📚 Acids, Bases and Salts | 酸、碱和盐
In the Edexcel IGCSE Science course, understanding acids, bases and salts is essential for both Paper 1 and Paper 2. This revision guide covers the core definitions, reactions and salt-preparation methods with clear explanations.
在 Edexcel IGCSE 科学课程中，理解酸、碱和盐是Paper 1和Paper 2的关键内容。本复习指南涵盖核心定义、反应和盐的制备方法，并配以清晰的解释。
1. Properties of Acids | 酸的性质
Acids are substances that release hydrogen ions (H⁺) when dissolved in water. They taste sour and can be corrosive. Common examples include hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃).
酸是溶于水时释放氢离子（H⁺）的物质。它们有酸味，可能具有腐蚀性。常见例子包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。
In aqueous solution, acids conduct electricity because the H⁺ ions act as charge carriers.
酸的水溶液能够导电，因为H⁺离子作为电荷载体。
| Property | Acid |
| Taste | Sour |
| pH | Less than 7 |
| Litmus | Red |
2. Properties of Bases and Alkalis | 碱和碱液的性质
Bases are oxides or hydroxides of metals that neutralise acids to form salt and water. An alkali is a base that dissolves in water, releasing hydroxide ions (OH⁻). Examples of alkalis include sodium hydroxide (NaOH) and potassium hydroxide (KOH).
碱是金属的氧化物或氢氧化物，能与酸反应生成盐和水。可溶于水的碱称为碱液，它释放氢氧根离子（OH⁻）。碱液的例子有氢氧化钠（NaOH）和氢氧化钾（KOH）。
Alkalis feel soapy and have a pH greater than 7. They turn red litmus blue.
碱液有滑腻感，pH大于7，能使红色石蕊变蓝。
Base + Acid → Salt + Water
碱 + 酸 → 盐 + 水
3. Indicators | 指示剂
Indicators are substances that change colour in the presence of acids or alkalis. The most common is litmus, but methyl orange and phenolphthalein are also used.
指示剂是在酸或碱存在下改变颜色的物质。最常见的是石蕊，另外还有甲基橙和酚酞。
| Indicator | Acid | Alkali |
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
4. The pH Scale | pH值标度
The pH scale ranges from 0 to 14. A pH of 7 is neutral. Acids have pH values below 7, and alkalis have pH values above 7. pH can be measured using universal indicator or a pH meter.
pH值标度范围为0到14。pH=7为中性。酸pH小于7，碱pH大于7。pH可用万用指示剂或pH计测量。
pH = −log₁₀[H⁺]
On the Edexcel specification, you only need to understand the qualitative relationship between pH and hydrogen ion concentration: the lower the pH, the higher the [H⁺].
在Edexcel考纲中，你只需要理解pH和氢离子浓度的定性关系：pH越低，[H⁺]越高。
5. Neutralisation Reactions | 中和反应
Neutralisation is the reaction between an acid and a base to produce salt and water only. For an acid and alkali, the ionic equation is:
中和反应是酸和碱之间生成盐和水的反应。酸和碱液反应的离子方程式为：
H⁺(aq) + OH⁻(aq) → H₂O(l)
Neutralisation is exothermic, so the temperature rises. This reaction is used in agriculture to treat acidic soil with lime (calcium oxide).
中和反应放热，因此温度会升高。该反应在农业上用于用石灰（氧化钙）改良酸性土壤。
6. Acids and Metals | 酸与金属
Acids react with certain metals to produce a salt and hydrogen gas. For example, zinc reacts with hydrochloric acid:
酸与某些金属反应生成盐和氢气。例如，锌与盐酸反应：
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
Only metals above hydrogen in the reactivity series react with dilute acids. Copper and silver do not react. The hydrogen gas can be tested with a lit splint – it burns with a squeaky pop.
只有金属活动性顺序中位于氢之前的金属才能与稀酸反应。铜和银不反应。氢气可用点燃的木条检验——会发出尖锐爆鸣声。
7. Acids, Bases and Carbonates | 酸、碱与碳酸盐
Acids react with carbonates and hydrogencarbonates to form a salt, water and carbon dioxide gas. For example, calcium carbonate reacts with nitric acid:
酸与碳酸盐和碳酸氢盐反应生成盐、水和二氧化碳气体。例如，碳酸钙与硝酸反应：
CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)
Carbon dioxide is tested by bubbling it through limewater – it turns milky.
二氧化碳可通过通入石灰水来检验——石灰水变浑浊。
8. Preparing Soluble Salts | 可溶性盐的制备
There are two main methods to prepare soluble salts. The first is reacting an acid with an insoluble base or carbonate. The base is added in excess until no more dissolves, then excess solid is filtered off.
制备可溶性盐有两种主要方法。第一种是让酸与不溶性碱或碳酸盐反应。碱过量加入直至不再溶解，然后过滤掉多余的固体。
The second method is reacting an acid with an alkali using titration. A burette is used to add the acid to the alkali with an indicator. Once the colour change is seen, the volumes are recorded. The experiment is repeated without indicator to obtain a pure salt solution.
第二种方法是使用滴定让酸与碱液反应。用滴定管将酸加入到含有指示剂的碱液中。当看到颜色变化时，记录体积。然后不加指示剂重复实验，得到纯盐溶液。
Acid + Insoluble base → Salt + Water
酸 + 不溶性碱 → 盐 + 水
9. Preparing Insoluble Salts | 不溶性盐的制备
Insoluble salts are prepared by precipitation. Two soluble salts are mixed so that the insoluble salt forms as a solid. For example, preparing barium sulphate:
不溶性盐通过沉淀反应制备。将两种可溶性盐混合，使不溶性盐以固体形式生成。例如，制备硫酸钡：
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
The precipitate is collected by filtration, washed with distilled water, and dried in an oven.
沉淀通过过滤收集，用蒸馏水洗涤，并在烘箱中干燥。
10. Common Salts and Their Uses | 常见盐及其用途
Salts have many everyday uses. Sodium chloride is used in food flavouring and preserving. Ammonium nitrate is used as a fertiliser. Copper sulphate is used as a fungicide and in electroplating.
盐有许多日常用途。氯化钠用于食品调味和保存。硝酸铵用作肥料。硫酸铜用作杀菌剂和电镀。
| Salt | Formula | Use |
| Sodium chloride | NaCl | Food preservation |
| Ammonium nitrate | NH₄NO₃ | Fertiliser |
| Copper sulphate | CuSO₄ | Fungicide / electroplating |
11. Exam Tips | 考试技巧
When writing ionic equations, always include state symbols: (s), (l), (g), (aq). For neutralisation, remember that only H⁺ and OH⁻ combine to form water. For salt preparation questions, identify whether the salt is soluble or insoluble, then choose the correct method.
在书写离子方程式时，务必包含状态符号：(s)、(l)、(g)、(aq)。对于中和反应，记住只有H⁺和OH⁻结合生成水。对于盐的制备题，先判断盐是可溶还是不溶，然后选择正确的方法。
Practise balancing equations and writing word equations. Common mistakes include forgetting to filter excess solid, or not recording the burette reading correctly.
练习配平方程式和书写文字方程式。常见错误包括忘记过滤多余的固体，或没有正确记录滴定管读数。
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