📚 IGCSE Edexcel Science: Acids, Bases and Salts — A Complete Revision Guide | IGCSE Edexcel 科学：酸、碱与盐——完整复习指南
Acids, bases and salts form a core topic in IGCSE Edexcel Science (Double Award and Triple Award). This guide covers the key definitions, reactions, pH scale, indicators, and salt preparation methods you need for your exams. Each section pairs an English explanation with a Chinese translation to help you master the concepts in both languages.
酸、碱与盐是 IGCSE Edexcel 科学（双奖和 triple award）中的核心主题。本指南涵盖了你考试需要的关键定义、反应、pH 标度、指示剂以及盐的制备方法。每个部分都配对了英文解释和中文翻译，帮助你用两种语言掌握概念。
1. Definitions of Acids and Bases | 酸和碱的定义
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid produces H⁺ and Cl⁻ ions in solution. Acids have a pH less than 7 and turn blue litmus paper red.
酸是溶于水时产生氢离子（H⁺）的物质。例如，盐酸在溶液中产生 H⁺ 和 Cl⁻ 离子。酸的 pH 小于 7，能使蓝色石蕊试纸变红。
A base is a substance that neutralises an acid to form water and a salt. Bases that are soluble in water are called alkalis. Alkalis produce hydroxide ions (OH⁻) when dissolved in water. Bases have a pH greater than 7 and turn red litmus paper blue.
碱是能与酸中和生成水和盐的物质。可溶于水的碱称为碱液（alkalis）。碱液溶于水时产生氢氧根离子（OH⁻）。碱的 pH 大于 7，能使红色石蕊试纸变蓝。
- Acid → H⁺ (aq) source | 酸 → 提供 H⁺ (aq)
- Alkali → OH⁻ (aq) source | 碱液 → 提供 OH⁻ (aq)
- Base = metal oxide / hydroxide / carbonate | 碱 = 金属氧化物 / 氢氧化物 / 碳酸盐
2. The pH Scale and Indicators | pH 标度和指示剂
The pH scale runs from 0 to 14. Values below 7 are acidic, 7 is neutral, and values above 7 are alkaline. The pH of a solution can be measured using universal indicator paper or a pH meter. Universal indicator gives a colour change depending on the pH: red/orange for strong to weak acids, green for neutral, and blue/purple for alkalis.
pH 标度从 0 到 14。低于 7 的值为酸性，7 为中性，高于 7 的值为碱性。溶液的 pH 可以用万能指示剂试纸或 pH 计测量。万能指示剂根据 pH 显示不同颜色：红色/橙色表示强酸到弱酸，绿色表示中性，蓝色/紫色表示碱。
Common indicators:
- Litmus: red in acid, blue in alkali | 石蕊：酸中变红，碱中变蓝
- Methyl orange: red in acid, yellow in alkali | 甲基橙：酸中红色，碱中黄色
- Phenolphthalein: colourless in acid, pink in alkali | 酚酞：酸中无色，碱中粉红色
- Universal indicator: full spectrum of colours | 万能指示剂：全色系
pH < 7 → acidic | pH = 7 → neutral | pH > 7 → alkaline
3. Reactions of Acids | 酸的反应
Acids react with many substances. You need to know these four general reactions and their word equations.
酸能与许多物质反应。你需要知道以下四种一般反应及其文字方程式。
3.1 Acid + Metal → Salt + Hydrogen | 酸 + 金属 → 盐 + 氢气
For example, hydrochloric acid + zinc → zinc chloride + hydrogen. A test for hydrogen is the squeaky pop test: a lit splint held near the gas produces a popping sound.
例如，盐酸 + 锌 → 氯化锌 + 氢气。氢气的测试方法是“爆鸣”测试：将点燃的火柴靠近气体，会发出“噗”的爆鸣声。
2HCl (aq) + Zn (s) → ZnCl₂ (aq) + H₂ (g)
3.2 Acid + Base (oxide/hydroxide) → Salt + Water | 酸 + 碱（氧化物/氢氧化物）→ 盐 + 水
This is called neutralisation. Example: sulfuric acid + copper(II) oxide → copper(II) sulfate + water.
这称为中和反应。例：硫酸 + 氧化铜（II）→ 硫酸铜（II）+ 水。
H₂SO₄ (aq) + CuO (s) → CuSO₄ (aq) + H₂O (l)
3.3 Acid + Alkali → Salt + Water | 酸 + 碱液 → 盐 + 水
Example: nitric acid + sodium hydroxide → sodium nitrate + water.
例：硝酸 + 氢氧化钠 → 硝酸钠 + 水。
HNO₃ (aq) + NaOH (aq) → NaNO₃ (aq) + H₂O (l)
3.4 Acid + Carbonate → Salt + Water + Carbon Dioxide | 酸 + 碳酸盐 → 盐 + 水 + 二氧化碳
Example: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide. Test for CO₂: bubble through limewater, which turns milky/cloudy.
例：盐酸 + 碳酸钙 → 氯化钙 + 水 + 二氧化碳。CO₂ 的测试：通入石灰水，石灰水变浑浊/乳白色。
2HCl (aq) + CaCO₃ (s) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)
4. Strong and Weak Acids | 强酸和弱酸
A strong acid is fully ionised in water, meaning all acid molecules dissociate into H⁺ ions. Examples: hydrochloric acid, sulfuric acid, nitric acid. A weak acid is only partially ionised; only some molecules dissociate. Examples: ethanoic acid, citric acid, carbonic acid.
强酸在水中完全电离，即所有酸分子都离解成 H⁺ 离子。例如：盐酸、硫酸、硝酸。弱酸仅部分电离；只有一些分子离解。例如：乙酸、柠檬酸、碳酸。
The pH of a strong acid is lower than a weak acid of the same concentration because it produces more H⁺ ions. However, the concentration of an acid is a separate measure (mol/dm³).
在相同浓度下，强酸的 pH 低于弱酸，因为它产生更多 H⁺ 离子。然而，酸的浓度是另一个单独的度量（mol/dm³）。
Concentration vs strength: concentration tells you how many acid molecules are in a given volume; strength tells you what fraction of those molecules are ionised.
浓度 vs 强度：浓度告诉你在给定体积中有多少酸分子；强度告诉你这些分子中电离的比例。
5. Bases, Alkalis and Neutralisation | 碱、碱液和中和反应
Neutralisation is the reaction between H⁺ ions and OH⁻ ions to form water. The ionic equation is:
中和反应是 H⁺ 离子与 OH⁻ 离子之间生成水的反应。离子方程式为：
H⁺ (aq) + OH⁻ (aq) → H₂O (l)
Soluble bases are called alkalis. Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)₂). Insoluble bases include copper(II) oxide and zinc oxide.
可溶的碱称为碱液。常见的碱液包括氢氧化钠（NaOH）、氢氧化钾（KOH）和氢氧化钙（Ca(OH)₂）。不溶的碱包括氧化铜（II）和氧化锌。
The general reaction of an acid with an insoluble base is used to prepare soluble salts. The base is added in excess to ensure all the acid is used up, then the excess solid is filtered off.
酸与不溶碱的一般反应用于制备可溶性盐。碱要过量加入，以确保所有酸都被消耗完，然后过滤掉多余的固体。
6. Salts: Naming and Solubility | 盐：命名和溶解性
A salt is formed when the hydrogen ions of an acid are replaced by metal ions (or ammonium ions). The name of a salt has two parts: the metal (or ammonium) first, then the acid root.
盐是在酸中的氢离子被金属离子（或铵离子）替代时形成的。盐的名称分为两部分：先是金属（或铵），然后是酸根。
- Hydrochloric acid → chloride (e.g., sodium chloride) | 盐酸 → 氯化物（如氯化钠）
- Sulfuric acid → sulfate (e.g., copper sulfate) | 硫酸 → 硫酸盐（如硫酸铜）
- Nitric acid → nitrate (e.g., potassium nitrate) | 硝酸 → 硝酸盐（如硝酸钾）
- Ethanoic acid → ethanoate (e.g., sodium ethanoate) | 乙酸 → 乙酸盐（如乙酸钠）
Solubility rules for common salts:
| Soluble | 可溶 | Insoluble | 不溶 |
| All sodium, potassium and ammonium salts | Silver chloride, lead(II) chloride |
| All nitrates | Silver sulfate, calcium sulfate (slightly) |
| Most chlorides | Barium sulfate, lead(II) sulfate |
| Most sulfates | Most carbonates (except Na, K, NH₄) |
| Sodium, potassium carbonates | Most hydroxides (except Na, K, NH₄, Ca partially) |
7. Preparing Salts: Methods | 制备盐：方法
There are two main methods for preparing salts in the lab, depending on whether the salt is soluble or insoluble.
实验室中制备盐主要有两种方法，取决于盐是可溶还是不溶。
7.1 Making soluble salts | 制备可溶性盐
Method A: Acid + insoluble base (oxide, hydroxide or carbonate)
方法 A：酸 + 不溶碱（氧化物、氢氧化物或碳酸盐）
- Warm a measured volume of dilute acid.
- Add the insoluble base in excess, stirring until no more reacts.
- Filter to remove excess solid.
- Evaporate gently to concentrate the solution.
- Leave to crystallise, then dry the crystals.
- 加热一定体积的稀酸。
- 加入过量不溶碱，搅拌直到不再反应。
- 过滤去除多余的固体。
- 温和蒸发以浓缩溶液。
- 静置结晶，然后干燥晶体。
Method B: Acid + alkali (titration) — used for soluble salts from soluble bases. A titration must be used because the alkali is soluble and you cannot simply add excess. Use an indicator to find the endpoint, then repeat without indicator to make the salt.
方法 B：酸 + 碱液（滴定） — 用于可溶碱制备可溶性盐。由于碱液可溶，不能简单加过量，必须用滴定。用指示剂找到终点，然后不用指示剂重复制备盐。
7.2 Making insoluble salts | 制备不溶性盐
Mix two soluble solutions that contain the ions needed. The insoluble salt precipitates out, then filter, wash and dry the precipitate. Example: silver nitrate + sodium chloride → silver chloride (precipitate) + sodium nitrate.
混合两种含有所需离子的可溶溶液。不溶性盐沉淀出来，然后过滤、洗涤并干燥沉淀。例：硝酸银 + 氯化钠 → 氯化银（沉淀）+ 硝酸钠。
AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)
8. Acidic and Alkaline Oxides | 酸性氧化物和碱性氧化物
Metal oxides are usually basic. They react with acids to form salts and water. Non-metal oxides are usually acidic. For example, carbon dioxide is an acidic oxide that dissolves in water to form carbonic acid, and sodium oxide is a basic oxide.
金属氧化物通常是碱性的。它们与酸反应生成盐和水。非金属氧化物通常是酸性的。例如，二氧化碳是酸性氧化物，溶于水生成碳酸；氧化钠是碱性氧化物。
Some oxides are amphoteric — they can act as both acids and bases. Aluminium oxide and zinc oxide are amphoteric. They react with both acids and strong alkalis. You do not need the equations for amphoteric oxides in IGCSE, but you should know the examples.
有些氧化物是两性的——既能表现出酸性也能表现出碱性。氧化铝和氧化锌是两性氧化物。它们既与酸反应，也与强碱反应。IGCSE 中不需要掌握两性氧化物的方程式，但需要知道例子。
9. Acids and Alkalis in Everyday Life | 酸和碱在日常生活中的应用
Many household substances are acids or alkalis. Vinegar contains ethanoic acid; lemon juice contains citric acid; car batteries contain sulfuric acid. Antacids like magnesium hydroxide or calcium carbonate neutralise excess stomach acid. Soap and bleach are alkaline.
许多家用物质是酸或碱。醋含有乙酸；柠檬汁含有柠檬酸；汽车电池含有硫酸。抗酸药如氢氧化镁或碳酸钙中和过量胃酸。肥皂和漂白剂是碱性的。
Crops are affected by soil pH. If soil is too acidic, farmers add lime (calcium oxide or calcium hydroxide) to neutralise it and raise the pH. This is an example of large-scale neutralisation.
作物受土壤 pH 影响。如果土壤太酸，农民会添加石灰（氧化钙或氢氧化钙）来中和并提高 pH。这是大规模中和反应的例子。
10. Sample Exam Questions and Tips | 模拟考试题和技巧
Question 1: Write the word equation for the reaction between sulfuric acid and magnesium oxide.
问题 1：写出硫酸与氧化镁反应的文字方程式。
Answer: Sulfuric acid + magnesium oxide → magnesium sulfate + water.
答案：硫酸 + 氧化镁 → 硫酸镁 + 水。
Question 2: Suggest why universal indicator is used instead of litmus to measure pH.
问题 2：说明为什么使用万能指示剂而不是石蕊来测量 pH。
Answer: Universal indicator can show the exact pH or a range (strong/weak acid or alkali), whereas litmus only shows whether a solution is acidic or alkaline.
答案：万能指示剂能显示准确的 pH 或范围（强/弱酸或碱），而石蕊只能显示溶液是酸性还是碱性。
Question 3: In preparing copper sulfate from copper(II) oxide and sulfuric acid, why must copper(II) oxide be added in excess?
问题 3：在从氧化铜（II）和硫酸制备硫酸铜时，为什么必须加入过量氧化铜（II）？
Answer: To ensure all the sulfuric acid is neutralised/reacted. Excess solid can be removed by filtration, leaving a pure solution of copper sulfate.
答案：为了确保所有的硫酸都被中和/反应掉。多余的固体可以通过过滤除去，留下纯净的硫酸铜溶液。
Common mistakes:
- Forgetting state symbols in equations | 在方程式中忘记状态符号
- Confusing acid strength with concentration | 混淆酸强度和浓度
- Saying that all bases are alkalis (alkalis are a subset) | 说所有碱都是碱液（碱液是碱的子集）
- Using the wrong method for soluble vs insoluble salts | 制备可溶盐和不溶盐时用错方法
Practise writing balanced equations for every reaction mentioned in this guide, and memorise the solubility rules. When answering exam questions, always quote pH values, observations (colour change, gas produced) and state symbols. Good luck!
练习为本指南中提到的每个反应写出配平的方程式，并记住溶解性规则。回答考试问题时，务必引用 pH 值、观察现象（颜色变化、产生的气体）和状态符号。祝你好运！
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