📚 Acids, Bases and Neutralisation Reactions | 酸、碱与中和反应
Acids and bases are among the most important families of chemical substances, playing essential roles in both laboratory chemistry and everyday life. From the hydrochloric acid in our stomachs to the sodium hydroxide used in soap‑making, these compounds undergo characteristic reactions – especially neutralisation – that produce salts and water. Understanding their properties, the pH scale, and the methods used to prepare pure salts is fundamental to success in IGCSE Edexcel Science (Chemistry). This article systematically explores the core concepts, practical techniques, and real‑world applications required by the syllabus.
酸和碱是化学物质中最重要的两大类别，在实验室化学和日常生活中都扮演着重要角色。从胃里的盐酸到制造肥皂所用的氢氧化钠，这些化合物会发生特征反应——尤其是中和反应——生成盐和水。理解它们的性质、pH 标度以及制备纯盐的方法是 IGCSE Edexcel 科学（化学）取得好成绩的基础。本文系统梳理了考纲所要求的核心概念、实验技术和实际应用。
1. Defining Acids and Bases | 酸和碱的定义
An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) ionises completely in water to give H⁺ and Cl⁻ ions. The presence of H⁺ ions is responsible for the typical acidic properties, such as sour taste and the ability to turn blue litmus paper red. A base, on the other hand, is a substance that can neutralise an acid to form a salt and water. Soluble bases that release hydroxide ions (OH⁻) in water are called alkalis. Sodium hydroxide (NaOH) dissolves in water to give Na⁺ and OH⁻, making it a strong alkali.
酸是一种溶于水时释放氢离子（H⁺）的物质。例如，盐酸（HCl）在水中完全电离，生成 H⁺ 和 Cl⁻ 离子。H⁺ 离子的存在赋予了酸典型的性质，比如酸味以及使蓝色石蕊试纸变红的能力。而碱是一种能与酸中和生成盐和水的物质。可溶于水并能释放氢氧根离子（OH⁻）的碱称为碱。氢氧化钠（NaOH）溶于水时产生 Na⁺ 和 OH⁻，因此是一种强碱。
2. The pH Scale and Universal Indicator | pH 标度与通用指示剂
The pH scale measures the acidity or alkalinity of an aqueous solution, ranging from 0 (strongly acidic) to 14 (strongly alkaline), with pH 7 being neutral. Each unit change represents a ten‑fold change in H⁺ concentration: a solution of pH 3 has 10 times more H⁺ ions than one of pH 4. Universal indicator is a mixture of dyes that produces a spectrum of colours across different pH values, from red in strong acids to violet in strong alkalis. It is more informative than litmus because it indicates the approximate pH rather than simply showing whether a substance is acidic or alkaline.
pH 标度用来衡量水溶液的酸碱度，范围从 0（强酸性）到 14（强碱性），pH 7 为中性。pH 值每变化 1 个单位，H⁺ 浓度就相差 10 倍：pH 3 的溶液中的 H⁺ 浓度是 pH 4 溶液的 10 倍。通用指示剂是一种混合染料，在不同 pH 值下呈现一系列不同颜色，从强酸中的红色到强碱中的紫色。它比石蕊指示剂提供的信息更多，因为它可以显示近似的 pH 值，而不仅仅是酸性或碱性。
3. Common Laboratory Acids and Alkalis | 常见实验室酸与碱
In the IGCSE laboratory, you will frequently encounter hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). These are strong acids that completely ionise in water. Ethanoic acid (CH₃COOH) is a weak acid found in vinegar that partially ionises. Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂) – the latter is used to make limewater for the carbon dioxide test. Ammonia solution (NH₃(aq)) is a weak alkali that partially ionises to give OH⁻ ions.
在 IGCSE 实验室中，你经常会接触到盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。它们都是强酸，在水中完全电离。乙酸（CH₃COOH）是一种弱酸，存在于食醋中，只能部分电离。常见的碱包括氢氧化钠（NaOH）、氢氧化钾（KOH）和氢氧化钙（Ca(OH)₂）——后者用来配制石灰水以检验二氧化碳。氨水（NH₃(aq)）是一种弱碱，部分电离产生 OH⁻ 离子。
4. Neutralisation – The Core Reaction | 中和反应——核心反应
gen ions from the acid combine with hydroxide ions from the alkali to form water molecules. The other ions present – for example, Na⁺ and Cl⁻ if hydrochloric acid and sodium hydroxide are used – vaporates. Neutralisation is an exothermic reaction; the temperature of the mixture rises.
中和反应是酸与碱生成盐和水的反应。酸与碱之间中和反应的基本离子方程为：H⁺(aq) + OH⁻(aq) → H₂O(l)。这表明酸中的氢离子与碱中的氢氧根离子结合生成水分子。其他存在的离子——比如使用盐酸和氢氧化钠时的 Na⁺ 和 Cl⁻——留在溶液中，待水分蒸发后形成盐。中和反应是放热反应，混合物的温度会升高。
5. Naming Salts and Choosing Starting Materials | 盐的命名与起始原料的选择
The name of a salt comes from the acid and the metal (or base) used. Hydrochloric acid produces chloride salts (e.g., sodium chloride, copper(II) chloride). Sulfuric acid gives sulfate salts (e.g., potassium sulfate, magnesium sulfate). Nitric acid forms nitrate salts (e.g., calcium nitrate). To make a specific salt, you select the acid containing the correct anion and a base (or metal, carbonate, or alkali) that supplies the metal cation. For example, to prepare copper(II) sulfate, you would react sulfuric acid with copper(II) oxide or copper(II) carbonate.
盐的名称来源于所用的酸和金属（或碱）。盐酸产生氯化物盐（如氯化钠、氯化铜(II)）。硫酸产生硫酸盐（如硫酸钾、硫酸镁）。硝酸形成硝酸盐（如硝酸钙）。若要制备某种特定的盐，你需要选择含有正确阴离子的酸以及能提供金属阳离子的碱（或金属、碳酸盐或碱溶液）。例如，要制备硫酸铜(II)，你可以用硫酸与氧化铜(II)或碳酸铜(II)反应。
6. Preparing Soluble Salts – Method 1: Acid + Excess Insoluble Base | 可溶性盐的制备——方法一：酸 + 过量不溶性碱
This method is suitable when the base is an insoluble metal oxide or metal carbonate. You add an excess of the solid base to a fixed volume of warm acid, stirring until no more dissolves and some solid remains. This ensures all the acid has been neutralised. The mixture is then filtered to remove the unreacted solid. The filtrate is a solution of the salt. Finally, you evaporate some of the water gently and leave the concentrated solution to crystallise. For example, copper(II) sulfate crystals can be prepared by reacting warm sulfuric acid with excess black copper(II)oxide powder.
当碱是不溶性金属氧化物或金属碳酸盐时适用此方法。你将过量的固体碱加入一定体积的温酸中，不断搅拌，直到不再溶解且有固体残留。这确保所有的酸都已被中和。然后将混合物过滤，除去未反应的固体。滤液即为盐溶液。最后，微热蒸发掉一部分水，将浓缩后的溶液静置结晶。例如，可用温热的硫酸与过量的黑色氧化铜(II)粉末反应来制备硫酸铜(II)晶体。
7. Preparing Soluble Salts – Method 2: Titration using an Alkali | 可溶性盐的制备——方法二：使用碱的滴定法
When both the acid and the base are soluble (e.g., sodium hydroxide and hydrochloric acid), you cannot use an excess of solid. Instead, a titration is carried out to determine the exact volumes needed for complete neutralisation. Using a pipette, a known volume of alkali is placed in a conical flask with a few drops of an indicator such as phenolphthalein. Acid is added from a burette until the indicator changes colour. The process is repeated without the indicator using the determined volumes to obtain a pure salt solution. The solution is then evaporated and crystallised to yield, for example, pure sodium chloride crystals.
当酸和碱都是可溶的（如氢氧化钠和盐酸），就不能使用过量固体。这时需要采用滴定法来确定恰好完全中和所需的确切体积。用移液管量取一定体积的碱溶液放入锥形瓶中，加入几滴酚酞等指示剂。从滴定管中加入酸，直到指示剂变色。在确定体积后，不用指示剂重复操作，得到纯净的盐溶液。再通过蒸发、结晶，即可获得如纯净氯化钠晶体等产物。
8. Preparing Insoluble Salts – Precipitation | 不溶性盐的制备——沉淀法
Some salts are insoluble in water, such as silver chloride, barium sulfate, and lead(II) iodide. These are prepared by mixing two aqueous solutions, each containing one of the required ions. An immediate precipitate forms. The solid is separated by filtration, washed with distilled water, and dried. For instance, to prepare lead(II) iodide (a bright yellow insoluble salt), you mix lead(II) nitrate solution with potassium iodide solution. The precipitation method is fast and does not require heating or evaporation.
有些盐不溶于水，如氯化银、硫酸钡和碘化铅(II)。这些盐的制备方法是将两种各含一种所需离子的水溶液混合，立即生成沉淀。通过过滤将固体分离，用蒸馏水洗涤后干燥。例如，要制备碘化铅(II)（一种亮黄色的不溶性盐），可以将硝酸铅(II)溶液与碘化钾溶液混合。沉淀法操作快捷，不需要加热或蒸发。
9. Reactions of Acids with Metals | 酸与金属的反应
en: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). This reaction is a classic test for hydrogen gas – acids. The reactivity series helps predict which metals can displace hydrogen from acids.
稀酸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(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)。这个反应是检验氢气的经典方法——点燃的木条会产生爆鸣声。并非所有金属都能反应，铜和银就不与稀酸反应。金属活动性顺序可以帮助预测哪些金属能从酸中置换出氢气。
10. Reactions of Acids with Carbonates | 酸与碳酸盐的反应
CO₂(g). The carbon dioxide evolved turns limewater milky, which is a standard chemical test. This reaction is also responsible for the weathering of limestone buildings by acid rain.
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。一般规律为：酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。例如，碳酸钙（石灰石）与盐酸反应：CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)。生成的二氧化碳会使石灰水变浑浊，这是一种标准的化学检验方法。酸雨对石灰岩建筑的侵蚀正是基于这一反应。
11. Everyday Applications of Neutralisation | 中和反应的日常应用
Neutralisation reactions are widely used to solve practical problems. Indigestion remedies are weak bases (such as magnesium hydroxide or calcium carbonate) that neutralise excess stomach acid. In agriculture, slaked lime (calcium hydroxide) is added to acidic soils to raise the pH for better crop growth. Wasp stings are alkaline and can be treated with vinegar (a weak acid), while bee stings are acidic and are treated with baking soda (a weak alkali). Effluent from factories is often neutralised before being released into rivers to prevent damage to aquatic life.
中和反应被广泛用于解决实际问题。消化不良药是弱碱性物质（如氢氧化镁或碳酸钙），能中和过多的胃酸。在农业上，常在酸性土壤中加入熟石灰（氢氧化钙）来提高 pH 值，以利于作物生长。黄蜂蜇伤是碱性的，可用醋（弱酸）处理；而蜜蜂蜇伤是酸性的，可用小苏打（弱碱）处理。工厂废水在排入河流前通常需要中和，以免危害水生生物。
12. Key Recall and Examination Tips | 重点回顾与应试技巧
or IGCSE Edexcel, you must confidently write word and balanced chemical equations for acid– curves for neutralisation, and be prepared for questions linking acid reactions to the reactivity series and tests for gases (H₂, CO₂). Use precise scientific language when describing practical procedures.
在 IGCSE Edexcel 考试中，你必须能够自信地写出酸–碱反应的文字方程式和配平的化学方程式。要能描述两种盐的制备方法，并解释为什么选择某种方法。熟记石蕊、酚酞和甲基橙在酸和碱中的颜色变化。牢记中和反应的离子方程式为 H⁺ + OH⁻ → H₂O。练习绘制中和反应的 pH 曲线，并准备好回答将酸的反应与金属活动性顺序以及气体检验（H₂、CO₂）相结合的题目。在描述实验步骤时，要使用精确的科学语言。
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