📚 Acids, Bases and Salts | 酸、碱和盐
Acids, bases and salts are fundamental chemical substances that shape the world around us, from the digestive juices in our stomach to the cleaning products we use at home. Understanding their properties, reactions and everyday applications is a key part of the IGCSE Edexcel Science (Chemistry) syllabus. This article explores the core concepts step by step, helping you build a solid foundation for exam success.
酸、碱和盐是构成我们周围世界的基本化学物质，从胃里的消化液到家庭清洁用品，无处不见它们的身影。理解它们的性质、反应和日常应用是IGCSE Edexcel科学（化学）课程的重要内容。本文将逐步解析核心概念，帮助你为考试打下扎实的基础。
1. Introduction to Acids and Bases | 酸和碱简介
Acids are substances that release hydrogen ions (H⁺) when dissolved in water. Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Bases are substances that can neutralise acids, producing a salt and water. Alkalis are soluble bases that release hydroxide ions (OH⁻) in water, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH).
酸是溶于水时会释放氢离子（H⁺）的物质。实验室中常见的酸包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。碱是能够中和酸、生成盐和水的物质。可溶性的碱被称为碱溶液，它们能在水中释放氢氧根离子（OH⁻），例如氢氧化钠（NaOH）和氢氧化钾（KOH）。
It is important to remember that all alkalis are bases, but not all bases are alkalis. For instance, copper(II) oxide (CuO) is a base because it reacts with acids, but it does not dissolve in water, so it is not an alkali.
重要的是要记住，所有的碱溶液都是碱，但并非所有的碱都是碱溶液。例如，氧化铜（CuO）是一种碱，因为它能与酸反应，但它不溶于水，因此不属于碱溶液。
2. The pH Scale | pH标度
The pH scale measures how acidic or alkaline a solution is. It ranges from 0 to 14. Solutions with a pH less than 7 are acidic; the lower the pH, the stronger the acid. A pH of 7 is neutral, such as pure water. Solutions with a pH greater than 7 are alkaline; the higher the pH, the stronger the alkali.
pH标度用来衡量溶液的酸碱性，范围从0到14。pH小于7的溶液呈酸性；pH越低，酸性越强。pH为7时为中性，如纯水。pH大于7的溶液呈碱性；pH越高，碱性越强。
Universal indicator is a mixture of dyes that turns different colours at different pH values. Litmus paper is another simple indicator: blue litmus turns red in acid, and red litmus turns blue in alkali. For precise measurements, a pH meter can be used.
通用指示剂是一种混合染料，在不同的pH值下会显示不同的颜色。石蕊试纸是另一种简单的指示剂：蓝色石蕊在酸中变红，红色石蕊在碱中变蓝。如需精确测量，可以使用pH计。
3. Indicators | 指示剂
Indicators are substances that change colour depending on the pH of the solution. Apart from universal indicator and litmus, some commonly used indicators in IGCSE chemistry include phenolphthalein and methyl orange. Phenolphthalein is colourless in acid and turns pink in alkali. Methyl orange is red in acid and turns yellow in alkali.
指示剂是根据溶液的pH值而变色的物质。除了通用指示剂和石蕊，IGCSE化学中常用的指示剂还包括酚酞和甲基橙。酚酞在酸中无色，在碱中变成粉红色。甲基橙在酸中呈红色，在碱中变为黄色。
The choice of indicator depends on the type of reaction being studied. For a strong acid-strong base titration, either phenolphthalein or methyl orange can be used. However, for a weak acid-strong base titration, phenolphthalein is more suitable because its colour change occurs in the basic range.
指示剂的选择取决于所研究的反应类型。对于强酸与强碱的滴定，酚酞或甲基橙都可以使用。但对于弱酸与强碱的滴定，酚酞更为合适，因为它的颜色变化发生在碱性范围内。
4. Reactions of Acids with Metals | 酸与金属的反应
ydrogen can be tested by introducing a lit splint, which will produce a squeaky pop sound.
稀酸能与某些金属反应，生成盐和氢气。一般方程式为：酸 + 金属 → 盐 + 氢气。例如，镁与盐酸反应生成氯化镁和氢气：Mg + 2HCl → MgCl₂ + H₂。氢气的检验方法是用燃烧的木条靠近，会听到爆鸣声（squeaky pop）。
Not all metals react with dilute acids. Metals such as copper, silver and gold are below hydrogen in the reactivity series and do not displace hydrogen from acids. Very reactive metals like potassium and sodium react too violently with acids, so these reactions are usually not demonstrated in the school lab.
并非所有金属都能与稀酸反应。铜、银和金等金属在金属活动性顺序表中排在氢之后，不能从酸中置换出氢。像钾和钠这样非常活泼的金属与酸反应过于剧烈，因此在学校实验室通常不进行演示。
5. Reactions of Acids with Bases (Neutralisation) | 酸与碱的反应（中和）
hydrogen ions from the acid combine with hydroxide ions from the alkali to produce water.
中和反应是酸与碱生成盐和水的反应。以盐酸与氢氧化钠反应为例，方程式为：HCl + NaOH → NaCl + H₂O。中和反应的离子方程式为：H⁺ + OH⁻ → H₂O。这表明酸中的氢离子与碱中的氢氧根离子结合生成了水。
Neutralisation reactions are exothermic, meaning they release heat. This is why antacid tablets containing bases like magnesium hydroxide can relieve indigestion by neutralising excess stomach acid (hydrochloric acid). In agriculture, lime (calcium oxide) is used to neutralise acidic soils.
中和反应是放热反应，即释放热量。这就是为什么含有氢氧化镁等碱的抗酸药片能通过中和过多的胃酸（盐酸）来缓解消化不良。在农业中，石灰（氧化钙）被用来中和酸性土壤。
6. Reactions of Acids with Carbonates | 酸与碳酸盐的反应
ium carbonate (limestone) reacts with hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
酸与金属碳酸盐和碳酸氢盐反应生成盐、水和二氧化碳气体。一般方程式为：酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。例如，碳酸钙（石灰石）与盐酸反应：CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂。
Carbon dioxide gas can be identified by bubbling it through limewater (calcium hydroxide solution), which turns milky or cloudy due to the formation of insoluble calcium carbonate. This reaction is also used in the laboratory preparation of CO₂ and in making salts called chlorides, sulfates and nitrates.
二氧化碳气体可以通过将其通入石灰水（氢氧化钙溶液）来检验，石灰水会因为生成不溶性的碳酸钙而变浑浊。这个反应也用于实验室制备二氧化碳，以及制备氯化物、硫酸盐和硝酸盐等盐类。
7. Making Salts | 制备盐
A salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion (NH₄⁺). There are several methods to prepare salts, and the choice depends on the solubility of the salt and the reactants. The main methods include: reaction of an acid with an insoluble base or metal, titration, and precipitation.
盐是酸中的氢离子被金属离子或铵离子（NH₄⁺）取代后形成的离子化合物。制备盐的方法有多种，选择哪种方法取决于盐和反应物的溶解性。主要方法包括：酸与不溶性碱或金属的反应、滴定法和沉淀法。
When making a soluble salt using an insoluble base (e.g. copper(II) oxide and sulfuric acid), the acid is warmed and the base is added until it no longer dissolves. The excess solid is then removed by filtration, and the filtrate is evaporated to obtain crystals of the salt (e.g. copper(II) sulfate pentahydrate, CuSO₄·5H₂O).
使用不溶性碱（如氧化铜和硫酸）制备可溶性盐时，需要将酸加热，然后加入碱直到不再溶解。通过过滤除去多余的固体，再蒸发滤液即可获得盐的晶体（例如五水合硫酸铜，CuSO₄·5H₂O）。
8. Soluble and Insoluble Salts | 可溶盐和不溶盐
Solubility rules help us 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. Most sulfates are soluble, except barium sulfate, lead sulfate and calcium sulfate. Most carbonates and hydroxides are insoluble, except those of sodium, potassium and ammonium.
溶解性规则帮助我们判断一种盐是否能溶于水。所有钠盐、钾盐和铵盐都是可溶的。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。大多数硫酸盐可溶，但硫酸钡、硫酸铅和硫酸钙除外。大多数碳酸盐和氢氧化物不溶，但钠、钾和铵的碳酸盐和氢氧化物除外。
Insoluble salts are prepared by precipitation. In this method, two soluble salt solutions are mixed, and the insoluble salt forms as a precipitate, which can be separated by filtration, washed with distilled water and dried. For example, barium sulfate can be made by mixing barium chloride solution and sodium sulfate solution.
不溶性盐通过沉淀法制备。该方法是将两种可溶性盐溶液混合，不溶性盐会以沉淀形式析出，经过滤、蒸馏水洗涤和干燥后得到产物。例如，将氯化钡溶液和硫酸钠溶液混合即可制得硫酸钡。
9. Titration | 滴定
Titration is a quantitative technique used to determine the concentration of an acid or an alkali by neutralising it with a standard solution of known concentration. A known volume of the alkali is pipetted into a conical flask, and a few drops of an indicator (e.g. phenolphthalein) are added. The acid is added from a burette until the indicator just changes colour, marking the end point.
滴定是一种定量技术，通过用已知浓度的标准溶液中和酸或碱来测定其浓度。用移液管量取一定体积的碱溶液到锥形瓶中，加入几滴指示剂（如酚酞）。从滴定管中加入酸，直到指示剂刚好变色，即为滴定终点。
Titration is especially useful for preparing salts of reactive metals like sodium, potassium and ammonium salts, where the metal or base cannot be used safely with an acid. The results from a titration are accurate and reproducible, making it a core skill in IGCSE chemistry practical work.
滴定法特别适用于制备活泼金属如钠、钾和铵的盐，因为这些金属或碱无法安全地与酸直接反应。滴定结果准确且可重复，是IGCSE化学实验操作中的一项核心技能。
10. Everyday Examples of Acids, Bases and Salts | 酸碱盐的日常应用
Acids are found in many foods and household products. Citric acid is present in citrus fruits, acetic acid is the main component of vinegar, and carbonic acid gives fizz to soft drinks. Car batteries contain sulfuric acid, while hydrochloric acid is used for cleaning brickwork and in the stomach for digestion.
许多食物和家用产品中都含有酸。柠檬酸存在于柑橘类水果中，醋酸是醋的主要成分，碳酸使软饮料产生气泡。汽车电池中含有硫酸，而盐酸用于清洁砖墙，也存在于胃中帮助消化。
Common bases and alkalis include sodium hydrogencarbonate (baking soda), which is used in cooking and as a mild antacid. Calcium hydroxide (slaked lime) is used in building and in water treatment. Sodium chloride (table salt) is the most familiar salt, while potassium nitrate is used in fertilisers, and magnesium sulfate (Epsom salt) is used in bath salts and medicine.
常见的碱和碱溶液包括碳酸氢钠（小苏打），用于烹饪和作为温和的抗酸剂。氢氧化钙（熟石灰）用于建筑和水处理。氯化钠（食盐）是最常见的盐，硝酸钾用于化肥，硫酸镁（泻盐）用于浴盐和医药。
11. Strong vs Weak Acids | 强酸与弱酸
A strong acid is one that completely dissociates into its ions in aqueous solution. Hydrochloric acid, sulfuric acid and nitric acid are strong acids. Because they fully ionise, a 1 mol/dm³ solution of a strong acid has a high concentration of H⁺ ions and a low pH. In contrast, a weak acid only partially dissociates. Ethanoic acid (CH₃COOH) and citric acid are weak acids. They produce a much lower concentration of H⁺ ions for the same concentration of acid, giving a higher pH than a strong acid.
强酸是指在水溶液中完全电离的酸。盐酸、硫酸和硝酸都属于强酸。由于完全电离，1 mol/dm³ 的强酸溶液中H⁺离子浓度高，pH值低。相反，弱酸仅部分电离。乙酸（CH₃COOH）和柠檬酸是弱酸。相同的酸的浓度下，它们产生的H⁺离子浓度低得多，因此pH值比强酸高。
The concept of strong and weak acids should not be confused with concentrated and dilute acids. 'Strong' and 'weak' refer to the degree of dissociation, whereas 'concentrated' and 'dilute' refer to the amount of acid dissolved in water. You can have a dilute strong acid or a concentrated weak acid.
强酸和弱酸的概念不应与浓酸和稀酸混淆。“强”和“弱”指的是电离程度，而“浓”和“稀”指的是溶于水中的酸的量。你可以有稀的强酸，也可以有浓的弱酸。
12. Summary and Exam Tips | 总结与考试技巧
Acids, bases and salts form a cornerstone of IGCSE Edexcel Chemistry. You should be able to define acids and bases in terms of ion production, use the pH scale, recall the reactions of acids with metals, bases and carbonates, and apply solubility rules to choosing salt preparation methods. Understanding neutralisation and titration is also essential for both theoretical and practical examinations.
酸、碱和盐是IGCSE Edexcel化学的基石。你应该能够根据离子的产生来定义酸和碱，会使用pH标度，记住酸与金属、碱和碳酸盐的反应，并能运用溶解性规则来选择盐的制备方法。理解中和反应和滴定对理论考试和实验考试都至关重要。
en answering questions on salt preparation, first check whether the salt is soluble; if it is soluble, use the method of acid + insoluble base (or metal or carbonate), but if it is an alkali metal or ammonium salt, use titration. For insoluble salts, choose precipitation.
写离子方程式时，始终只展示实际发生变化的微粒。对于中和反应，离子方程式始终为 H⁺ + OH⁻ → H₂O。回答关于盐制备的问题时，首先要检查盐是否可溶；如果可溶，使用酸加不溶性碱（或金属或碳酸盐）的方法，但如果是碱金属盐或铵盐，则使用滴定法。对于不溶性盐，选择沉淀法。
Keep practising past paper questions on these topics, and use the correct chemical terminology. Accuracy in writing formulae and balancing equations will earn you marks. Good luck with your revision!
请持续练习这些主题的历年真题，并使用正确的化学术语。书写化学式和配平方程式的准确性会为你赢得分数。祝你复习顺利！
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