Acids, Bases, and Salts | 酸、碱和盐

📚 Acids, Bases, and Salts | 酸、碱和盐

Acids, bases, and salts form one of the most important topics in Cambridge IGCSE Chemistry. Understanding how these substances react helps you explain everything from indigestion tablets to the formation of fertilisers. This article follows the key concepts on pages 131–146 of the Cambridge IGCSE Chemistry coursebook, with clear definitions, reaction patterns, and practical preparation methods.

酸、碱和盐是剑桥IGCSE化学中最重要的话题之一。理解这些物质的反应方式，可以帮助你解释从消化不良药片到肥料形成等各种现象。本文内容对应于剑桥IGCSE化学教材第131–146页，涵盖关键定义、反应规律和实验制备方法，并配有中文对照，便于复习。

1. What Are Acids? | 什么是酸？

In aqueous solution, an acid is a substance that produces hydrogen ions, H⁺(aq). In the Brønsted–Lowry definition, an acid is a proton donor. For example, when hydrogen chloride dissolves in water, it ionises to release H⁺ ions.

在溶液中，酸是指能产生氢离子 H⁺(aq) 的物质。根据布朗斯特-劳里定义，酸是质子的给予体。例如，氯化氢溶于水时会离子化，释放出H⁺离子。

HCl(aq) → H⁺(aq) + Cl⁻(aq)

Acids have several characteristic properties: they taste sour, turn litmus red, have pH below 7, and react with reactive metals, carbonates, and bases. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).

酸具有许多典型性质：味道酸，使石蕊变红，pH小于7，并能与活泼金属、碳酸盐和碱反应。常见酸包括盐酸(HCl)、硫酸(H₂SO₄)和硝酸(HNO₃)。

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2. What Are Bases and Alkalis? | 什么是碱和碱液？

A base is a substance that neutralises an acid to give salt and water only. Bases are usually metal oxides or metal hydroxides. In the Brønsted–Lowry model, a base is a proton acceptor. A soluble base is called an alkali and produces hydroxide ions, OH⁻(aq), in water.

碱是指能中和酸并只生成盐和水的物质。碱通常是金属氧化物或金属氢氧化物。在布朗斯特-劳里模型中，碱是质子的接受体。可溶于水的碱称为碱液(alkali)，它在水中产生氢氧根离子 OH⁻(aq)。

NaOH(s) → Na⁺(aq) + OH⁻(aq)

Examples of bases: copper(II) oxide, CuO; zinc oxide, ZnO; sodium hydroxide, NaOH; and ammonia solution, NH₃(aq). Ammonia is a weak alkali because it only partially ionises in water.

碱的例子有：氧化铜 CuO；氧化锌 ZnO；氢氧化钠 NaOH；氨水 NH₃(aq)。氨是弱碱，因为它在水中仅部分电离。

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3. The pH Scale and Indicators | pH标度和指示剂

The pH scale measures how acidic or alkaline a solution is. It ranges from 0 to 14. A pH of 7 is neutral; pH less than 7 is acidic; pH greater than 7 is alkaline.

pH标度用于测量溶液的酸性或碱性，范围从0到14。pH=7为中性；pH<7为酸性；pH>7为碱性。

• pH 0–6: acidic; lower pH means more acidic | pH 0–6：酸性；pH越小酸性越强
• pH 7: neutral | pH 7：中性
• pH 8–14: alkaline; higher pH means more alkaline | pH 8–14：碱性；pH越大碱性越强

Indicators are dyes that change colour depending on pH. The most common indicators are litmus, methyl orange, and phenolphthalein. Universal indicator is a mixture that gives a continuous range of colours across the pH scale.

指示剂是根据pH改变颜色的染料。最常用的指示剂是石蕊、甲基橙和酚酞。广谱指示剂是一种混合物，能在整个pH范围内显示连续的色阶。

Indicator	Colour in acid	Colour in alkali
Litmus | 石蕊	Red | 红	Blue | 蓝
Methyl orange | 甲基橙	Red | 红	Yellow | 黄
Phenolphthalein | 酚酞	Colourless | 无色	Pink | 粉红

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4. Strong and Weak Acids | 强酸和弱酸

The strength of an acid depends on how completely it ionises in water. A strong acid fully ionises into H⁺ ions, whereas a weak acid only partially ionises. This is different from concentration, which simply describes the amount of acid dissolved in a given volume of solution.

酸的强度取决于它在水中电离的完全程度。强酸完全电离产生H⁺离子，而弱酸仅部分电离。这不同于浓度，浓度只描述一定体积溶液中溶解的酸量。

• Strong acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃) | 强酸：盐酸(HCl)、硫酸(H₂SO₄)、硝酸(HNO₃)
• Weak acids: ethanoic acid (CH₃COOH), citric acid, carbonic acid (H₂CO₃) | 弱酸：乙酸(CH₃COOH)、柠檬酸、碳酸(H₂CO₃)

For weak acids, the ionisation is reversible, so an equilibrium is established, for example:

对弱酸而言，电离是可逆的，因此会建立平衡，例如：

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

A 1 mol/dm³ weak acid has a higher pH than a 1 mol/dm³ strong acid because fewer H⁺ ions are produced. The same distinction applies to strong and weak alkalis, such as NaOH (strong) and NH₃ (weak).

1 mol/dm³的弱酸比1 mol/dm³的强酸具有更高的pH，因为产生的H⁺离子更少。相同的区分也适用于强碱与弱碱，如NaOH（强碱）和NH₃（弱碱）。

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5. Neutralization Reactions | 中和反应

Neutralization is the reaction between an acid and a base to produce salt and water only. The general equation is:

中和反应是酸与碱之间生成盐和水只的反应。通式为：

Acid + Base → Salt + Water

酸 + 碱 → 盐 + 水

The ionic equation for neutralization is simply the reaction between hydrogen ions and hydroxide ions:

中和反应的离子方程式就是氢离子与氢氧根离子之间的反应：

H⁺(aq) + OH⁻(aq) → H₂O(l)

Neutralization is used in many everyday applications, including treating acid indigestion with antacids (e.g., Mg(OH)₂) and treating stings (e.g., vinegar for bee stings, or baking soda for wasp stings).

中和反应在日常生活中有许多应用，如用抗酸药（如Mg(OH)₂）治疗胃酸过多，以及处理叮咬（如用醋处理蜂蜇、用小苏打处理黄蜂蜇）。

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6. Acids and Metals | 酸与金属

Reactive metals react with dilute acids to produce a salt and hydrogen gas. The general word equation is:

活泼金属能与稀酸反应生成盐和氢气。通式如下：

Acid + Metal → Salt + Hydrogen

酸 + 金属 → 盐 + 氢气

The hydrogen gas is tested by collecting it in a test tube and holding a lighted splint to the mouth of the tube. A positive test gives a ‘squeaky pop’ sound. For example:

可通过将收集氢气的试管口靠近火焰来检验氢气；若发出“噗”的爆鸣声，则证明是氢气。例如：

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂↑

However, copper and other metals below hydrogen in the reactivity series do not react with dilute acids because they cannot displace hydrogen from the acid solution.

然而，铜以及金属活动性顺序中氢之后的金属不能与稀酸反应，因为它们不能从酸溶液中置换出氢。

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7. Acids and Carbonates | 酸与碳酸盐

Acids react with carbonates and hydrogencarbonates to produce a salt, water, and carbon dioxide gas. This reaction is often used to test for carbonate ions.

酸与碳酸盐和碳酸氢盐反应生成盐、水和二氧化碳气体。该反应常用于检验碳酸根离子。

Acid + Carbonate → Salt + Water + Carbon Dioxide

酸 + 碳酸盐 → 盐 + 水 + 二氧化碳

For example:

例如：

2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g)

The carbon dioxide gas can be tested by bubbling it through limewater (calcium hydroxide solution). If CO₂ is present, the limewater turns milky/cloudy.

可以将产生的气体通入石灰水（氢氧化钙溶液）中检验二氧化碳。若CO₂存在，石灰水会变浑浊。

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8. Salts | 盐类

Salts are ionic compounds formed when the hydrogen ions of an acid are replaced by metal ions (or ammonium ions). The naming of salts depends on the acid used:

盐是由酸中的氢离子被金属离子（或铵根离子）取代而形成的离子化合物。盐的命名取决于所用的酸：

• Hydrochloric acid → chloride (e.g., NaCl) | 盐酸 → 氯化物（如NaCl）
• Sulfuric acid → sulfate (e.g., CuSO₄) | 硫酸 → 硫酸盐（如CuSO₄）
• Nitric acid → nitrate (e.g., KNO₃) | 硝酸 → 硝酸盐（如KNO₃）

If the metal can form more than one ion, the name may include a Roman numeral, for example iron(II) sulfate (FeSO₄) and iron(III) sulfate [Fe₂(SO₄)₃]. Ammonium salts are formed when ammonia, a weak base, reacts with acids: NH₃ + HCl → NH₄Cl.

如果金属能形成多种离子，名称中会包含罗马数字，例如硫酸亚铁(FeSO₄)和硫酸铁[Fe₂(SO₄)₃]。当弱碱氨与酸反应时则生成铵盐：NH₃ + HCl → NH₄Cl。

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9. Preparing Soluble Salts | 制备可溶性盐

Soluble salts can be prepared by reacting an acid with an appropriate insoluble base or carbonate. The method uses an excess of the solid to ensure all the acid is used up, then filters away the excess solid, and finally crystallises the salt from the filtrate.

可溶性盐可通过使酸与适当的难溶性碱或碳酸盐反应来制备。该方法使用过量的固体以确保酸完全反应，然后滤除剩余的固体，最后从滤液中结晶出盐。

Example: preparing copper(II) sulfate from copper(II) oxide and sulfuric acid.

例：用氧化铜和硫酸制备硫酸铜。

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

• Add excess black CuO to warm sulfuric acid, stirring until no more reacts | 向加热的稀硫酸中加入过量黑色CuO，搅拌至不再反应
• Filter to remove unreacted CuO | 过滤以除去未反应的CuO
• Evaporate some water to concentrate the blue solution, then leave to crystallise | 蒸发部分水使蓝色溶液浓缩，然后冷却结晶

For salts derived from soluble bases, such as sodium chloride from sodium hydroxide and hydrochloric acid, a titration method is used with an indicator to find the exact acid–alkali ratio. After the end point is known, the indicator is left out and the mixture is evaporated to obtain the pure salt.

对于由可溶性碱生成的盐，如氢氧化钠与盐酸生成氯化钠，常用滴定法借助指示剂确定酸碱的精确比例。知道终点后，再次混合时不加指示剂，蒸发溶液即可得到纯净盐。

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10. Preparing Insoluble Salts | 制备不溶性盐

Insoluble salts are prepared by precipitation. Two suitable aqueous solutions are mixed so that a solid precipitate forms.

不溶性盐通过沉淀反应制备。将两种合适的溶液混合，生成固体沉淀。

Example: silver chloride from silver nitrate and sodium chloride.

例：用硝酸银和氯化钠制备氯化银。

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

The precipitate is collected by filtration, washed with distilled water, and then dried. The ionic equation for the precipitation is:

沉淀通过过滤收集，用蒸馏水洗涤并干燥。该沉淀反应的离子方程式为：

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Precipitation reactions can also be used to test for halide ions, sulfate ions, and carbonate ions. For example, adding barium nitrate to a solution containing sulfate ions produces a white precipitate of barium sulfate, BaSO₄.

沉淀反应还可用于检验卤离子、硫酸根离子和碳酸根离子。例如，向含硫酸根离子的溶液中加入硝酸钡，会生成白色沉淀——硫酸钡(BaSO₄)。

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11. Types of Oxides | 氧化物的类型

Oxides of elements show periodic trends. They can be classified into acidic, basic, amphoteric, or neutral oxides based on their reactions with acids and bases.

元素的氧化物显示出周期律。根据与酸和碱的反应，氧化物可分为酸性氧化物、碱性氧化物、两性氧化物或中性氧化物。

Type | 类型	Examples | 例子	Reaction with acid	Reaction with alkali
Acidic | 酸性	CO₂, SO₂, SO₃, P₂O₅, NO₂	No reaction | 不反应	React to form salt and water | 反应生成盐和水
Basic | 碱性	Na₂O, CaO, CuO, ZnO	React to form salt and water | 反应生成盐和水	No reaction (except amphoteric oxides) | 不反应（两性氧化物除外）
Amphoteric | 两性	Al₂O₃, ZnO	React to form salt and water | 反应生成盐和水	React to form salt and water | 反应生成盐和水
Neutral | 中性	CO, NO, H₂O	No reaction | 不反应	No reaction | 不反应

Acidic oxides are usually non-metal oxides, while basic oxides are metal oxides. Amphoteric oxides such as Al₂O₃ behave as both acidic and basic. For example:

酸性氧化物通常是非金属氧化物，碱性氧化物是金属氧化物。两性氧化物如Al₂O₃既能表现酸性又能表现碱性。例如：

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄ (or 2NaAlO₂ + 3H₂O)

Neutral oxides such as CO and NO do not react with acids or bases solutions under normal conditions.

中性氧化物如CO和NO在通常条件下不与酸或碱溶液反应。

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