📚 Acids, Bases and Salts | 酸、碱和盐
Acids, bases and salts are fundamental to understanding chemical reactions in IGCSE Science. They appear everywhere – from digestion in our stomachs to the manufacture of fertilisers and cleaning products. Grasping the properties of acids and alkalis, the pH scale, and the methods for preparing salts is essential for success in the Edexcel IGCSE examination.
酸、碱和盐是理解 IGCSE 科学中化学反应的基础。它们无处不在——从我们胃里的消化过程到化肥和清洁产品的制造。掌握酸和碱的性质、pH 标度以及制备盐的方法，对于在爱德思 IGCSE 考试中取得成功至关重要。
1. Defining Acids and Bases | 酸和碱的定义
Acids are substances that release hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) fully dissociates into H⁺ and Cl⁻ ions. Bases are substances that can neutralise acids to produce a salt and water. Alkalis are soluble bases that release hydroxide ions (OH⁻) in water, such as sodium hydroxide (NaOH).
酸是溶于水时释出氢离子（H⁺）的物质。例如，盐酸（HCl）完全解离成 H⁺ 和 Cl⁻ 离子。碱是能够中和酸生成盐和水的物质。可溶性碱在水中释出氢氧根离子（OH⁻），例如氢氧化钠（NaOH）。
Common laboratory acids include sulfuric acid (H₂SO₄), nitric acid (HNO₃) and ethanoic acid (CH₃COOH). Common alkalis include sodium hydroxide, potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)₂).
常见的实验室酸包括硫酸（H₂SO₄）、硝酸（HNO₃）和乙酸（CH₃COOH）。常见的碱包括氢氧化钠、氢氧化钾（KOH）和氢氧化钙（Ca(OH)₂）。
2. The pH Scale and Indicators | pH 标度与指示剂
The pH scale is a measure of the acidity or alkalinity of an aqueous solution. It runs from 0 to 14: pH less than 7 is acidic, pH 7 is neutral, and pH greater than 7 is alkaline. The scale is logarithmic, meaning each change of 1 pH unit represents a ten‑fold change in H⁺ ion concentration.
pH 标度是衡量水溶液酸碱性的尺度，从 0 到 14：pH 小于 7 为酸性，pH 等于 7 为中性，pH 大于 7 为碱性。该标度是对数的，意味着每变化 1 个 pH 单位，H⁺ 离子浓度就扩大或缩小 10 倍。
Indicators are substances that change colour depending on pH. For Edexcel IGCSE, you need to know litmus, phenolphthalein and methyl orange. Universal indicator is a mixture of dyes that gives a range of colours across the pH scale and can be used to estimate pH.
指示剂是能随 pH 变化而改变颜色的物质。在爱德思 IGCSE 中，你需要知道石蕊、酚酞和甲基橙。通用指示剂是多种染料的混合物，能在整个 pH 范围内呈现不同的颜色，可用于估计 pH 值。
| Indicator | Colour in Acid | Colour in Neutral | Colour in Alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
Table 1: Common indicators for IGCSE | 表 1：IGCSE 常见指示剂
3. Reactions of Acids with Metals, Bases and Carbonates | 酸与金属、碱和碳酸盐的反应
Acids react with reactive metals to produce a salt and hydrogen gas. For example, magnesium reacts with hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The test for hydrogen is a lighted splint producing a squeaky pop.
酸与活泼金属反应生成盐和氢气。例如，镁与盐酸反应：Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)。氢气的检验方法是用点燃的木条放在管口，会听到爆鸣声。
Acids neutralise bases (including alkalis) to form a salt and water. The reaction between hydrochloric acid and sodium hydroxide is: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). This is a neutralisation reaction.
酸与碱（包括可溶性碱）发生中和反应，生成盐和水。盐酸与氢氧化钠的反应：HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)。这是一个中和反应。
Acids react with carbonates to produce a salt, water and carbon dioxide gas. For instance, calcium carbonate reacts with hydrochloric acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Carbon dioxide turns limewater milky.
酸与碳酸盐反应生成盐、水和二氧化碳气体。例如，碳酸钙与盐酸反应：CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)。二氧化碳能使石灰水变浑浊。
4. Reactivity Series and Acid–Metal Reactions | 金属活动性顺序与酸–金属反应
The reactivity series places metals in order of their tendency to form positive ions. For acid–metal reactions, only metals above hydrogen in the series will react with dilute acids to produce hydrogen. Metals below hydrogen, such as copper, silver and gold, do not react with dilute acids.
金属活动性顺序将金属按照它们形成正离子的倾向排列。在酸与金属的反应中，只有排位在氢之前的金属才能与稀酸反应产生氢气。排在氢之后的金属，如铜、银和金，不与稀酸反应。
Potassium, sodium and lithium are too reactive to be added safely to acids in a school laboratory. Magnesium, aluminium, zinc and iron react at different rates, with magnesium reacting vigorously.
钾、钠和锂过于活泼，在学校实验室中不能安全地加入酸中。镁、铝、锌和铁的反应速率各不相同，其中镁的反应最剧烈。
5. Making Soluble Salts: Methods and Techniques | 可溶性盐的制备：方法和技术
There are several methods to prepare a soluble salt, depending on the reactants. The most common method for making a salt from an acid and an insoluble base (or metal, or carbonate) is the excess solid method. You add an excess of the solid to a fixed volume of acid, warm gently, then filter off the unreacted solid and crystallise the salt from the filtrate.
制备可溶性盐有几种方法，取决于所用的反应物。最常用的方法是用酸与不溶性碱（或金属、或碳酸盐）反应——即过量固体法。将过量的固体加入一定体积的酸中，稍微加热，然后过滤掉未反应的固体，再从滤液中结晶出盐。
For example, to make copper(II) sulfate crystals, you add excess copper(II) oxide to warm dilute sulfuric acid, stirring until no more dissolves. The mixture is filtered while hot to remove unreacted copper(II) oxide. The blue filtrate is then heated gently to evaporate some water and then left to cool so that blue copper(II) sulfate crystals form.
例如，要制备硫酸铜晶体，向温热稀硫酸中加入过量的氧化铜，搅拌至不再溶解。趁热过滤，除去未反应的氧化铜。把蓝色滤液用小火加热蒸发掉一部分水，然后放置冷却，让蓝色的硫酸铜晶体析出。
Crystallisation involves dissolving the salt in a minimum amount of hot water, filtering while hot, and then allowing the solution to cool slowly. Slow cooling produces larger, purer crystals.
结晶的过程是把盐溶解在尽可能少的热水中，趁热过滤，然后让溶液慢慢冷却。缓慢冷却可以生成更大、更纯的晶体。
6. Making Insoluble Salts by Precipitation | 用沉淀法制备不溶性盐
Insoluble salts, such as barium sulfate (BaSO₄) and silver chloride (AgCl), can be prepared by a precipitation reaction. Two aqueous solutions, each containing one of the ions that will form the insoluble salt, are mixed. The insoluble salt precipitates out and can be separated by filtration, washed with distilled water, and dried.
不溶性盐，如硫酸钡（BaSO₄）和氯化银（AgCl），可以通过沉淀反应制备。将两种水溶液混合，每种溶液含有将要生成的不溶性盐的一种离子。不溶性盐沉淀析出，可以通过过滤分离，用蒸馏水洗涤后干燥。
For instance, to prepare barium sulfate, you mix barium chloride solution and sodium sulfate solution: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). The white precipitate of barium sulfate is then filtered, washed and dried in an oven at a low temperature.
例如，要制备硫酸钡，将氯化钡溶液与硫酸钠溶液混合：BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)。生成的硫酸钡白色沉淀经过过滤、洗涤，并在低温烘箱中干燥。
7. Acid–Alkali Titrations and Concentration Calculations | 酸碱滴定与浓度计算
Titration is a technique used to determine the concentration of an unknown acid or alkali solution by neutralising it with a solution of known concentration. A pipette measures a fixed volume of the unknown solution into a conical flask, and a burette delivers the standard solution. An indicator, such as phenolphthalein or methyl orange, signals the end point.
滴定是一种通过用已知浓度的溶液进行中和来测定未知酸或碱溶液浓度的技术。用移液管量取一定体积的未知溶液放入锥形瓶中，用滴定管滴加标准溶液。指示剂（如酚酞或甲基橙）指示终点。
Titration calculations use the formula: moles = concentration × volume (in dm³) s of NaOH = 0.00300 mol, so concentration of NaOH = 0.00300 / (25.0/1000) = 0.120 mol/dm³.
滴定计算的公式是：摩尔数 = 浓度 × 体积（dm³）。根据配平的化学方程式，利用摩尔比求出未知浓度。例如，25.0 cm³ NaOH 被 30.0 cm³ 0.100 mol/dm³ 的 HCl 中和，则 HCl 的摩尔数 = 0.100 × (30.0/1000) = 0.00300 mol。1:1 的反应摩尔比说明 NaOH 的摩尔数也是 0.00300 mol，因此 NaOH 的浓度 = 0.00300 / (25.0/1000) = 0.120 mol/dm³。
Concentration (mol/dm³) = moles ÷ volume (dm³)
浓度 (mol/dm³) = 摩尔数 ÷ 体积 (dm³)
8. Strong and Weak Acids and Bases | 强酸、弱酸、强碱与弱碱
Strong acids, such as hydrochloric, sulfuric and nitric acids, dissociate completely in water, giving a high concentration of H⁺ ions. Weak acids, such as ethanoic, citric and carbonic acids, only partially dissociate, leaving many molecules undissociated. As a result, weak acids have a higher pH than a strong acid of the same concentration.
强酸，如盐酸、硫酸和硝酸，在水中完全解离，产生高浓度的 H⁺ 离子。弱酸，如乙酸、柠檬酸和碳酸，只部分解离，溶液中仍有大量未解离的分子。因此，相同浓度的弱酸的 pH 比强酸高。
Similarly, strong bases like sodium hydroxide ionise completely to give a high concentration of OH⁻ ions, while weak bases like ammonia solution only partially ionise.
同理，氢氧化钠等强碱完全电离，产生高浓度的 OH⁻ 离子，而氨水等弱碱只部分电离。
It is important to distinguish between strength and concentration. A concentrated weak acid still contains a high mass of acid per volume, but most of it is undissociated, so the concentration of H⁺ is relatively low.
区分酸碱的强度与浓度非常重要。浓的弱酸仍然含有单位体积较高质量分数的酸，但大部分未解离，因此 H⁺ 浓度相对较低。
9. The Importance of pH in Soil and Biology | pH 在土壤和生物学中的重要性
The pH of soil significantly affects plant growth. Most plants prefer a slightly acidic to neutral soil (pH 6–7). If the soil is too acidic, farmers add lime (calcium oxide or calcium hydroxide) to raise the pH. If too alkaline, organic matter or ammonium sulfate can be used to lower the pH.
土壤的 pH 显著影响植物生长。大多数植物喜欢微酸性到中性的土壤（pH 6–7）。如果土壤过酸，农民会施加石灰（氧化钙或氢氧化钙）来提高 pH。如果过碱，可以添加有机质或硫酸铵来降低 pH。
In the human body, the stomach produces hydrochloric acid to aid digestion and kill bacteria. The pH inside the stomach is around 1–2. The small intestine operates at a pH of about 8, where pancreatic juice neutralises stomach acid.
在人体中，胃分泌盐酸以帮助消化和杀死细菌。胃内 pH 约为 1–2。小肠内的 pH 约为 8，胰液在此中和胃酸。
10. Everyday Applications of Acids, Bases and Salts | 酸、碱和盐的日常应用
Acids are used in car batteries (sulfuric acid), food preservation (ethanoic acid in vinegar) and rust removal (phosphoric acid). Bases like sodium hydroxide are used in soap making and drain cleaners. Ammonia solution is a common household cleaner. Salts have countless uses: sodium chloride as table salt and de‑icer, ammonium nitrate as fertiliser, and calcium sulfate in plaster of Paris.
酸用于汽车电池（硫酸）、食品保存（食醋中的乙酸）以及除锈（磷酸）。碱如氢氧化钠用于制皂和通渠剂。氨水是常见的家用清洁剂。盐的用途数不胜数：氯化钠用作食盐和除冰剂，硝酸铵用作肥料，硫酸钙用于制作熟石膏。
A thorough understanding of the properties and reactions of acids, bases and salts is not only an IGCSE requirement but also a step towards appreciating the chemistry that shapes our world.
深入理解酸、碱和盐的性质与反应不仅是 IGCSE 的要求，也是进一步欣赏塑造我们世界的化学的一步。
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