📚 Acids, Bases and Salts: Core Concepts for Cambridge Science | 酸、碱与盐：剑桥科学核心概念
Understanding acids, bases and salts is fundamental to Cambridge Science, especially in the Chemistry sections of Checkpoint, IGCSE and Coordinated Sciences. This article revises the key definitions, reactions, practical methods and common exam pitfalls you need to master.
理解酸、碱和盐是剑桥科学的基础，特别是在 Checkpoint、IGCSE 和综合科学的化学部分。本文复习关键定义、反应、实验方法以及你需要掌握的常见考试易错点。
1. What Are Acids? | 什么是酸？
An acid is a substance that releases hydrogen ions, H⁺, when dissolved in water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, HCl(aq), which contains H⁺ and Cl⁻ ions. The presence of H⁺ ions is what makes a solution acidic.
酸是溶于水时释放出氢离子 H⁺ 的物质。例如，氯化氢气体溶于水形成盐酸 HCl(aq)，其中含有 H⁺ 和 Cl⁻ 离子。H⁺ 离子的存在使溶液呈酸性。
Common laboratory acids include hydrochloric acid, sulfuric acid and nitric acid. Their formulas are HCl, H₂SO₄ and HNO₃ respectively. You should remember these because exam questions often ask you to write balanced neutralisation equations using them.
常见的实验室酸包括盐酸、硫酸和硝酸。它们的化学式分别是 HCl、H₂SO₄ 和 HNO₃。你应该记住这些，因为考试题目经常要求用它们书写配平的中和方程式。
- Acid definition: proton donor / H⁺ ion producer. | 酸的定义：质子供体 / 产生 H⁺ 离子。
- Examples: HCl, H₂SO₄, HNO₃, ethanoic acid. | 实例：HCl、H₂SO₄、HNO₃、乙酸。
2. What Are Bases and Alkalis? | 什么是碱和可溶性碱？
A base is a substance that reacts with an acid to form a salt and water. Bases include metal oxides, metal hydroxides and metal carbonates. 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。碱包括金属氧化物、金属氢氧化物和金属碳酸盐。碱溶液（可溶性碱）是溶于水时释放出氢氧根离子 OH⁻ 的物质。
Common alkalis are sodium hydroxide, NaOH, potassium hydroxide, KOH, and aqueous ammonia, NH₃(aq). Although ammonia has no OH⁻ in its formula, it produces OH⁻ ions when it reacts with water, so it is a weak alkali.
常见的碱溶液包括氢氧化钠 NaOH、氢氧化钾 KOH 和氨水 NH₃(aq)。虽然氨的化学式中没有 OH⁻，但它与水反应会生成 OH⁻ 离子，因此它是一种弱碱。
- Base: neutralises an acid. | 碱：能中和酸。
- Alkali: soluble base producing OH⁻. | 碱溶液：产生 OH⁻ 的可溶性碱。
3. The pH Scale and Universal Indicator | pH 标度与通用指示剂
The pH scale measures how acidic or alkaline a solution is. It ranges from 0 to 14. A pH below 7 is acidic, pH 7 is neutral, and a pH above 7 is alkaline. The lower the pH, the higher the concentration of H⁺ ions.
pH 标度用来衡量溶液的酸性或碱性程度，范围从 0 到 14。pH 低于 7 为酸性，pH 等于 7 为中性，pH 高于 7 为碱性。pH 越低，H⁺ 离子浓度越高。
Universal indicator is a mixture of dyes that changes colour gradually across the pH range. It is useful for estimating pH values, but it is not suitable for titrations because the colour change is gradual rather than sharp.
通用指示剂是一种染料混合物，在 pH 范围内颜色逐渐变化。它适合估计 pH 值，但不适合用于滴定，因为颜色变化是渐进而非突变的。
| pH range | Colour with universal indicator | Nature |
|---|---|---|
| 0-2 | Red | Strongly acidic |
| 3-5 | Orange / yellow | Weakly acidic |
| 6-7 | Green | Neutral |
| 8-11 | Blue / purple | Weakly alkaline |
| 12-14 | Violet / deep purple | Strongly alkaline |
4. Common Laboratory Indicators | 常见实验室指示剂
Besides universal indicator, litmus and phenolphthalein are frequently used. Litmus turns red in acidic solutions and blue in alkaline solutions. It is often used as litmus paper for quick tests.
除通用指示剂外，石蕊和酚酞也经常使用。石蕊在酸性溶液中变红，在碱性溶液中变蓝。通常以石蕊试纸形式用于快速检测。
Phenolphthalein is colourless in acidic and neutral solutions but turns pink in alkaline solutions. It is especially useful in acid-base titrations because it gives a sharp colour change.
酚酞在酸性和中性溶液中无色，但在碱性溶液中变成粉红色。它在酸碱滴定中特别有用，因为它能给出敏锐的颜色变化。
- Litmus: red in acid, blue in alkali. | 石蕊：酸中红，碱中蓝。
- Phenolphthalein: colourless in acid, pink in alkali. | 酚酞：酸中无色，碱中粉红。
5. Neutralisation Reactions | 中和反应
Neutralisation is the reaction between an acid and a base to produce a salt and water. The essential ionic equation is H⁺(aq) + OH⁻(aq) → H₂O(l). This equation shows that acidity and alkalinity cancel each other out.
中和反应是酸与碱反应生成盐和水的过程。其核心离子方程式为 H⁺(aq) + OH⁻(aq) → H₂O(l)。这个方程式表明酸性和碱性相互抵消。
A typical neutralisation is the reaction between hydrochloric acid and sodium hydroxide: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The salt formed is sodium chloride, NaCl, which is common table salt.
典型的中和反应是盐酸与氢氧化钠的反应：HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)。生成的盐是氯化钠 NaCl，也就是常见的食盐。
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
6. Making Salts: Methods and Examples | 盐的制备：方法与实例
Salts can be prepared by several methods, depending on whether the base is soluble or insoluble. The method you choose affects the steps you must describe in a practical exam question.
盐可以通过多种方法制备，取决于所用的碱是可溶的还是不可溶的。你选择的方法会影响你在实验考试题中需要描述的步骤。
For a soluble base such as sodium hydroxide, titration is used. For an insoluble base such as copper(II) oxide, the method is: add excess base to warm acid, filter off the excess solid, and evaporate the filtrate to obtain salt crystals.
对于可溶性碱如氢氧化钠，使用滴定法。对于不可溶性碱如氧化铜，方法是：将过量碱加入温热的酸中，过滤掉过量固体，蒸发滤液得到盐晶体。
- Soluble base + acid: titration. | 可溶性碱 + 酸：滴定法。
- Insoluble base + acid: add excess, filter, evaporate. | 不可溶性碱 + 酸：加过量、过滤、蒸发。
7. Solubility Rules for Salts | 盐的溶解性规则
Solubility rules help you predict whether a salt will dissolve in water. All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Most chlorides are soluble except silver chloride and lead chloride.
溶解性规则帮助你预测盐是否能溶于水。所有钠盐、钾盐和铵盐都可溶。所有硝酸盐都可溶。大多数氯化物可溶，但氯化银和氯化铅除外。
Most sulfates are soluble, but barium sulfate and lead sulfate are insoluble. Most carbonates are insoluble, except sodium carbonate, potassium carbonate and ammonium carbonate.
大多数硫酸盐可溶，但硫酸钡和硫酸铅不可溶。大多数碳酸盐不可溶，但碳酸钠、碳酸钾和碳酸铵除外。
| Salt type | Soluble? | Exceptions |
|---|---|---|
| Nitrates | All soluble | None |
| Chlorides | Most soluble | AgCl, PbCl₂ |
| Sulfates | Most soluble | BaSO₄, PbSO₄ |
| Carbonates | Most insoluble | Na₂CO₃, K₂CO₃, (NH₄)₂CO₃ |
8. Everyday Applications of Acids and Bases | 酸碱的日常应用
Acids and bases are used in everyday life, from food preservation to cleaning products. Ethanoic acid gives vinegar its sour taste, while citric acid is found in citrus fruits. Hydrochloric acid in the stomach helps digest food.
酸碱在日常生活中应用广泛，从食品防腐到清洁产品。乙酸使醋具有酸味，柠檬酸存在于柑橘类水果中。胃中的盐酸帮助消化食物。
Alkalis such as sodium hydroxide are used in soap making and drain cleaners. Calcium hydroxide is used to neutralise acidic soils in agriculture, while magnesium hydroxide is an ingredient in antacids to relieve indigestion.
氢氧化钠等碱用于制造肥皂和下水道清洁剂。氢氧化钙在农业中用于中和酸性土壤，而氢氧化镁是抗酸药中的成分，用于缓解消化不良。
- Acids in food: ethanoic acid, citric acid, ascorbic acid. | 食品中的酸：乙酸、柠檬酸、抗坏血酸。
- Alkalis at home: cleaning products, antacids, soil treatment. | 家用碱：清洁产品、抗酸药、土壤处理。
9. Common Mistakes and Exam Tips | 常见错误与考试技巧
A common mistake is to say that all bases are alkalis. An alkali is a soluble base, so insoluble bases such as copper(II) oxide are not alkalis. Be precise with terminology in written answers.
一个常见错误是说所有碱都是碱溶液。碱溶液是可溶性碱，因此像氧化铜这样的不可溶性碱不是碱溶液。在书面答案中要准确使用术语。
When writing ionic equations, cancel spectator ions such as Na⁺ and Cl⁻. For example, in HCl + NaOH, the full ionic equation is H⁺ + Cl⁻ + Na⁺ + OH⁻ → Na⁺ + Cl⁻ + H₂O. Removing spectator ions leaves H⁺ + OH⁻ → H₂O.
书写离子方程式时，要删除旁观离子如 Na⁺ 和 Cl⁻。例如，在 HCl + NaOH 中，完整离子方程式为 H⁺ + Cl⁻ + Na⁺ + OH⁻ → Na⁺ + Cl⁻ + H₂O。删除旁观离子后得到 H⁺ + OH⁻ → H₂O。
H⁺(aq) + OH⁻(aq) → H₂O(l)
10. Quick Revision Summary | 快速复习总结
Acids produce H⁺ ions, alkalis produce OH⁻ ions, and neutralisation produces a salt plus water. The pH scale classifies solutions from acidic to alkaline, with universal indicator showing colours from red to violet.
酸产生 H⁺ 离子，碱溶液产生 OH⁻ 离子，中和反应生成盐和水。pH 标度将溶液从酸性到碱性进行分类，通用指示剂颜色从红到紫。
Salt preparation depends on the solubility of the base. Use titration for soluble bases and the excess-solid method for insoluble bases. Learn solubility rules to predict precipitation reactions and write correct formulas.
盐的制备取决于碱的溶解性。可溶性碱用滴定法，不可溶性碱用过量固体法。学习溶解性规则可以预测沉淀反应并书写正确的化学式。
- Acid + metal hydroxide → salt + water. | 酸 + 金属氢氧化物 → 盐 + 水。
- Acid + metal carbonate → salt + water + carbon dioxide. | 酸 + 金属碳酸盐 → 盐 + 水 + 二氧化碳。
- Memorise solubility exceptions: AgCl, PbCl₂, BaSO₄, PbSO₄. | 记住溶解性例外：AgCl、PbCl₂、BaSO₄、PbSO₄。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导