacts with hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The test for carbon dioxide is bubbling the gas through limewater, which turns milky (formation of calcium carbonate precipitate).

通用文字方程式：碳酸盐 + 酸 → 盐 + 水 + 二氧化碳。例如，碳酸钙（石灰石）与盐酸反应：CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂。检验二氧化碳的方法是将气体通入石灰水中，石灰水变浑浊（生成碳酸钙沉淀）。

Ionic equation: 2H⁺ + CO₃²⁻ → H₂O + CO₂

The ionic equation simplifies the reaction to show that hydrogen ions from the acid react with carbonate ions to produce water and carbon dioxide. This is independent of the specific acid or carbonate used, as long as they are soluble or react accordingly.

离子方程式精简了反应，表明酸中的氢离子与碳酸根离子反应生成水和二氧化碳。只要反应物是可溶的或能发生反应，这一离子方程式与具体使用的酸或碳酸盐种类无关。

6. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base (or alkali) to form a salt and water. The hydrogen ions from the acid combine with hydroxide ions from the base to produce water. This is an exothermic process, releasing heat.

中和反应是指酸和碱（或碱液）反应生成盐和水的过程。酸中的氢离子与碱中的氢氧根离子结合生成水。这是一个放热过程，会释放热量。

General equation: acid + base → salt + water. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water: HCl + NaOH → NaCl + H₂O. The overall ionic equation for neutralisation is: H⁺ + OH⁻ → H₂O. This is the core reaction and is the same for all strong acid–strong base neutralisations.

通用方程式：酸 + 碱 → 盐 + 水。例如，盐酸 + 氢氧化钠 → 氯化钠 + 水：HCl + NaOH → NaCl + H₂O。中和反应的总离子方程式为：H⁺ + OH⁻ → H₂O。这是所有强酸与强碱中和反应的核心反应，完全相同。

Neutralisation has many practical applications, such as in agriculture (adding lime (calcium oxide) to acidic soil), in medicine (antacid tablets containing magnesium hydroxide to neutralise excess stomach acid), and in industry (treating acidic waste).

中和反应有许多实际应用，例如农业中向酸性土壤添加石灰（氧化钙），医学中使用含氢氧化镁的抗酸片中和过多的胃酸，以及工业中处理酸性废水等。

7. Reactions of Acids with Bases | 酸与碱的反应

While metal oxides and metal hydroxides are typical bases, their reactions with acids are also neutralisations. Metal oxides react with acids to form a salt and water, without producing carbon dioxide. This is similar to the reaction with alkalis but often slower because the base may be insoluble.

金属氧化物和金属氢氧化物是典型的碱，它们与酸的反应同样属于中和反应。金属氧化物与酸反应生成盐和水，不会产生二氧化碳。这类似于与碱液的反应，但由于碱可能不溶，反应通常较慢。

For example, copper(II) oxide (a black solid, insoluble) reacts with warm sulfuric acid to produce blue copper(II) sulfate solution and water: CuO + H₂SO₄ → CuSO₄ + H₂O. The ionic equation is: CuO + 2H⁺ → Cu²⁺ + H₂O. This reaction is often used to prepare soluble salts through the method of adding excess base and filtering.

例如，氧化铜（黑色固体，不溶于水）与温热稀硫酸反应生成蓝色的硫酸铜溶液和水：CuO + H₂SO₄ → CuSO₄ + H₂O。离子方程式为：CuO + 2H⁺ → Cu²⁺ + H₂O。该反应常用来通过加入过量碱并过滤的方法制备可溶性盐。

Similarly, metal hydroxides such as calcium hydroxide react with acids: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O. Here, the hydroxide ions neutralise the hydrogen ions, leaving the corresponding salt in solution.

类似地，金属氢氧化物如氢氧化钙与酸反应：Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O。其中，氢氧根离子中和了氢离子，溶液中留下相应的盐。

8. Strong and Weak Acids | 强酸与弱酸

The strength of an acid depends on the degree of dissociation into ions in water. A strong acid fully dissociates into H⁺ and its anion. Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). In contrast, a weak acid only partially dissociates, so at equilibrium there are mostly un-dissociated molecules and fewer H⁺ ions. Common weak acids are ethanoic acid (CH₃COOH), citric acid, and carbonic acid.

酸的强弱取决于它在水中解离成离子的程度。强酸在水中完全解离为 H⁺ 和相应的阴离子。例如盐酸 (HCl)、硫酸 (H₂SO₄) 和硝酸 (HNO₃)。相反，弱酸仅部分解离，因此在平衡体系中，主要以未解离的分子形式存在，H⁺ 离子浓度较低。常见的弱酸有乙酸 (CH₃COOH)、柠檬酸和碳酸。

Write the dissociation using the reversible arrow for weak acids: CH₃COOH ⇌ CH₃COO⁻ + H⁺. The double arrow indicates an incomplete dissociation. It is essential to distinguish between the strength of an acid and its concentration. A concentrated weak acid can still have a relatively high number of acid molecules, but only a small fraction releases H⁺ ions.

弱酸的解离使用可逆箭头：CH₃COOH ⇌ CH₃COO⁻ + H⁺。双向箭头表示不完全解离。区分酸的强度与浓度至关重要。浓的弱酸虽然酸分子数量很多，但只有少部分能释放 H⁺ 离子。

Property	Strong acid	Weak acid
Dissociation	Complete (→)	Partial (⇌)
H⁺ ion concentration (same concentration)	High	Low
pH (for 0.1 mol dm⁻³)	~1	~3
Electrical conductivity	Good	Poor
Rate of reaction with metals	Faster	Slower

9. Concentration versus Strength | 浓度与强度的区别

Concentration refers to the amount of acid dissolved in a certain volume of water, usually expressed in mol dm⁻³. A concentrated acid contains a large number of acid molecules per unit volume, while a dilute acid contains fewer. Strength, on the other hand, is about the proportion of acid molecules that produce H⁺ ions.

浓度是指在一定体积的水中溶解的酸的量，通常以 mol dm⁻³ 表示。浓酸在单位体积中含有大量酸分子，稀酸则含量较少。而强度则是指酸分子产生 H⁺ 离子的比例。

It is possible to have a concentrated weak acid (many molecules but low percentage ionised) and a dilute strong acid (few molecules but all ionised). For example, 2 mol dm⁻³ ethanoic acid still has a lower H⁺ ion concentration and higher pH than 0.5 mol dm⁻³ hydrochloric acid. Exam questions often ask you to compare pH, reaction rates, and electrical conductivity of different acid solutions.

有可能存在浓的弱酸（分子数量多但电离比例低）和稀的强酸（分子数量少但全部电离）。例如，2 mol dm⁻³ 的乙酸，其 H⁺ 离子浓度仍然低于 0.5 mol dm⁻³ 的盐酸，因而 pH 值更高。考试中常要求比较不同酸溶液的 pH、反应速率和导电性等。

The key takeaway: do not equate ‘strong’ with ‘concentrated’. Always check the dissociation behaviour before making predictions.

关键点：不要将“强”与“浓”等同起来。在做出推断之前，务必先确认其解离行为。

10. Preparing Soluble Salts | 可溶性盐的制备

Salts are ionic compounds formed when the hydrogen ion of an acid is replaced by a metal or ammonium ion. The method of preparation depends on the solubility of the salt and the reactants. For soluble salts, four main methods are used: (1) acid + metal (for reactive metals), (2) acid + insoluble base, (3) acid + carbonate, and (4) titration (for soluble base).

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。盐的制备方法取决于盐和反应物的溶解性。可溶性盐的制备主要有四种方法：（1）酸 + 金属（用于活泼金属），（2）酸 + 不溶性碱，（3）酸 + 碳酸盐，以及（4）滴定法（用于可溶性碱）。

Method 1-3 involve adding an excess of the solid reactant (metal, base, or carbonate) to the acid, allowing the reaction to complete, then removing the excess solid by filtration. The filtrate is then evaporated to crystallisation point to obtain pure salt crystals. Method 4 (titration) is used when both acid and base are soluble, as there is no excess solid to filter. The exact neutralising volumes are determined using an indicator, and then the solution is crystallised.

方法 1-3 是将过量的固体反应物（金属、碱或碳酸盐）加入酸中，待反应完成后，过滤除去剩余固体。然后将滤液蒸发至结晶点，得到纯净的盐晶体。方法 4（滴定）用于酸和碱均为可溶物时的情形，因为没有过量的固体可供过滤。通过指示剂确定恰好中和的体积，然后将溶液结晶。

For example, to prepare copper(II) sulfate crystals, you would add excess black copper(II) oxide to warm sulfuric acid until no more dissolves, filter, and crystallise. The ionic equation remains CuO + 2H⁺ → Cu²⁺ + H₂O, and the salt is obtained by crystallisation.

例如，制备硫酸铜晶体：向温热稀硫酸中加入过量黑色氧化铜，搅拌直至不再溶解，过滤，将滤液结晶。离子方程式仍为 CuO + 2H⁺ → Cu²⁺ + H₂O，通过结晶获得盐。

11. Everyday Acids and Bases | 日常生活中的酸与碱

Acids and bases are not just laboratory chemicals; they are found everywhere in daily life. Common household acids include vinegar (ethanoic acid), lemon juice (citric acid), fizzy drinks (carbonic acid), and car batteries (sulfuric acid). Bases include toothpaste (mildly alkaline), baking soda (sodium hydrogencarbonate), bleach (sodium hypochlorite, basic), and oven cleaners (sodium hydroxide).

酸和碱并不只是实验室化学品，它们在日常中随处可见。常见的家用酸包括醋（乙酸）、柠檬汁（柠檬酸）、汽水（碳酸）和汽车电池中的硫酸。碱包括牙膏（弱碱性）、小苏打（碳酸氢钠）、漂白剂（次氯酸钠，呈碱性）以及烤箱清洁剂（氢氧化钠）。

Many natural substances act as indicators, such as red cabbage juice, which changes colour across the pH range. The stings of bees and wasps can be treated using neutralisation: bee stings are acidic and can be neutralised with baking soda, while wasp stings are alkaline and can be treated with vinegar. This demonstrates the practical application of acids and bases knowledge.

许多天然物质也可作为指示剂，如紫甘蓝汁，其颜色随 pH 而变化。蜜蜂和黄蜂的蜇伤也可通过中和反应处理：蜜蜂的蜇伤是酸性的，可用小苏打中和；黄蜂的蜇伤是碱性的，可用醋处理。这充分展示了酸碱知识的实际应用。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

When answering WJEC IGCSE questions on acids and bases, always write state symbols (s), (l), (g), (aq) for chemical equations where appropriate. Use the correct names for salts: sulfuric acid → sulfate, hydrochloric acid → chloride, nitric acid → nitrate. Never confuse strong acid with concentrated acid, and always explain using the idea of dissociation.

在回答 WJEC IGCSE 中关于酸与碱的题目时，如适用，化学方程式应标注状态符号 (s), (l), (g), (aq)。正确命名盐类：硫酸→硫酸盐，盐酸→氯化物，硝酸→硝酸盐。切勿混淆强酸与浓酸，并且始终从解离的角度进行解释。

A common mistake is writing hydrogen as ‘H’ instead of ‘H₂’ or forgetting that reactive metals produce hydrogen gas with acids. Many students also incorrectly say that all bases are alkalis — remember, alkalis are soluble bases. In titrations, be precise about the choice of indicator: phenolphthalein for strong acid vs. strong base, not methyl orange if the pH change is at 7. Practice writing ionic equations showing the actual species involved in the reaction.

常见错误包括将氢气写成“H”而非“H₂”，或忘记活泼金属与酸反应会产生氢气。许多学生还错误地认为所有碱都是碱液——请记住，碱液是可溶性碱。在滴定实验中，要精确选择指示剂：强酸对强碱用酚酞，若 pH 变化在 7 左右则不宜用甲基橙。多加练习书写离子方程式，展示反应实际涉及的微粒。

Finally, always link the properties (pH, conductivity, reactivity) to the presence and concentration of mobile H⁺ ions. Being able to interpret a pH curve and identify the point of neutralisation is also a key skill for the examination.

最后，始终将性质（pH、导电性、反应活性）与可移动 H⁺ 离子的存在和浓度联系起来。能够解读 pH 曲线并识别中和点也是考试中的关键技能。

Published by TutorHao | IGCSE WJEC Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

IGCSE WJEC Science: Acids and Bases Key Points | IGCSE WJEC 科学：酸与碱 考点精讲