📚 Acids, Bases and Salts | 酸、碱和盐
Acids, bases and salts form the core of many IGCSE Chemistry topics, from describing everyday substances to calculating concentrations. In this revision guide, we will explore the definitions, properties, reactions, and preparation methods of salts, all aligned with the Edexcel IGCSE Science specification.
酸、碱和盐是IGCSE化学的核心内容，从描述日常物质到计算浓度都离不开它们。在本复习指南中，我们将围绕Edexcel IGCSE科学考纲，深入探讨酸、碱和盐的定义、性质、反应以及盐的制备方法。
1. What Are Acids? | 什么是酸？
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid ionises to H⁺ and Cl⁻. In aqueous solution, these H⁺ ions are responsible for the characteristic acidic properties.
酸是指溶于水时能产生氢离子（H⁺）的物质。例如，盐酸电离为H⁺和Cl⁻。在水溶液中，这些H⁺离子是酸典型性质的来源。
- Physical properties: sour taste; turn blue litmus paper red; pH less than 7.
- 化学性质：酸味；使蓝色石蕊试纸变红；pH小于7。
- Common acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃).
- 常见酸：盐酸（HCl）、硫酸（H₂SO₄）、硝酸（HNO₃）。
2. What Are Bases and Alkalis? | 什么是碱和可溶性碱？
A base is a substance that neutralises an acid to form a salt and water. Bases are usually metal oxides and metal hydroxides. An alkali is a soluble base that releases hydroxide ions (OH⁻) in water.
碱是能与酸反应生成盐和水的物质。碱通常是金属氧化物和金属氢氧化物。可溶性碱（alkali）是溶于水后能释放氢氧根离子（OH⁻）的碱。
- Properties: soapy feel; turn red litmus paper blue; pH greater than 7.
- 性质：有滑腻感；使红色石蕊试纸变蓝；pH大于7。
- Examples of alkalis: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂).
- 可溶性碱示例：氢氧化钠（NaOH）、氢氧化钾（KOH）、氢氧化钙（Ca(OH)₂）。
3. The pH Scale and Indicators | pH值标度和指示剂
The pH scale measures how acidic or alkaline a solution is, usually from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline.
pH值标度用于测量溶液的酸碱性，通常范围是0到14。pH等于7为中性，小于7为酸性，大于7为碱性。
pH < 7 (acidic) | pH = 7 (neutral) | pH > 7 (alkaline)
- Litmus: red in acid, blue in alkali.
- 石蕊：遇酸变红，遇碱变蓝。
- Phenolphthalein: colourless in acid, pink in alkali (pH > 8.2).
- 酚酞：在酸性溶液中无色，在碱性溶液中呈粉色（pH > 8.2）。
- Methyl orange: red in acid, yellow in alkali.
- 甲基橙：在酸性溶液中呈红色，在碱性溶液中呈黄色。
4. Neutralisation Reactions | 中和反应
an acid and a base, producing a salt and water only. The ionic equation is H⁺ + OH⁻ → H₂O.
中和反应是酸与碱反应生成盐和水的过程。离子方程式为 H⁺ + OH⁻ → H₂O。
Acid + Base → Salt + Water
Example: HCl + NaOH → NaCl + H₂O (hydrochloric acid + sodium hydroxide → sodium chloride + water).
示例：HCl + NaOH → NaCl + H₂O（盐酸 + 氢氧化钠 → 氯化钠 + 水）。
5. Acids and Metals | 酸与金属
Acids react with certain reactive metals to produce a salt and hydrogen gas. This is a displacement reaction where the metal replaces hydrogen in the acid.
酸与活泼金属反应生成盐和氢气。这是一种置换反应，金属取代了酸中的氢。
Acid + Metal → Salt + Hydrogen
Example: Zn + 2HCl → ZnCl₂ + H₂ (zinc + hydrochloric acid → zinc chloride + hydrogen).
示例：Zn + 2HCl → ZnCl₂ + H₂（锌 + 盐酸 → 氯化锌 + 氢气）。
- Testing for hydrogen: a lighted splint produces a squeaky pop.
- 氢气检验：用点燃的火柴靠近，发出“噗”的爆鸣声。
- Reactivity: hydrogen is only displaced by metals above it in the reactivity series.
- 活泼性：只有比氢活泼的金属才能置换出酸中的氢。
6. Acids and Carbonates | 酸与碳酸盐
Acids react with carbonates and hydrogencarbonates to produce a salt, water and carbon dioxide gas. This reaction is often used to test for carbonate ions.
酸与碳酸盐和碳酸氢盐反应生成盐、水和二氧化碳气体。该反应常用于检验碳酸根离子。
Acid + Carbonate → Salt + Water + Carbon Dioxide
Example: + CO₂ (calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide).
示例：CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂（碳酸钙 + 盐酸 → 氯化钙 + 水 + 二氧化碳）。
- Test for carbon dioxide: gas turns limewater (calcium hydroxide solution) milky.
- 二氧化碳检验：气体使澄清石灰水（氢氧化钙溶液）变浑浊。
7. Types of Oxides | 氧化物的分类
Oxides are compounds containing oxygen and one other element. They are classified based on their reaction with acids and bases.
氧化物是氧元素与另一种元素形成的化合物，根据与酸和碱的反应情况分类。
| Type | Example | Reaction |
| Acidic oxide | CO₂, SO₂ | React with bases to form salt and water |
| Basic oxide | CuO, MgO | React with acids to form salt and water |
| Amphoteric oxide | Al₂O₃, ZnO | React with both acids and bases |
| Neutral oxide | CO, H₂O | No reaction with acids or bases |
Note: The English table headings are "Type", "Example", "Reaction"; Chinese equivalents are 类型、示例、反应. This structure helps revision.
注意：表格标题分别为“类型”“示例”“反应”。这种结构有助于复习记忆。
8. Salts and Their Preparation | 盐及其制备
A salt is formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion or ammonium ion (NH₄⁺). For example, H₂SO₄ can form two series of salts: sulfates and hydrogen sulfates.
盐是酸中的氢离子（H⁺）被金属离子或铵根离子（NH₄⁺）取代后生成的化合物。例如，H₂SO₄能形成两系列盐：硫酸盐和硫酸氢盐。
Insoluble salts are prepared by precipitation. Soluble salts are prepared either by reacting an acid with an insoluble base or by titrating an acid with an alkali.
不溶性盐通过沉淀反应制备。可溶性盐可以通过酸与不溶性碱反应或酸与碱的滴定反应制备。
- Precipitation: Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)
- 沉淀反应：Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)
- Acid + insoluble base: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
- 酸 + 不溶性碱：CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
- Titration: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
- 滴定：NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
9. Solubility Rules | 溶解性规律
Knowing which salts are soluble in water is essential for predicting precipitation reactions and choosing preparation methods.
了解哪些盐可溶于水，对于预测沉淀反应和选择制备方法至关重要。
| Soluble salts | Exceptions |
| All sodium, potassium and ammonium salts | None |
| All nitrates | None |
| Most chlorides | AgCl, PbCl₂ only slightly soluble |
| Most sulfates | BaSO₄, PbSO₄ insoluble; CaSO₄ slightly soluble |
| Most carbonates | Only sodium, potassium and ammonium carbonates soluble |
Chinese translation of the table header: 第一列为“可溶性盐”，第二列为“例外情况”。
表格标题中文对照：第一列为“可溶性盐”，第二列为“例外情况”。
10. Testing for Ions | 离子检验
Identification of ions in salts uses characteristic reactions. For cations, flame tests and sodium hydroxide precipitation are common. For anions, specific gas tests are used.
通过特征反应可以鉴定盐中的离子。对于阳离子，常用焰色反应和氢氧化钠沉淀法；对于阴离子，则使用特定气体检验。
- Flame tests: Li⁺ (crimson), Na⁺ (yellow), K⁺ (lilac), Ca²⁺ (brick red), Cu²⁺ (blue-green).
- 焰色反应：Li⁺（深红）、Na⁺（黄）、K⁺（淡紫）、Ca²⁺（砖红）、Cu²⁺（蓝绿色）。
- Carbonate (CO₃²⁻): add dilute acid → bubbles of CO₂ (limewater turns milky).
- 碳酸根（CO₃²⁻）：加入稀酸 → 产生CO₂气泡（石灰水变浑浊）。
- Sulfate (SO₄²⁻): add dilute HCl and BaCl₂ → white precipitate of BaSO₄.
- 硫酸根（SO₄²⁻）：加入稀HCl和BaCl₂ → 产生白色BaSO₄沉淀。
- Halides (Cl⁻, Br⁻, I⁻): add dilute HNO₃ and AgNO₃ → white (AgCl), cream (AgBr) or yellow (AgI) precipitate.
- 卤离子（Cl⁻、Br⁻、I⁻）：加入稀HNO₃和AgNO₃ → 分别产生白色（AgCl）、淡黄色（AgBr）或黄色（AgI）沉淀。
11. Acid Rain | 酸雨
Acid rain is formed when sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels dissolve in rainwater. This causes environmental damage.
酸雨是化石燃料燃烧产生的二氧化硫（SO₂）和氮氧化物（NOₓ）溶于雨水形成的，会对环境造成破坏。
- Effects: corrosion of buildings, damage to trees and aquatic life, soil acidification.
- 影响：腐蚀建筑物、危害树木和水生生物、使土壤酸化。
- Prevention: remove sulphur from fuel before burning, use low-nitrogen combustion conditions, fit flue-gas desulfurisation.
- 防治：燃烧前脱硫、控制贫氧燃烧条件、安装烟气脱硫装置。
12. Summary and Exam Tips | 总结与考试要点
Remember the three key reactions: acid + base, acid + metal, acid + carbonate. Always balance equations and include state symbols. Use solubility rules to decide whether a precipitate forms.
牢记三个关键反应：酸+碱、酸+金属、酸+碳酸盐。书写方程式时务必配平并注明状态符号。利用溶解性规律判断是否有沉淀生成。
- Quick checklist: H⁺ for acid, OH⁻ for alkali, salt = H replaced by metal.
- 速查清单：酸提供H⁺，碱提供OH⁻，盐是H被金属取代的产物。
- Practice: write ionic equations by cancelling spectator ions.
- 练习：通过删去旁观离子写出离子方程式。
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