📚 Acids, Bases and the pH Scale | 酸、碱与 pH 标度
This revision guide covers acids, bases and the pH scale for Cambridge IGCSE / O Level Science. You will learn how to define acids and bases, use the pH scale, identify common indicators, write neutralisation equations, and apply titration calculations. These concepts appear frequently in Paper 2 and Paper 4, so practise both recall and application.
本复习指南涵盖剑桥 IGCSE / O Level 科学中的酸、碱与 pH 标度。你将学习如何定义酸和碱、使用 pH 标度、识别常见指示剂、书写中和反应方程式，并应用滴定计算。这些概念在 Paper 2 和 Paper 4 中经常出现，因此既要记忆，也要练习应用。
1. Defining Acids and Bases | 酸与碱的定义
In the Cambridge syllabus, an acid is a proton donor. When hydrogen chloride gas dissolves in water, it releases hydrogen ions (H⁺). A base is a proton acceptor. Metal oxides and metal hydroxides are typical bases. An alkali is a soluble base that releases hydroxide ions (OH⁻) in water.
在剑桥课程中，酸是质子供体。氯化氢气体溶于水时会释放氢离子（H⁺）。碱是质子受体。金属氧化物和金属氢氧化物是典型的碱。可溶性碱在水中释放氢氧根离子（OH⁻），称为 alkali（可溶性碱）。
HCl → H⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
2. The pH Scale and Its Meaning | pH 标度及其含义
The pH scale runs from 0 to 14. A pH below 7 indicates an acidic solution; a pH above 7 indicates an alkaline solution; pH 7 is neutral. Each unit change in pH represents a tenfold change in hydrogen ion concentration.
pH 标度范围为 0 至 14。pH 低于 7 表示酸性溶液；pH 高于 7 表示碱性溶液；pH 7 为中性。pH 每变化一个单位，氢离子浓度就改变十倍。
pH = -log₁₀[H⁺]
For example, a solution with pH 3 has ten times the H⁺ concentration of a solution with pH 4. Universal indicator turns green in a neutral solution.
例如，pH 3 的溶液中 H⁺ 浓度是 pH 4 溶液的十倍。通用指示剂在中性溶液中呈绿色。
3. Common Indicators and Colour Changes | 常见指示剂与颜色变化
Indicators are dyes that change colour depending on pH. Litmus turns red in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Phenolphthalein is colourless in acid and pink in alkali. Universal indicator gives a range of colours from red to violet.
指示剂是随 pH 改变颜色的染料。石蕊在酸性中变红、碱性中变蓝。甲基橙在酸性中呈红色、碱性中呈黄色。酚酞在酸性中无色、碱性中呈粉红色。通用指示剂呈现从红到紫的一系列颜色。
| Indicator | Acid colour | Neutral colour | Alkaline colour |
|---|---|---|---|
| Litmus | red | purple | blue |
| Methyl orange | red | orange | yellow |
| Phenolphthalein | colourless | colourless | pink |
4. Strong and Weak Acids | 强酸与弱酸
A strong acid, such as hydrochloric acid, completely ionises in water. A weak acid, such as ethanoic acid, only partially ionises, forming an equilibrium mixture. Strong and weak do not mean concentrated and dilute; concentration refers to the amount of acid dissolved, while strength refers to the degree of ionisation.
强酸（如盐酸）在水中完全电离。弱酸（如乙酸）仅部分电离，形成平衡混合物。强与弱不等于浓与稀；浓度指溶解的酸量，强度指电离程度。
CH₃COOH ⇌ H⁺ + CH₃COO⁻
Common strong acids include HCl, H₂SO₄ and HNO₃. Common weak acids include ethanoic acid, citric acid and carbonic acid. Weak acids have a higher pH than strong acids of the same concentration.
常见的强酸包括 HCl、H₂SO₄ 和 HNO₃。常见的弱酸包括乙酸、柠檬酸和碳酸。相同浓度下，弱酸的 pH 高于强酸。
5. Properties of Acids | 酸的性质
Acids have sour taste (do not taste in lab), turn blue litmus red, and react with reactive metals to produce hydrogen gas. They react with carbonates to produce carbon dioxide, water and a salt. They also react with bases to form salt and water.
酸有酸味（实验室中不可品尝）、使蓝色石蕊变红，并与活泼金属反应产生氢气。酸与碳酸盐反应生成二氧化碳、水和盐。酸与碱反应生成盐和水。
- metal + acid → salt + hydrogen | 金属 + 酸 → 盐 + 氢气
- carbonate + acid → salt + water + carbon dioxide | 碳酸盐 + 酸 → 盐 + 水 + 二氧化碳
- base + acid → salt + water | 碱 + 酸 → 盐 + 水
Example equations: Zn + 2HCl → ZnCl₂ + H₂; CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O.
示例方程式：Zn + 2HCl → ZnCl₂ + H₂；CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O。
6. Properties of Bases and Alkalis | 碱与可溶性碱的性质
Bases neutralise acids to form salt and water. Alkalis feel soapy and turn red litmus blue. Ammonia gas is a common weak base; when dissolved in water it forms ammonium hydroxide, which partially ionises. Strong alkalis such as sodium hydroxide are corrosive.
碱能中和酸生成盐和水。可溶性碱有滑腻感，使红色石蕊变蓝。氨气是常见的弱碱；溶于水形成氨水，部分电离。强碱如氢氧化钠具有腐蚀性。
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Common bases include copper(II) oxide, iron(III) oxide and calcium hydroxide. Common alkalis include sodium hydroxide, potassium hydroxide and aqueous ammonia.
常见的碱包括氧化铜(II)、氧化铁(III)和氢氧化钙。常见的可溶性碱包括氢氧化钠、氢氧化钾和氨水。
7. Neutralisation Reactions | 中和反应
ali is H⁺ + OH⁻ → H₂O. Neutralisation is exothermic; the temperature of the mixture rises.
中和是酸与碱反应生成盐和水的过程。强酸与强碱反应的离子方程式为 H⁺ + OH⁻ → H₂O。中和反应放热，混合物的温度升高。
HCl + NaOH → NaCl + H₂O
In a full ionic equation, spectator ions are omitted. For example, Na⁺ and Cl⁻ remain unchanged and are not included in the net ionic equation.
在完整离子方程式中，旁观离子被省略。例如 Na⁺ 和 Cl⁻ 保持不变，不包含在净离子方程式中。
8. Salts and Their Preparation | 盐及其制备
A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. Soluble salts can be prepared by titration or by reacting an acid with a metal, insoluble base or carbonate. Insoluble salts are made by precipitation.
盐是酸中的氢被金属离子或铵根离子取代后形成的离子化合物。可溶性盐可通过滴定或酸与金属、不溶性碱或碳酸盐反应制备。不溶性盐通过沉淀法制备。
| Salt | Parent acid | Parent base |
|---|---|---|
| sodium chloride | hydrochloric acid | sodium hydroxide |
| copper(II) sulfate | sulfuric acid | copper(II) oxide |
| potassium nitrate | nitric acid | potassium hydroxide |
Titration is used for salts of Group 1 metals and ammonium salts because they are soluble. Excess solid method is used for transition metal salts using insoluble bases such as CuO or MgO.
滴定法用于第 1 族金属盐和铵盐，因为它们可溶。过量固体法用于过渡金属盐，使用不溶性碱如 CuO 或 MgO。
9. Titration Calculations | 滴定计算
= 0.100 × 0.025 = 0.0025 mol. Ratio 1:1, so moles HCl = 0.0025 mol. Concentration HCl = 0.0025 / 0.020 = 0.125 mol/dm³.
在滴定中，已知体积的酸用指示剂与碱中和。浓度可用公式：浓度 = 物质的量 / 体积。示例：25.0 cm³ 的 0.100 mol/dm³ NaOH 中和 20.0 cm³ HCl。计算 HCl 的浓度。NaOH 物质的量 = 0.100 × 0.025 = 0.0025 mol。因 1:1，HCl 物质的量 = 0.0025 mol。HCl 浓度 = 0.0025 / 0.020 = 0.125 mol/dm³。
c = n / V
Remember to convert cm³ to dm³ by dividing by 1000. Always check the mole ratio from the balanced equation before calculating unknown concentrations.
记住将 cm³ 转换为 dm³ 需除以 1000。在计算未知浓度前，务必根据配平方程式检查物质的量之比。
10. Everyday Applications and Exam Tips | 日常应用与备考提示
Acids and bases are used in everyday life: stomach acid contains hydrochloric acid, antacids contain bases to neutralise excess acid, and soil pH is adjusted with lime. In exams, always write balanced symbol equations and state symbols where asked. Avoid giving vague answers such as 'neutralises' without naming the products.
酸和碱在日常生活中广泛应用：胃酸含盐酸，抗酸药含碱以中和过多胃酸，土壤 pH 用石灰调节。考试中要书写配平的符号方程式，并在要求时标注状态符号。避免给出“中和”这类模糊答案而不说明产物。
rmed from a named acid and base, explain why phenolphthalein is suitable for a strong acid– icator at a given pH. Practise using the ionic equation H⁺ + OH⁻ → H₂O for neutralisation.
常见考题要求你根据指定的酸和碱判断生成的盐、解释为什么酚酞适用于强酸–强碱滴定，或预测给定 pH 下通用指示剂的颜色。练习使用中和反应的离子方程式 H⁺ + OH⁻ → H₂O。
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