📚 Acids, Bases and Salts | 酸、碱与盐
Acids, bases and salts are among the most important groups of compounds in Cambridge IGCSE Science. They appear in laboratory experiments, industrial processes and everyday materials such as vinegar, soap, antacids and fertilisers. Understanding their properties, reactions and safe handling is essential for both the written paper and practical assessment.
酸、碱和盐是剑桥 IGCSE 科学中最重要的化合物类别之一。它们出现在实验室实验、工业过程以及醋、肥皂、抗酸药和化肥等日常物质中。理解它们的性质、反应和安全操作对笔试和实验考核都至关重要。
1. Defining Acids and Bases | 酸和碱的定义
In the Cambridge IGCSE Science syllabus, an acid is defined as a substance that produces hydrogen ions, H⁺, when dissolved in water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, which ionises completely. A base is a substance that can neutralise an acid, while a soluble base is called an alkali. Alkalis are substances that produce hydroxide ions, OH⁻, in aqueous solution.
在剑桥 IGCSE 科学大纲中，酸被定义为溶于水时产生氢离子 H⁺ 的物质。例如，氯化氢气体溶于水形成盐酸，并完全电离。碱是指能中和酸的物质，而可溶的碱称为碱（alkali）。碱是在水溶液中产生氢氧根离子 OH⁻ 的物质。
The ionisation of hydrochloric acid and sodium hydroxide can be written as:
盐酸和氢氧化钠的电离可以写作：
HCl → H⁺ + Cl⁻
NaOH → Na⁺ + OH⁻
All acids contain hydrogen, but not all hydrogen-containing compounds are acids. For example, water and methane contain hydrogen but do not release H⁺ ions in solution. Similarly, all alkalis contain hydroxide ions, but most metal hydroxides are insoluble bases rather than alkalis.
所有酸都含有氢，但并非所有含氢化合物都是酸。例如，水和甲烷含有氢，但在溶液中不会释放 H⁺ 离子。同样，所有碱都含有氢氧根离子，但大多数金属氢氧化物是不溶性碱而不是可溶性碱。
2. The pH Scale and Indicators | pH 值与指示剂
The pH scale measures how acidic or alkaline a solution is, ranging from 0 to 14. A pH below 7 indicates an acidic solution, pH 7 is neutral, and a pH above 7 indicates an alkaline solution. The lower the pH, the higher the concentration of hydrogen ions; the higher the pH, the higher the concentration of hydroxide ions.
pH 值用于衡量溶液的酸性或碱性程度，范围为 0 到 14。pH 低于 7 表示溶液呈酸性，pH 等于 7 表示中性，pH 高于 7 表示溶液呈碱性。pH 越低，氢离子浓度越高；pH 越高，氢氧根离子浓度越高。
Acid-base indicators change colour according to the pH of the solution. Litmus is red in acidic solution and blue in alkaline solution. Universal indicator gives a range of colours from red for strongly acidic solutions through green for neutral to purple for strongly alkaline solutions. Phenolphthalein is colourless in acid and pink in alkali, making it useful for titrations.
酸碱指示剂会根据溶液的 pH 值改变颜色。石蕊在酸性溶液中呈红色，在碱性溶液中呈蓝色。通用指示剂从强酸性溶液的红色，到中性溶液的绿色，再到强碱性溶液的紫色，呈现一系列颜色变化。酚酞在酸性溶液中无色，在碱性溶液中呈粉红色，因此常用于滴定实验。
| pH range | Colour with universal indicator | Nature of solution |
|---|---|---|
| 0-2 | Red | Strongly acidic |
| 3-4 | Orange | Weakly acidic |
| 5-6 | Yellow | Very weakly acidic |
| 7 | Green | Neutral |
| 8-9 | Blue | Weakly alkaline |
| 10-12 | Indigo | Moderately alkaline |
| 13-14 | Purple | Strongly alkaline |
3. Neutralisation Reactions | 中和反应
Neutralisation is the reaction between an acid and a base to form a salt and water. In terms of ions, hydrogen ions from the acid react with hydroxide ions from the alkali to form water. The ionic equation for any neutralisation reaction between a strong acid and a strong alkali is:
中和反应是酸与碱反应生成盐和水的反应。从离子的角度看，酸中的氢离子与碱中的氢氧根离子结合生成水。强酸与强碱之间任何中和反应的离子方程式为：
H⁺ + OH⁻ → H₂O
The general word equation for neutralisation is:
中和反应的一般文字方程式为：
acid + base → salt + water
For example, when hydrochloric acid reacts with sodium hydroxide, sodium chloride and water are formed. The name of the salt comes from the metal in the base and the acid used. If sulfuric acid is used instead of hydrochloric acid, the salt formed is a sulfate; if nitric acid is used, the salt is a nitrate.
例如，盐酸与氢氧化钠反应生成氯化钠和水。盐的名称来自碱中的金属和所使用的酸。如果使用硫酸代替盐酸，生成的盐是硫酸盐；如果使用硝酸，生成的盐是硝酸盐。
Neutralisation has many practical applications. Farmers add lime (calcium oxide or calcium hydroxide) to acidic soil to raise its pH. Antacid tablets containing magnesium hydroxide or aluminium hydroxide neutralise excess stomach acid. Effluent from factories can be treated with acid or alkali before being released into rivers to avoid damaging aquatic life.
中和反应有许多实际应用。农民向酸性土壤中添加石灰（氧化钙或氢氧化钙）以提高 pH 值。含有氢氧化镁或氢氧化铝的抗酸药片可以中和过多的胃酸。工厂废水在排入河流前可以用酸或碱处理，以避免危害水生生物。
4. Common Acids and Bases in the Laboratory | 实验室中常见的酸和碱
Common laboratory acids include hydrochloric acid, HCl, sulfuric acid, H₂SO₄, and nitric acid, HNO₃. Hydrochloric acid is used to prepare chlorides, sulfuric acid is used to prepare sulfates, and nitric acid is used to prepare nitrates. These acids must be handled with care because they are corrosive, especially when concentrated.
实验室常见的酸包括盐酸 HCl、硫酸 H₂SO₄ 和硝酸 HNO₃。盐酸用于制备氯化物，硫酸用于制备硫酸盐，硝酸用于制备硝酸盐。这些酸具有腐蚀性，尤其是浓酸，因此必须小心操作。
Common bases include sodium hydroxide, NaOH, potassium hydroxide, KOH, calcium oxide, CaO, and ammonia solution, NH₃(aq). Sodium hydroxide and potassium hydroxide are strong alkalis because they ionise completely in water. Ammonia solution is a weak alkali because it only partially ionises to produce hydroxide ions.
常见的碱包括氢氧化钠 NaOH、氢氧化钾 KOH、氧化钙 CaO 和氨水 NH₃(aq)。氢氧化钠和氢氧化钾是强碱，因为它们在水中完全电离。氨水是弱碱，因为它只能部分电离产生氢氧根离子。
In the laboratory, acids are usually stored in glass bottles with ground-glass stoppers, while alkalis are often stored in plastic bottles because they can react slowly with glass. When diluting concentrated acid, the acid should always be added slowly to water, never water to acid, to avoid dangerous splashing caused by the large amount of heat released.
在实验室中，酸通常存放在带磨口玻璃塞的玻璃瓶中，而碱通常存放在塑料瓶中，因为碱会与玻璃缓慢反应。稀释浓酸时，应始终将酸缓慢加入水中，绝不能将水加入酸中，以避免释放大量热量引起危险飞溅。
5. Making Salts: Acid + Metal | 制备盐：酸 + 金属
Dilute acids react with reactive metals to produce a salt and hydrogen gas. This method is suitable for metals that are above hydrogen in the reactivity series, such as magnesium, zinc and iron. However, it is not suitable for very reactive metals like sodium and potassium because the reaction with acid is explosive, nor for unreactive metals like copper because no reaction occurs.
稀酸与活泼金属反应生成盐和氢气。此方法适用于金属活动性顺序中位于氢之前的金属，如镁、锌和铁。但不适用于钠和钾等非常活泼的金属，因为它们与酸反应会爆炸；也不适用于铜等不活泼金属，因为不会发生反应。
The general word equation for the reaction is:
该反应的一般文字方程式为：
acid + metal → salt + hydrogen
For example, zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas:
例如，锌与稀盐酸反应生成氯化锌和氢气：
Zn + 2HCl → ZnCl₂ + H₂
Hydrogen gas can be tested using a lighted splint, which produces a squeaky pop sound. In the laboratory, this method is often used when the salt is soluble and the excess metal can be removed by filtration after the reaction has stopped.
氢气可以用点燃的木条进行检验，会产生尖锐的爆鸣声。在实验室中，当盐可溶且反应停止后可以通过过滤除去过量金属时，通常使用这种方法制备盐。
6. Making Salts: Acid + Carbonate | 制备盐：酸 + 碳酸盐
Dilute acids react with metal carbonates and hydrogencarbonates to form a salt, water and carbon dioxide gas. This method works for all carbonates, including those of unreactive metals, because carbonates are bases. The general word equation is:
稀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。此方法适用于所有碳酸盐，包括不活泼金属的碳酸盐，因为碳酸盐是碱。一般文字方程式为：
acid + carbonate → salt + water + carbon dioxide
For example, calcium carbonate reacts with dilute hydrochloric acid to form calcium chloride, water and carbon dioxide:
例如，碳酸钙与稀盐酸反应生成氯化钙、水和二氧化碳：
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
Carbon dioxide is tested by bubbling the gas through limewater, which turns milky or cloudy due to the formation of insoluble calcium carbonate. This reaction is often used to prepare salts in the laboratory because the excess carbonate can be removed easily by filtration, and the carbon dioxide escapes as a gas.
二氧化碳的检验方法是将气体通入石灰水中，石灰水因生成不溶性碳酸钙而变浑浊或呈乳白色。该反应常用于实验室制备盐，因为过量的碳酸盐可以通过过滤轻松除去，二氧化碳作为气体逸出。
7. Making Salts: Acid + Base (Titration) | 制备盐：酸 + 碱（滴定法）
When preparing a salt from a soluble base such as an alkali, a titration is used because there is no visible excess of solid to remove. In a titration, a known volume of alkali is measured into a conical flask using a pipette, and a few drops of phenolphthalein indicator are added. Acid is then added slowly from a burette until the indicator just changes from pink to colourless, showing that neutralisation is complete.
当使用可溶性碱（如碱溶液）制备盐时，需要使用滴定法，因为看不到明显的过量固体需要去除。在滴定中，用移液管量取一定体积的碱液加入锥形瓶中，并加入几滴酚酞指示剂。然后从滴定管中缓慢加入酸，直到指示剂刚好由粉红色变为无色，表明中和反应已完成。
The word equation for the reaction between hydrochloric acid and sodium hydroxide is:
盐酸与氢氧化钠反应的文字方程式为：
hydrochloric acid + sodium hydroxide → sodium chloride + water
After the neutralisation point is found, the titration is repeated without the indicator using the same volumes of acid and alkali. The pure salt solution is then evaporated gently to obtain salt crystals. This method is especially useful for preparing sodium chloride, potassium nitrate and other salts of alkali metals.
找到中和点后，使用相同体积的酸和碱，在不加指示剂的情况下重复滴定。然后将纯盐溶液缓慢蒸发，得到盐晶体。这种方法特别适用于制备氯化钠、硝酸钾以及其他碱金属的盐。
8. Solubility of Salts and Precipitation | 盐的溶解性与沉淀
The solubility of salts in water is an important concept in salt preparation. Most sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Most chlorides, bromides and iodides 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.
盐在水中的溶解性是盐制备中的一个重要概念。大多数钠盐、钾盐和铵盐可溶。所有硝酸盐均可溶。大多数氯化物、溴化物和碘化物可溶，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。
Precipitation is a reaction in which two soluble salts react in aqueous solution to form an insoluble salt, called a precipitate. For example, when solutions of silver nitrate and sodium chloride are mixed, a white precipitate of silver chloride is formed:
沉淀反应是指两种可溶性盐在水溶液中反应生成不溶性盐（称为沉淀）的反应。例如，将硝酸银溶液与氯化钠溶液混合，会生成氯化银的白色沉淀：
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Precipitation is used to prepare insoluble salts such as silver chloride, barium sulfate and lead iodide. The precipitate is separated by filtration, washed with distilled water and dried. This method is quick and does not require heating or evaporation.
沉淀法用于制备氯化银、硫酸钡和碘化铅等不溶性盐。沉淀通过过滤分离，用蒸馏水洗涤并干燥。这种方法快速，不需要加热或蒸发。
9. Acid Rain and Environmental Impact | 酸雨及其环境影响
Acid rain is rainwater that has a pH lower than about 5.6 because of dissolved acidic gases. The main gases responsible are sulfur dioxide, SO₂, and nitrogen oxides, NOₓ, which come from burning fossil fuels in power stations, factories and vehicles. These gases dissolve in water droplets in the atmosphere and form sulfuric acid and nitric acid.
酸雨是指 pH 值低于约 5.6 的雨水，因为其中溶解了酸性气体。主要的致酸气体是二氧化硫 SO₂ 和氮氧化物 NOₓ，它们来自发电站、工厂和车辆燃烧化石燃料。这些气体溶解在大气中的水滴中，形成硫酸和硝酸。
The formation of acid rain can be represented by the following equations:
酸雨的形成可以用以下方程式表示：
SO₂ + H₂O → H₂SO₃
2NO₂ + H₂O → HNO₂ + HNO₃
Acid rain damages forests, acidifies lakes and rivers so that fish and other aquatic life cannot survive, and erodes limestone buildings and statues. The acidity in lakes can be neutralised by adding slaked lime, calcium hydroxide, but the most effective long-term solution is to reduce emissions of sulfur dioxide and nitrogen oxides by using cleaner fuels and installing scrubbers in chimneys.
酸雨会破坏森林，使湖泊和河流酸化，导致鱼类和其他水生生物无法生存，并侵蚀石灰岩建筑和雕像。可以通过加入熟石灰（氢氧化钙）来中和湖泊中的酸性，但最有效的长期解决方案是通过使用更清洁的燃料和在烟囱中安装脱硫装置来减少二氧化硫和氮氧化物的排放。
10. Summary of Key Reactions | 关键反应总结
The following table summarises the main reactions of acids that are assessed in Cambridge IGCSE Science. Remember to name the salt correctly by combining the metal or ammonium part of the base with the acid type: chloride from hydrochloric acid, sulfate from sulfuric acid, and nitrate from nitric acid.
下表总结了剑桥 IGCSE 科学中考查的酸的主要反应。请记住，用碱中的金属或铵根部分与酸的类型组合来正确命名盐：盐酸生成氯化物，硫酸生成硫酸盐，硝酸生成硝酸盐。
| Reactants | Products | Example |
|---|---|---|
| Acid + metal | Salt + hydrogen | Zn + 2HCl → ZnCl₂ + H₂ |
| Acid + carbonate | Salt + water + carbon dioxide | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ |
| Acid + base | Salt + water | HCl + NaOH → NaCl + H₂O |
| Acid + metal oxide | Salt + water | CuO + 2HCl → CuCl₂ + H₂O |
In salt preparation, choose the method based on whether the base is soluble or insoluble. For insoluble bases such as copper oxide or calcium carbonate, add excess solid to acid and filter. For soluble bases such as sodium hydroxide, use titration. For insoluble salts, use precipitation. Balanced chemical equations and accurate salt names are essential in written answers.
在制备盐时，应根据碱是可溶还是不可溶来选择方法。对于氧化铜或碳酸钙等不溶性碱，将过量固体加入酸中并过滤。对于氢氧化钠等可溶性碱，使用滴定法。对于不溶性盐，使用沉淀法。在书面答案中，配平化学方程式和准确的盐名称必不可少。
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