📚 Acids, Bases and Salts: Key Concepts for Edexcel IGCSE | 酸、碱与盐：Edexcel IGCSE 核心概念
Acids, bases and salts form a central topic in Edexcel IGCSE Science. Understanding their properties, reactions and methods of preparation allows you to explain many everyday processes and laboratory procedures. This article covers the essential definitions, the pH scale, reaction equations and practical salt preparation techniques required by the syllabus.
酸、碱与盐是 Edexcel IGCSE 科学中的核心主题。掌握它们的性质、反应和制备方法，可以帮助你解释许多日常过程与实验室操作。本文内容涵盖基本定义、pH 标度、反应方程式以及教学大纲所要求的盐的制备实验技术。
1. What are acids and bases? | 什么是酸和碱？
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid dissociates into H⁺ and Cl⁻ ions. Common acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).
酸是溶解于水时产生氢离子（H⁺）的物质。例如，盐酸解离生成 H⁺ 与 Cl⁻ 离子。常见酸包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。
Bases are substances that can neutralise an acid. A base that dissolves in water to release hydroxide ions (OH⁻) is called an alkali. Examples of bases are copper(II) oxide (CuO) and sodium hydroxide (NaOH). Alkalis are soluble bases; copper(II) oxide is not soluble in water, so it is not an alkali.
碱是能够中和酸的物质。可溶于水并释放氢氧根离子（OH⁻）的碱称为碱。碱的例子有氧化铜（CuO）和氢氧化钠（NaOH）。可溶性的碱是碱；氧化铜不溶于水，因此它不是碱。
2. The pH scale and indicators | pH 标度与指示剂
The pH scale runs from 0 to 14. A pH below 7 indicates an acidic solution, pH 7 is neutral, and pH above 7 is alkaline. The pH of a solution can be measured using universal indicator paper, which gives a colour change that is compared with a colour chart.
pH 标度范围为 0 至 14。pH 低于 7 表示酸性溶液，7 为中性，高于 7 为碱性。溶液的 pH 可使用万能指示剂试纸测量，试纸颜色变化与比色卡对照即可得出读数。
Indicators are chemicals that change colour depending on the pH of a solution. Common ones include:
指示剂是根据溶液 pH 改变颜色的化学物质。常见的有：
- Litmus: red in acid, blue in alkali. 石蕊：遇酸变红，遇碱变蓝。
- Phenolphthalein: colourless in acid, pink in alkali. 酚酞：遇酸无色，遇碱变粉红色。
- Methyl orange: red in acid, yellow in alkali. 甲基橙：遇酸变红，遇碱变黄。
Universal indicator gives a full range of colours from red (strongly acidic) to violet (strongly alkaline), allowing an approximate pH value to be determined.
万能指示剂呈现从红色（强酸性）到紫色（强碱性）的完整颜色范围，从而可估算出 pH 数值。
3. Reactions of acids with metals | 酸与金属的反应
When an acid reacts with a reactive metal, the products are a salt and hydrogen gas. The general equation is:
当酸与活泼金属反应时，产物是盐和氢气。通式如下：
Acid + Metal → Salt + Hydrogen
For example, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen:
例如，锌与盐酸反应生成氯化锌和氢气：
Zn + 2HCl → ZnCl₂ + H₂
Observation: the metal fizzes and bubbles of hydrogen gas are produced; the metal gradually disappears as it reacts; the solution warms up. Hydrogen can be tested with a lighted splint, which gives a squeaky pop.
现象：金属表面产生气泡并发出嘶嘶声；金属逐渐溶解消失；溶液温度升高。氢气可用点燃的木条检验，会发出尖细的爆鸣声。
4. Reactions of acids with bases and alkalis | 酸与碱和碱的反应
Acids react with bases (metal oxides, metal hydroxides) to produce a salt and water only. This is called neutralisation.
酸与碱（金属氧化物、金属氢氧化物）反应只生成盐和水，这称为中和反应。
Acid + Base → Salt + Water
Example: hydrochloric acid reacts with sodium hydroxide (a soluble base) to give sodium chloride and water:
例如：盐酸与氢氧化钠（可溶性碱）反应生成氯化钠和水：
HCl + NaOH → NaCl + H₂O
The ionic equation for any neutralisation is:
任何中和反应的离子方程式为：
H⁺ + OH⁻ → H₂O
This shows that the net reaction is between hydrogen ions from the acid and hydroxide ions from the alkali.
这表明反应的实质是酸中的氢离子与碱中的氢氧根离子结合。
5. Reactions of acids with carbonates | 酸与碳酸盐的反应
Acids react with carbonates (or hydrogencarbonates) to produce a salt, water and carbon dioxide gas.
酸与碳酸盐（或碳酸氢盐）反应生成盐、水和二氧化碳气体。
Acid + Carbonate → Salt + Water + Carbon dioxide
Example: calcium carbonate reacts with dilute hydrochloric acid:
例如：碳酸钙与稀盐酸反应：
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Carbon dioxide is tested by bubbling the gas through limewater; it turns limewater milky (a white precipitate of calcium carbonate forms).
检验二氧化碳的方法：将气体通入澄清石灰水中，石灰水变浑浊（生成白色碳酸钙沉淀）。
6. Neutralisation | 中和作用
Neutralisation is the reaction between an acid and a base to form a neutral solution of salt and water. It is used in many everyday situations.
中和反应是酸与碱生成中性盐溶液和水的反应。在日常生活中应用广泛。
For example, bee stings are acidic; they can be treated with a mild alkali such as baking soda. Wasp stings are alkaline and can be treated with vinegar (acetic acid). Farmers add lime (calcium hydroxide) to acidic soil to neutralise excess acid and improve crop growth.
例如，蜜蜂蛰伤处呈酸性，可用小苏打等弱碱处理。黄蜂蛰伤处呈碱性，可用醋（醋酸）处理。农民在酸性土壤中加入熟石灰（氢氧化钙）以中和过多酸并改善作物生长。
Antacids contain bases such as magnesium hydroxide to neutralise excess stomach acid and relieve indigestion.
抗酸药含有氢氧化镁等碱，可中和胃中过多酸缓解消化不良。
7. Preparing soluble salts | 制备可溶性盐
Soluble salts can be prepared by reacting an acid with an alkali or with an insoluble base/carbonate. There are two main methods.
可溶性盐可通过酸与碱或不溶性碱/碳酸盐反应制备。主要有两种方法。
Titration method is used with an alkali (soluble base). The acid is placed in a burette and added dropwise to a known volume of alkali containing an indicator. The indicator is added to show the exact point of neutralisation. Once the end point is reached, the volume of acid used is noted. A fresh batch is then mixed without the indicator, so the salt solution is pure. The water is evaporated to obtain solid salt crystals.
滴定法用于碱（可溶性碱）的反应。在滴定管中加入酸，逐滴加入含有指示剂的已知体积的碱中。指示剂用于指示中和反应恰好完全的时刻。到达终点后记录消耗酸的体积。然后取新的碱溶液，不加指示剂，按相同体积混合酸和碱，得到纯净盐溶液。蒸发水分即得固体盐晶体。
Excess solid method is used with insoluble bases or carbonates. The acid is warmed, and the insoluble solid (e.g. copper(II) oxide) is added in excess until no more reacts. The mixture is filtered to remove unreacted solid; the filtrate is then evaporated to crystallise the salt. This method avoids the need for an indicator because the excess solid shows that the acid is fully used up.
过量固体法用于不溶性碱或碳酸盐。将酸加热，加入过量的不溶性固体（如氧化铜），直到不再反应。过滤混合物除去剩余固体；滤液蒸发结晶即可得到盐。这种方法不需要指示剂，因为有剩余固体就说明酸已完全反应。
8. Preparing insoluble salts | 制备不溶性盐
Insoluble salts are generally made by precipitation. Two soluble solutions are mixed so that the cations and anions combine to form an insoluble salt, which appears as a precipitate.
不溶性盐通常通过沉淀反应制备。将两种可溶溶液混合，使阳离子与阴离子结合生成不溶性盐，以沉淀形式析出。
Example: lead(II) nitrate solution reacts with potassium iodide solution to form yellow lead(II) iodide precipitate:
例如：硝酸铅溶液与碘化钾溶液反应生成黄色碘化铅沉淀：
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
The mixture is filtered, and the precipitate is washed with distilled water and dried in a warm oven.
将混合液过滤，沉淀用蒸馏水洗涤，并在温箱中烘干。
9. The importance of salts in daily life | 盐在日常生活中的重要性
Salts have many important uses in agriculture, industry and medicine.
盐在农业、工业和医药方面有许多重要用途。
- Ammonium nitrate (NH₄NO₃) is a common fertiliser because it provides nitrogen for plant growth. 硝酸铵（NH₄NO₃）是常见肥料，因为它为植物生长提供氮元素。
- Sodium chloride (NaCl) is used to preserve food and season meals. 氯化钠（NaCl）用于保存食物和调味。
- Calcium carbonate (CaCO₃) is used as an antacid and as a filler in paints and plastics. 碳酸钙（CaCO₃）用作抗酸药，也用作油漆和塑料的填料。
- Copper(II) sulfate (CuSO₄) is used in agriculture as a fungicide and in chemistry for testing water (white to blue with water). 硫酸铜（CuSO₄）在农业上用作杀菌剂，在化学中用于检验水（无水时白色，遇水变蓝）。
Understanding how salts are formed and their properties helps us appreciate both laboratory chemistry and real-world applications. For Edexcel IGCSE, be able to write word equations and balanced symbol equations for the reactions described above, and know the practical steps for salt preparation.
了解盐的生成方式与性质，有助于我们认识实验室化学与实际应用。对于 Edexcel IGCSE，你需要能够写出上述反应的文字方程式和配平的符号方程式，并掌握盐的制备实验步骤。
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