📚 Acid-Base Theory: IGCSE OCR Chemistry Key Points | IGCSE OCR 化学：酸碱理论 考点精讲
Understanding acids, bases and their reactions is fundamental to IGCSE Chemistry. This guide distills the OCR specification into clear, exam-focused explanations of acid-base theory, from definitions and properties to pH, indicators and neutralisation. We cover exactly what you need to know, with paired bilingual paragraphs so you can master the concepts in both English and Chinese. Whether you are revising for mocks or the final exam, these structured notes will sharpen your knowledge and boost your confidence.
理解酸、碱及其反应是IGCSE化学的基础。本文紧扣OCR考纲，将酸碱理论浓缩为清晰、面向考试的讲解，涵盖定义、性质、pH、指示剂和中和反应等核心考点。文中每个要点均以中英双语段落配对呈现，帮助你彻底掌握概念。无论你是在准备模拟考还是最终考试，这些结构化笔记都会让知识更扎实，信心更足。
1. Defining Acids and Bases | 酸和碱的定义
In IGCSE Chemistry, an acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) ionises completely in water to give H⁺ and Cl⁻. This is the simplest definition you must know.
在IGCSE化学中，酸是指溶于水时释放出氢离子（H⁺）的物质。例如，盐酸（HCl）在水中完全电离，生成H⁺和Cl⁻。这是你必须掌握的最基本定义。
A base is any substance that can neutralise an acid to produce a salt and water. Alkalis are a subset of bases: they are soluble in water and release hydroxide ions (OH⁻). Sodium hydroxide (NaOH) dissolves in water to give Na⁺ and OH⁻, making it an alkali.
碱是任何能够与酸中和生成盐和水的物质。碱类是可溶于水并能释放氢氧根离子（OH⁻）的碱。氢氧化钠（NaOH）溶于水时产生Na⁺和OH⁻，因此它是一种碱。
Memorising common strong acids (HCl, H₂SO₄, HNO₃) and strong alkalis (NaOH, KOH) is essential for writing equations and predicting salt names.
记住常见的强酸（HCl、H₂SO₄、HNO₃）和强碱（NaOH、KOH）对于书写方程式和推断盐的名称至关重要。
2. Physical Properties of Acids | 酸的物理性质
Dilute acids have a sour taste and turn blue litmus paper red. However, you must never taste chemicals in the lab! The litmus test is a simple and reliable method to identify an acidic solution. Acids are also typically good conductors of electricity because they contain mobile ions in aqueous solution.
稀酸有酸味，能使蓝色石蕊试纸变红。但在实验室严禁品尝化学品！石蕊试纸是鉴别酸性溶液简单可靠的方法。酸溶液通常还具有良好的导电性，因为它们在水溶液中含有可自由移动的离子。
All acids share these observable properties, but their reactivity varies with strength and concentration, which we will explore later. In the exam, you may be asked to describe a simple test for an acid using litmus or universal indicator.
所有酸都表现出这些可观察的性质，但其反应活性因强度和浓度而异，我们将在后文探讨。考试中可能要求你描述用石蕊试纸或通用指示剂检验酸的简单方法。
3. Physical Properties of Alkalis | 碱溶液的物理性质
Alkalis feel slippery or soapy to the touch and turn red litmus paper blue. Like acids, they conduct electricity due to the presence of mobile OH⁻ ions. Common alkalis such as sodium hydroxide solution are corrosive and must be handled with care.
碱溶液摸起来有滑腻感，能使红色石蕊试纸变蓝。与酸类似，由于存在可移动的OH⁻离子，碱溶液也能导电。常见的碱如氢氧化钠溶液具有腐蚀性，操作时必须小心。
The characteristic soapy feel comes from the reaction of alkalis with natural oils on your skin, which is another reminder that direct contact should be avoided. Both litmus and universal indicator can distinguish alkalis from acids by colour change.
滑腻感来自碱与皮肤上天然油脂的反应，这再次提醒我们应避免直接接触。石蕊试纸和通用指示剂均可通过颜色变化区分碱和酸。
4. The pH Scale – Measuring Acidity and Alkalinity | pH标度——衡量酸碱度
The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate an acidic solution (lower pH = more acidic), while values above 7 indicate an alkaline solution (higher pH = more alkaline). Pure water has a pH of 7 at 25 °C.
pH标度范围从0到14，7为中性。pH值低于7表明溶液为酸性（pH越低酸性越强），高于7则为碱性（pH越高碱性越强）。25 °C时纯水的pH为7。
Each unit change in pH corresponds to a tenfold change in H⁺ ion concentration. For example, a solution with pH 3 has 10 times more H⁺ ions than a solution with pH 4. This logarithmic relationship is a common source of calculation questions.
pH值每变化1个单位，H⁺离子浓度就改变10倍。例如，pH 3的溶液所含H⁺离子浓度是pH 4溶液的10倍。这种对数关系是常见的计算题来源。
| pH range | Type of solution | Example |
|---|---|---|
| 0 – 2 | Strong acid | Hydrochloric acid, HCl |
| 3 – 5 | Weak acid | Ethanoic acid (vinegar) |
| 6 – 7 | Very weak acid / neutral | Rainwater, pure water |
| 8 – 10 | Weak alkali | Ammonia solution |
| 11 – 12 | Strong alkali | Sodium hydroxide solution |
| 13 – 14 | Concentrated strong alkali | NaOH (conc.) |
Understanding the pH scale allows you to compare the acidity of different solutions and predict the colour change of indicators.
理解pH标度后，你就能比较不同溶液的酸度，并预测指示剂的颜色变化。
5. Common Indicators and Their Colours | 常见指示剂及其颜色变化
Indicators are substances that change colour depending on the pH of the solution. Litmus is the most basic: red in acid, blue in alkali. Methyl orange and phenolphthalein are frequently used in titrations.
指示剂是能根据溶液pH变化而改变颜色的物质。石蕊是最基本的指示剂：酸性下呈红色，碱中呈蓝色。甲基橙和酚酞常用于滴定实验。
The table below summarises the key colours you must learn for the exam. Knowing the exact colour at the end point of a titration is especially important for practical-based questions.
下表总结了考试必须掌握的关键颜色。掌握滴定终点时的准确颜色对实验类题目尤为重要。
| Indicator | Colour in acid | Colour in neutral | Colour in alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Methyl orange | Red | Orange | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
Universal indicator, a mixture of several indicators, shows a gradual colour change across the full pH range, from red (strong acid) to green (neutral) to purple (strong alkali).
通用指示剂由多种指示剂混合而成，在整个pH范围内呈现逐渐变化的颜色，从红色（强酸）到绿色（中性）再到紫色（强碱）。
6. Strength vs Concentration | 酸的强度与浓度
Acid strength and acid concentration are distinct concepts often confused by students. Strength refers to the degree of ionisation in water. A strong acid, such as HCl, fully dissociates into ions: HCl → H⁺ + Cl⁻. A weak acid, such as ethanoic acid (CH₃COOH), only partially dissociates, establishing an equilibrium: CH₃COOH ⇌ H⁺ + CH₃COO⁻.
酸的强度与浓度是两个截然不同的概念，常被学生混淆。强度是指酸在水中的电离程度。强酸（如HCl）完全电离：HCl → H⁺ + Cl⁻。弱酸（如乙酸CH₃COOH）仅部分电离，形成平衡：CH₃COOH ⇌ H⁺ + CH₃COO⁻。
Concentration simply describes how much acid is dissolved in a given volume of water. You can have a concentrated weak acid (lots of ethanoic acid molecules per dm³, but only a small fraction ionised) or a dilute strong acid (few HCl molecules per dm³, but all are ionised). Exam questions frequently test this distinction.
浓度仅描述在一定体积水中溶解了多少酸。你可以有浓的弱酸（每 dm³ 含大量乙酸分子，但只有小部分电离），也可以有稀的强酸（每 dm³ 含少量 HCl 分子，但全部电离）。考题经常考查这一区别。
The same logic applies to alkalis: NaOH is a strong alkali (fully ionised) while ammonia solution (NH₃) is a weak alkali because only a small proportion of ammonia molecules react with water to form NH₄⁺ and OH⁻.
同样的逻辑也适用于碱：NaOH是强碱（完全电离），而氨水（NH₃）是弱碱，因为只有一小部分氨分子与水反应生成NH₄⁺和OH⁻。
7. Neutralisation Reactions | 中和反应
Neutralisation is the reaction between an acid and a base to form a salt and water. The essential ionic equation for neutralisation is: H⁺(aq) + OH⁻(aq) → H₂O(l). This equation is the foundation for all acid-base titrations.
中和反应是酸与碱之间生成盐和水的反应。中和反应的核心离子方程式为：H⁺(aq) + OH⁻(aq) → H₂O(l)。该方程式是所有酸碱滴定的基础。
For example, when hydrochloric acid reacts with sodium hydroxide, the salt produced is sodium chloride: HCl + NaOH → NaCl + H₂O. To name the salt, take the metal from the base and the non-metal/radical from the acid. Sulfuric acid gives sulfates, nitric acid gives nitrates, and so on.
例如，盐酸与氢氧化钠反应时，生成的盐是氯化钠：HCl + NaOH → NaCl + H₂O。盐的命名方法：取碱中的金属部分和酸中的非金属/酸根部分。硫酸生成硫酸盐，硝酸生成硝酸盐，依此类推。
Neutralisation is exothermic; the temperature rises when an acid and alkali mix. This temperature change can be used to follow the progress of a titration in a ‘thermometric titration’.
中和反应是放热反应；酸碱混合时温度升高。该温度变化可用于在“热滴定”中追踪滴定进程。
8. Reactions of Acids with Metals | 酸与金属的反应
Dilute acids react with reactive metals (those above hydrogen in the reactivity series) to produce a salt and hydrogen gas. The general word equation is: acid + metal → salt + hydrogen. For instance, magnesium + hydrochloric acid gives magnesium chloride and hydrogen: Mg + 2HCl → MgCl₂ + H₂.
稀酸能与活泼金属（金属活动性顺序中排在氢前的金属）反应，生成盐和氢气。一般文字表达式为：酸 + 金属 → 盐 + 氢气。例如，镁与盐酸反应生成氯化镁和氢气：Mg + 2HCl → MgCl₂ + H₂。
You can test for hydrogen gas by holding a lit splint at the mouth of the test tube; hydrogen burns with a squeaky pop sound. This is an excellent way to confirm that the reaction has occurred. Unreactive metals such as copper and silver do not react with dilute acids.
检验氢气的方法：用点燃的木条靠近试管口；氢气燃烧会发出尖锐的爆鸣声。这是确认反应发生的极佳方法。不活泼金属（如铜和银）不与稀酸反应。
9. Reactions of Acids with Carbonates and Hydrogencarbonates | 酸与碳酸盐及碳酸氢盐的反应
Acids react with carbonates and hydrogencarbonates to form a salt, water and carbon dioxide. The general equations are: acid + carbonate → salt + water + CO₂; acid + hydrogencarbonate → salt + water + CO₂. Observations include vigorous effervescence (fizzing) due to the release of CO₂ gas.
酸与碳酸盐、碳酸氢盐反应生成盐、水和二氧化碳。一般方程式为：酸 + 碳酸盐 → 盐 + 水 + CO₂；酸 + 碳酸氢盐 → 盐 + 水 + CO₂。实验现象包括因放出 CO₂ 气体而产生的剧烈冒泡现象。
You can test for carbon dioxide by bubbling the gas through limewater (calcium hydroxide solution); it turns milky or cloudy. In the exam, linking the observation of milky limewater to the presence of a carbonate is a classic mark-scoring point.
检验二氧化碳的方法是将气体通入石灰水（氢氧化钙溶液）；石灰水变浑浊或呈乳白色。考试中，将石灰水变浑浊的现象与碳酸盐的存在联系起来，是经典得分点。
10. Reactions of Bases and Alkalis with Ammonium Salts | 碱与铵盐的反应
Alkalis (and some bases) react with ammonium salts on warming to produce ammonia gas. For example, ammonium chloride + sodium hydroxide → sodium chloride + water + ammonia: NH₄Cl + NaOH → NaCl + H₂O + NH₃. This reaction is a specific test for ammonium ions (NH₄⁺).
碱（及某些碱类）在加热条件下与铵盐反应生成氨气。例如，氯化铵与氢氧化钠反应生成氯化钠、水和氨气：NH₄Cl + NaOH → NaCl + H₂O + NH₃。该反应是检验铵离子（NH₄⁺）的特效方法。
Ammonia gas has a sharp, pungent smell and turns damp red litmus paper blue. It is the only common alkaline gas, making this test straightforward. Be careful when smelling gases: waft the vapour towards your nose rather than inhaling directly.
氨气具有刺鼻性气味，能使湿润的红色石蕊试纸变蓝。它是唯一常见的碱性气体，因此这个检验方法简单直接。闻气体时需谨慎：用手将蒸气轻轻扇向鼻子，切勿直接吸入。
11. Making Salts – Choosing a Method | 制备盐——选择合适的方法
The method for making a salt depends on the solubility of the salt and the reactivity of the metal. For soluble salts from a soluble base (alkali), titration is used because no solid excess remains. For insoluble salts, precipitation (double decomposition) is the method of choice.
制备盐的方法取决于盐的溶解度以及金属的活泼性。由可溶性碱（碱）制备可溶性盐时使用滴定法，因为不会残留过量固体。制备不溶性盐则首选沉淀法（复分解反应）。
When making a soluble salt from an insoluble base (e.g. metal oxide or carbonate), the excess solid method works best: add excess solid to warm acid, filter off the unreacted solid, then crystallise the salt from the filtrate. Copper(II) sulfate from copper(II) oxide and sulfuric acid is a classic example.
从不溶性碱（如金属氧化物或碳酸盐）制备可溶性盐时，“过量固体法”效果最佳：将过量固体加入温热酸中，过滤掉未反应固体，然后从滤液中结晶析出盐。用氧化铜和硫酸制备硫酸铜(II)即为典型例子。
Selecting the correct method often appears in practical exam questions. Remember: titration for soluble salt from soluble base; excess solid + filtration + crystallisation for soluble salt from insoluble base; precipitation for insoluble salts.
选择正确的方法常出现在实验考题中。请记住：可溶性碱制可溶性盐用滴定法；不溶性碱制可溶性盐用过量固体+过滤+结晶法；不溶性盐用沉淀法。
12. Water of Crystallisation and Hydrated Salts | 结晶水与含水盐
Many salts crystallise from aqueous solution with water molecules trapped in the crystal lattice. These are called hydrated salts, and the associated water is water of crystallisation. For example, copper(II) sulfate pentahydrate has the formula CuSO₄·5H₂O.
许多盐从水溶液中结晶时会带有水分子嵌入晶格。这类盐称为水合盐，所带的水称为结晶水。例如，五水合硫酸铜(II)的化学式为 CuSO₄·5H₂O。
Heating a hydrated salt can drive off the water, leaving an anhydrous salt. CuSO₄·5H₂O (blue) → CuSO₄ (white) + 5H₂O. This reversible colour change is a test for water: anhydrous copper(II) sulfate turns blue in the presence of water.
加热水合盐可除去结晶水，得到无水盐。CuSO₄·5H₂O（蓝色）→ CuSO₄（白色）+ 5H₂O。这种可逆颜色变化可用于检验水：无水硫酸铜(II)遇水变蓝。
Calculations involving water of crystallisation require you to use mass data from heating to determine the value of n in the formula X·nH₂O. This combines mole calculations with practical gravimetric analysis.
涉及结晶水的计算要求你根据加热实验的质量数据确定化学式 X·nH₂O 中 n 的值。这需要综合运用摩尔计算与重量分析。
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