📚 Acids, Bases and Salts: The Complete IGCSE Guide | 酸、碱与盐：IGCSE 完整指南
Acids, bases and salts form one of the most fundamental and frequently tested topics in the Edexcel IGCSE Science curriculum. This guide covers every key concept you need, from the pH scale and neutralisation reactions to salt preparation techniques and everyday applications.
酸、碱与盐是 Edexcel IGCSE 科学课程中最基础也最常考的主题之一。本指南涵盖你所需的所有核心概念，从 pH 标度、中和反应到盐的制备方法和日常应用，一应俱全。
1. Defining Acids and Bases | 酸与碱的定义
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and ethanoic acid (CH₃COOH). Acids have a pH below 7 and turn blue litmus paper red.
酸是溶于水时产生氢离子（H⁺）的物质。常见例子包括盐酸（HCl）、硫酸（H₂SO₄）和乙酸（CH₃COOH）。酸的 pH 低于 7，能使蓝色石蕊试纸变红。
A base is a substance that neutralises an acid to form a salt and water. Bases that dissolve in water are called alkalis, which produce hydroxide ions (OH⁻) in solution. Common alkalis include sodium hydroxide (NaOH) and potassium hydroxide (KOH). Bases have a pH above 7 and turn red litmus paper blue.
碱是能与酸中和生成盐和水的物质。溶于水的碱称为碱液（alkali），在溶液中产生氢氧根离子（OH⁻）。常见的碱液包括氢氧化钠（NaOH）和氢氧化钾（KOH）。碱的 pH 高于 7，能使红色石蕊试纸变蓝。
Acid: H⁺ donor in solution | 酸：溶液中产生 H⁺
Base: OH⁻ donor in solution (for alkalis) | 碱（碱液）：溶液中产生 OH⁻
2. The pH Scale and Indicators | pH 标度与指示剂
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are alkaline. Universal indicator is a mixture of dyes that gives a colour range across the pH scale.
pH 标度范围从 0 到 14，用于测量溶液的酸碱程度。pH 为 7 表示中性，低于 7 为酸性，高于 7 为碱性。万能指示剂是多种染料的混合物，能在不同 pH 下呈现一系列颜色。
| pH Value | pH 值 | 0-2 | 3-5 | 6-8 | 9-11 | 12-14 |
| Indicator Colour | 指示剂颜色 | Red 红 | Orange/Yellow 橙/黄 | Green 绿 | Blue 蓝 | Purple 紫 |
Methyl orange changes from red (acid) to yellow (alkali), while phenolphthalein changes from colourless (acid) to pink (alkali). You must know both of these for titration experiments.
甲基橙在酸性中呈红色，在碱性中呈黄色；酚酞在酸性中呈无色，在碱性中呈粉红色。滴定实验必须熟悉这两种指示剂。
3. Strong vs Weak Acids | 强酸与弱酸
H⁺ + Cl⁻ is a complete reaction. A weak acid only partially ionises, so only some molecules release H⁺ ions at equilibrium.
强酸在水中完全电离，即每个酸分子都释放出氢离子。例如，HCl → H⁺ + Cl⁻ 是彻底的反应。弱酸仅部分电离，因此只有部分分子在平衡状态下释放出 H⁺ 离子。
High concentration does not equal strong acidity. Concentration measures how much acid is dissolved, while strength measures the proportion that ionises. A dilute strong acid has a lower pH than a concentrated weak acid containing the same number of moles.
高浓度不等于强酸性。浓度衡量溶解了多少酸，而强度衡量电离的比例。相同物质的量的情况下，稀释的强酸比浓缩的弱酸 pH 更低。
- Strong acids: hydrochloric, sulfuric, nitric | 强酸：盐酸、硫酸、硝酸
- Weak acids: ethanoic, citric, carbonic | 弱酸：乙酸、柠檬酸、碳酸
- Strong alkalis: sodium hydroxide, potassium hydroxide | 强碱：氢氧化钠、氢氧化钾
- Weak alkalis: ammonia solution | 弱碱：氨水
4. Neutralisation Reactions | 中和反应
equation is: H⁺ + OH⁻ → H₂O. This is the fundamental reaction behind many IGCSE questions.
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。这是许多 IGCSE 考题背后的基本反应。
Acid + Base → Salt + Water | 酸 + 碱 → 盐 + 水
r(II) oxide is a black solid that dissolves to form a blue solution of copper(II) sulfate.
当酸与金属氧化物或金属氢氧化物反应时，产物始终是盐和水。例如：H₂SO₄ + CuO → CuSO₄ + H₂O。氧化铜是黑色固体，反应后溶解形成蓝色的硫酸铜溶液。
t + Water + CO₂. This reaction produces effervescence, which can be tested with limewater.
酸与金属碳酸盐反应生成盐、水和二氧化碳气体。通式为：酸 + 碳酸盐 → 盐 + 水 + CO₂。该反应产生气泡，可用石灰水检验生成的二氧化碳。
5. Acids and Metals | 酸与金属的反应
Salt + Hydrogen. The hydrogen gas can be tested by holding a lighted splint near the tube — it produces a squeaky pop.
酸与某些金属反应生成盐和氢气。通式为：酸 + 金属 → 盐 + 氢气。可用点燃的火柴靠近试管口检验氢气——会发出"噗"的爆鸣声。
2HCl + Zn → ZnCl₂ + H₂↑ | 2HCl + Zn → ZnCl₂ + H₂↑
However, not all metals react with acids. Copper and silver are below hydrogen in the reactivity series, so they do not displace hydrogen from acids. This links directly to the reactivity series topic — you must know the order: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, copper, silver, gold.
然而，并非所有金属都能与酸反应。铜和银在金属活动性顺序中位于氢之后，因此不能从酸中置换出氢气。这与金属活动性顺序直接相关——你必须记住顺序：钾、钠、钙、镁、铝、锌、铁、铅、铜、银、金。
6. Solubility Rules and Salt Preparation | 溶解度规则与盐的制备
Not all salts are soluble in water. You must memorise the solubility rules for Edexcel IGCSE, as they determine which preparation method to use. Most sodium, potassium and ammonium salts are soluble, and all nitrates are soluble.
并非所有盐都溶于水。Edexcel IGCSE 要求记住溶解度规则，因为溶解性决定了使用哪种制备方法。大多数钠盐、钾盐和铵盐都可溶，所有硝酸盐都可溶。
- All sodium, potassium, ammonium salts: soluble | 所有钠盐、钾盐、铵盐：可溶
- All nitrates: soluble | 所有硝酸盐：可溶
- Most chlorides: soluble (except AgCl and PbCl₂) | 大多数氯化物：可溶（AgCl 和 PbCl₂ 除外）
- Most sulfates: soluble (except BaSO₄, PbSO₄, CaSO₄) | 大多数硫酸盐：可溶（BaSO₄、PbSO₄、CaSO₄ 除外）
- Most carbonates and hydroxides: insoluble (except sodium, potassium, ammonium) | 大多数碳酸盐和氢氧化物：不溶（钠、钾、铵的除外）
Soluble salts are prepared by titration or by reacting an acid with an excess of insoluble reactant. For example, to make copper(II) sulfate, add excess copper(II) oxide to warm sulfuric acid, filter off the excess solid, and evaporate the filtrate.
可溶性盐通过滴定法或酸与过量不溶物反应来制备。例如，制备硫酸铜时，向温热的硫酸中加入过量氧化铜，过滤掉多余的固体，然后蒸发滤液。
7. Titration Technique | 滴定技术
Titration is a precise method for preparing soluble salts and finding exact concentrations. In a titration, a known volume of alkali is measured with a pipette into a conical flask, and a few drops of indicator are added. The acid is added from a burette until the indicator changes colour.
滴定是制备可溶性盐和测定精确浓度的精密方法。在滴定中，用移液管量取已知体积的碱液加入锥形瓶，并滴加几滴指示剂。从滴定管中加入酸，直到指示剂变色。
The endpoint is reached when the indicator changes colour. For phenolphthalein, the colour change is pink to colourless; for methyl orange, yellow to red. The volume of acid added is recorded from the burette readings.
当指示剂变色时即为滴定终点。酚酞由粉红色变为无色；甲基橙由黄色变为红色。记录滴定管读数以确定加入酸的体积。
moles of acid = concentration × volume (mol = mol/dm³ × dm³) | 酸的物质的量 = 浓度 × 体积
ase (for monoprotic acid and monohydroxy base). Concentration can then be calculated using this relationship.
你必须理解计算：物质的量 = 浓度 × 体积。在终点时，酸的物质的量 = 碱的物质的量（对于一元酸和一元碱）。然后可以利用该关系计算浓度。
8. Insoluble Salt Preparation by Precipitation | 沉淀法制备不溶性盐
Insoluble salts are prepared by precipitation — mixing two soluble salt solutions so that the insoluble salt forms as a solid. For example, silver chloride is prepared by mixing silver nitrate solution with sodium chloride solution.
不溶性盐通过沉淀法制备——将两种可溶性盐溶液混合，使不溶性盐以固体形式析出。例如，氯化银通过混合硝酸银溶液和氯化钠溶液制备。
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) | AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
The precipitate is collected by filtration, washed with distilled water, and dried in an oven. This method works for any salt identified as insoluble in the solubility rules, such as barium sulfate (BaSO₄) and lead(II) iodide (PbI₂).
沉淀通过过滤收集，用蒸馏水洗涤，并在烘箱中干燥。此方法适用于任何在溶解度规则中判定为不溶的盐，例如硫酸钡（BaSO₄）和碘化铅（PbI₂）。
9. Everyday Uses of Acids and Bases | 酸与碱的日常用途
Acids and bases are everywhere in daily life. Hydrochloric acid is found in the human stomach and helps digest food. Sulfuric acid is used in car batteries. Ethanoic acid is the main component of vinegar. Citric acid is present in citrus fruits.
酸和碱在日常生活中无处不在。盐酸存在于人体胃中，帮助消化食物。硫酸用于汽车电池。乙酸是食醋的主要成分。柠檬酸存在于柑橘类水果中。
Magnesium hydroxide and calcium carbonate are used as antacids to neutralise excess stomach acid. Ammonia solution is used in cleaning products. Sodium hydroxide is used to make soap. These applications directly test your understanding of neutralisation in context.
氢氧化镁和碳酸钙用作抗酸剂，中和胃中过多的酸。氨水用于清洁产品。氢氧化钠用于制造肥皂。这些应用直接考查你在具体情境中对中和反应的理解。
10. Acid Rain and Environmental Impact | 酸雨与环境影响
Acid rain forms when sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in rainwater. These gases react with oxygen and water in the atmosphere, producing sulfuric and nitric acids, which fall as precipitation with a pH below 5.6.
燃烧化石燃料产生的二氧化硫和氮氧化物溶于雨水形成酸雨。这些气体在大气中与氧气和水反应，生成硫酸和硝酸，以 pH 低于 5.6 的降水形式降落。
Acid rain damages buildings made of limestone and marble because calcium carbonate reacts with acid. It also harms aquatic life by lowering the pH of lakes and rivers, and damages forests by leaching aluminium ions into the soil.
酸雨腐蚀石灰石和大理石建筑，因为碳酸钙会与酸反应。它还通过降低湖泊和河流的 pH 值来危害水生生物，并通过将铝离子淋溶到土壤中而破坏森林。
CaSO₄ + CO₂ + H₂O. Understanding this helps you answer application questions in the exam.
你应该能够写出这些反应的方程式。例如：CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O。理解这一点有助于你回答考试中的应用题。
11. Oxides and Their Classification | 氧化物的分类
Metal oxides are basic oxides — they neutralise acids to form salt and water. Non-metal oxides are acidic oxides — they dissolve in water to form acidic solutions. Carbon dioxide forms carbonic acid, and sulfur dioxide forms sulfurous acid.
金属氧化物是碱性氧化物——它们与酸中和生成盐和水。非金属氧化物是酸性氧化物——它们溶于水形成酸性溶液。二氧化碳形成碳酸，二氧化硫形成亚硫酸。
Some oxides are amphoteric, meaning they can act as both acids and bases. Aluminium oxide and zinc oxide are the two you need to know. They react with both acids and strong alkalis.
有些氧化物是两性的，既能表现酸性也能表现碱性。你需要掌握氧化铝和氧化锌。它们既能与酸反应，也能与强碱反应。
Zinc oxide dissolves in hydrochloric acid to form zinc chloride, and dissolves in sodium hydroxide to form sodium zincate. Neutral oxides such as carbon monoxide and water are neither acidic nor basic.
氧化锌溶于盐酸生成氯化锌，溶于氢氧化钠生成锌酸钠。中性氧化物如二氧化碳（实际上指 CO 一氧化碳）和水既非酸性也非碱性，需要注意 CO 不溶于水不做酸碱反应。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
The most frequent errors in this topic include confusing concentration with strength, forgetting to balance ionic equations, and writing the wrong products for acid-metal carbonate reactions. Always check that your salt formula matches the acid and the metal, including correct valencies.
本主题最常见的错误包括混淆浓度与强度、忘记配平离子方程式、写错酸与碳酸盐反应的产物。务必检查盐的化学式是否与酸和金属匹配，包括正确的化合价。
Pay attention to state symbols. Knowing whether a substance is aqueous (aq), solid (s), liquid (l) or gas (g) is worth marks in the exam. For example, in precipitation reactions, the insoluble salt must be written with (s).
注意状态符号。判断物质是水溶液（aq）、固体（s）、液体（l）还是气体（g）在考试中能得分数。例如，在沉淀反应中，不溶性盐必须标注（s）。
When calculating pH from hydrogen ion concentration, remember that each unit decrease in pH represents a tenfold increase in H⁺ concentration. This means a solution of pH 3 is ten times more acidic than one of pH 4.
从氢离子浓度计算 pH 时，记住 pH 每降低一个单位，H⁺ 浓度增加十倍。这意味着 pH 为 3 的溶液比 pH 为 4 的溶液酸性强十倍。
Finally, practise writing full equations, ionic equations and half equations for each reaction type. Master the solubility rules completely and practise titration calculations until they become automatic. With these skills, you are well prepared for any question on acids, bases and salts in your IGCSE exam.
最后，练习为每种反应类型写出完整方程式、离子方程式和半方程式。彻底掌握溶解度规则，反复练习滴定计算直到运用自如。具备这些技能，你就为 IGCSE 考试中有关酸、碱与盐的任何题目做好了充分准备。
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