📚 Acids, Bases and Salts | 酸、碱与盐
In IGCSE chemistry (Edexcel), the study of acids, bases and salts forms a core foundation for understanding chemical reactions in aqueous solutions. This article will guide you through the essential definitions, properties, reaction types, and practical techniques needed for your exams.
在 IGCSE 化学（Edexcel）课程中，酸、碱与盐的学习是理解水溶液中化学反应的核心基础。本文将带你梳理必备的定义、性质、反应类型以及考试所需的实验技能。
1. Definitions of Acids and Bases | 酸和碱的定义
An acid is a substance that dissolves in water to produce hydrogen ions (H⁺) as the only positive ions. For example, hydrochloric acid gives H⁺ and Cl⁻ in aqueous solution. In modern theory, a proton donor.
酸是指溶于水时只产生氢离子（H⁺）作为唯一正离子的物质。例如，盐酸在水溶液中产生 H⁺ 和 Cl⁻。按现代理论，酸是质子给予体。
A base is a substance that neutralises an acid to form a salt and water only. A soluble base is called an alkali, for example sodium hydroxide (NaOH). An alkali dissolves in water to produce hydroxide ions (OH⁻).
碱是指能与酸中和生成盐和水的物质。可溶性碱称为碱溶液（alkali），例如氢氧化钠（NaOH）。碱溶于水时产生氢氧根离子（OH⁻）。
Common examples: acids include sulfuric acid (H₂SO₄), nitric acid (HNO₃) and ethanoic acid (CH₃COOH); bases include copper(II) oxide (CuO), calcium carbonate (CaCO₃) and ammonia solution (NH₃(aq)).
常见例子：酸包括硫酸（H₂SO₄）、硝酸（HNO₃）和乙酸（CH₃COOH）；碱包括氧化铜（CuO）、碳酸钙（CaCO₃）和氨水（NH₃(aq)）。
2. Physical and Chemical Properties | 物理和化学性质
Acids taste sour, turn blue litmus red, and have a pH below 7. Strong acids such as HCl are fully ionised in water, while weak acids like ethanoic acid are only partially ionised.
酸有酸味，能使蓝色石蕊变红，pH 小于 7。像 HCl 这样的强酸在水中完全电离，而像乙酸这样的弱酸仅部分电离。
Bases feel soapy, turn red litmus blue, and have a pH above 7. Alkalis are bases that dissolve in water; insoluble bases include metal oxides and hydroxides like zinc oxide (ZnO).
碱有滑腻感，能使红色石蕊变蓝，pH 大于 7。碱溶液是可溶于水的碱；不溶性碱包括金属氧化物和氢氧化物，如氧化锌（ZnO）。
It is important to note that all alkalis are bases, but not all bases are alkalis. For example, copper(II) oxide is a base but it does not dissolve in water to give OH⁻ ions.
需要特别注意的是：所有碱溶液都是碱，但并非所有碱都能溶于水。例如，氧化铜是一种碱，但它不溶于水，不能产生 OH⁻ 离子。
3. The pH Scale and Indicators | pH 标度与指示剂
The pH scale runs from 0 to 14. pH 0–2 strong acid, 3–6 weak acid, 7 neutral, 8–11 weak alkali, 12–14 strong alkali. pH is a measure of the concentration of H⁺ ions in solution.
pH 标度从 0 到 14。pH 0–2 为强酸，3–6 为弱酸，7 为中性，8–11 为弱碱，12–14 为强碱。pH 是溶液中 H⁺ 离子浓度的量度。
Universal indicator gives a colour range: red for strong acid, orange/yellow for weak acid, green for neutral, blue for weak alkali, purple for strong alkali. A pH probe gives a numerical value more precisely than indicator paper.
广谱指示剂显示一系列颜色：强酸呈红色、弱酸呈橙黄色、中性呈绿色、弱碱呈蓝色、强碱呈紫色。pH 探头能给出比试纸更精确的数值。
For titrations, common indicators include methyl orange (red in acid, yellow in alkali) and phenolphthalein (colourless in acid, pink in alkali). Choosing the correct indicator depends on the strength of the acid and alkali.
在滴定中，常用指示剂包括甲基橙（酸中红色、碱中黄色）和酚酞（酸中无色、碱中粉红色）。选择指示剂需根据酸碱的强弱而定。
4. Reactions of Acids with Metals | 酸与金属的反应
Acid + metal → salt + hydrogen. For example, zinc with sulfuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂. This is a redox reaction where the metal loses electrons to form positive ions.
酸 + 金属 → 盐 + 氢气。例如，锌与硫酸反应：Zn + H₂SO₄ → ZnSO₄ + H₂。这是一个氧化还原反应，金属失去电子形成正离子。
To test for hydrogen gas, place a lighted splint near the mouth of the test tube; a squeaky pop confirms hydrogen. Only metals above hydrogen in the reactivity series react with dilute acids.
检验氢气的方法是：将点燃的木条放在试管口，若有尖锐的爆鸣声则证明是氢气。只有金属活动性顺序中排在氢之前的金属才能与稀酸反应。
Copper and silver do not react with dilute hydrochloric or sulfuric acid because they are less reactive than hydrogen. This explains why copper vessels can safely hold dilute acids in some demonstrations.
铜和银不与稀盐酸或稀硫酸反应，因为它们比氢不活泼。这也解释了在某些演示中铜容器为何能安全盛放稀酸。
5. Reactions of Acids with Bases and Carbonates | 酸与碱及碳酸盐的反应
Acid + base → salt + water (neutralisation). For example, HCl + NaOH → NaCl + H₂O. The ionic equation is H⁺ + OH⁻ → H₂O.
酸 + 碱 → 盐 + 水（中和反应）。例如，HCl + NaOH → NaCl + H₂O。离子方程式为 H⁺ + OH⁻ → H₂O。
Acid + carbonate → salt + water + carbon dioxide. Example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. Carbon dioxide gas can be tested by bubbling through limewater, which turns milky.
酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。例如：2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂。可通过通入石灰水中变浑浊来检验二氧化碳。
Acid + hydrogencarbonate → salt + water + carbon dioxide. The reaction is similar but often more vigorous, and it is used in antacid tablets containing sodium hydrogencarbonate (NaHCO₃).
酸 + 碳酸氢盐 → 盐 + 水 + 二氧化碳。反应相似但通常更剧烈，含碳酸氢钠（NaHCO₃）的抗酸药片基于此原理。
6. Neutralisation and Exothermic Nature | 中和反应与放热性质
Neutralisation is the reaction between an acid and a base to produce a salt and water, and sometimes other products. It is always exothermic, meaning heat energy is released to the surroundings.
中和是酸与碱反应生成盐和水（有时还有其他产物）的过程。中和反应总是放热的，即向周围环境释放热能。
In an experiment, mixing 25 cm³ of 1 mol/dm³ NaOH with 25 cm³ of 1 mol/dm³ HCl causes the temperature to rise by about 6–7 °C. The maximum temperature is recorded when the acid and alkali have completely reacted.
在实验中，将 25 cm³ 的 1 mol/dm³ NaOH 与 25 cm³ 的 1 mol/dm³ HCl 混合，温度会升高约 6–7 °C。当酸和碱完全反应时记录到最高温度。
Neutralisation is not only a chemical concept but also has real-life applications: treating wasp stings (alkaline) with vinegar (weak acid), and treating bee stings (acidic) with baking soda solution (alkaline).
中和反应不仅是化学概念，还有实际应用：用醋（弱酸）处理黄蜂蜇伤（碱性），用小苏打溶液（碱性）处理蜜蜂蜇伤（酸性）。
7. Solubility Rules for Salts | 盐的溶解性规则
All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Most chlorides are soluble except silver chloride (AgCl) and lead(II) chloride (PbCl₂).
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（AgCl）和氯化铅（PbCl₂）除外。
Most sulfates are soluble except barium sulfate (BaSO₄), lead(II) sulfate (PbSO₄) and calcium sulfate (CaSO₄, only slightly soluble). Most carbonates are insoluble except those of sodium, potassium and ammonium.
大多数硫酸盐可溶，但硫酸钡（BaSO₄）、硫酸铅（PbSO₄）和硫酸钙（CaSO₄，微溶）除外。大多数碳酸盐不溶，但钠盐、钾盐和铵盐除外。
These rules help predict whether a precipitate will form when two solutions are mixed. For example, mixing silver nitrate and sodium chloride produces a white precipitate of AgCl because it is insoluble.
这些规则有助于预测两种溶液混合时是否会产生沉淀。例如，硝酸银与氯化钠混合会产生白色 AgCl 沉淀，因为它不溶。
8. Preparation of Soluble Salts | 可溶性盐的制备
To prepare a soluble salt from an acid and an insoluble base, add excess solid base to warm acid. When the acid is fully neutralised, filter to remove excess insoluble base, then evaporate the filtrate to crystallise the salt.
要从不溶性碱和酸制备可溶性盐，需将过量固体碱加入温热的酸中。当酸完全中和后，过滤除去剩余的不溶性碱，然后蒸发滤液结晶析出盐。
If the base is soluble (e.g. NaOH), you cannot use this excess method because it is difficult to detect the endpoint. Instead, use titration: add exactly the required volume of alkali using a burette.
如果碱可溶（如 NaOH），则无法使用过量法，因为终点不易判断。此时应使用滴定法：用滴定管精确加入所需体积的碱液。
For example, to prepare sodium chloride from HCl and NaOH, titrate until neutralisation, then evaporate the solution gently to obtain white crystals. The crystals should be dried between filter papers or in a warm oven.
例如，由 HCl 和 NaOH 制备氯化钠时，滴定至恰好中和，然后温和蒸发溶液得到白色晶体。晶体应在滤纸之间或用温箱干燥。
9. Titration Procedure and Calculations | 滴定步骤与计算
A typical titration uses a conical flask containing a known volume of alkali with indicator, and a burette filled with acid. Acid is added dropwise until the indicator changes colour permanently.
典型滴定使用锥形瓶盛装已知体积的碱液和指示剂，滴定管盛装酸液。逐滴加入酸液，直到指示剂颜色永久改变。
Repeat the titration until two results are within 0.10 cm³ of each other. Use the average titre to calculate concentration using the equation: concentration (mol/dm³) = amount (mol) / volume (dm³).
重复滴定直到两次结果相差不超过 0.10 cm³。使用平均消耗体积，通过公式计算浓度：浓度（mol/dm³）= 物质的量（mol）/ 体积（dm³）。
Example: 25.0 cm³ of NaOH was titrated with 0.100 mol/dm³ HCl and the average titre was 20.0 cm³. Moles of HCl = 0.100 × 20.0/1000 = 0.00200 mol. Since NaOH reacts 1:1 with HCl, the concentration of NaOH = 0.00200 / (25.0/1000) = 0.0800 mol/dm³.
例如：25.0 cm³ NaOH 与 0.100 mol/dm³ HCl 滴定，平均消耗 20.0 cm³。HCl 的物质的量 = 0.100 × 20.0/1000 = 0.00200 mol。由于 NaOH 与 HCl 按 1:1 反应，NaOH 的浓度 = 0.00200 / (25.0/1000) = 0.0800 mol/dm³。
10. Applications and Environmental Significance | 应用与环境意义
Acids and bases are essential in daily life. Sulfuric acid is used in car batteries and fertiliser manufacturing; nitric acid is used in making explosives; hydrochloric acid is found in stomach juice to help digestion.
酸和碱在日常生活中至关重要。硫酸用于汽车电池和化肥制造；硝酸用于制造炸药；盐酸存在于胃液中帮助消化。
Reacting acids with alkalis helps treat acidic soil. Farmers add calcium oxide (lime) or calcium hydroxide to neutralise acidic soil, improving crop growth. Similarly, acidic waste from factories must be neutralised before discharge.
酸碱反应可用于改良酸性土壤。农民添加氧化钙（生石灰）或氢氧化钙来中和酸性土壤，促进作物生长。同样，工厂的酸性废液在排放前必须中和。
Understanding the rules of salt solubility is also key to water purification and medicine. For instance, barium sulfate is used in medical imaging because it is insoluble and safe for swallowing.
理解盐的溶解性规则也是水净化和医学的关键。例如，硫酸钡因不溶且可安全吞服，用于医学影像造影。
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