IGCSE Edexcel Science: Acids, Bases and Salts | IGCSE Edexcel 科学：酸、碱和盐

📚 IGCSE Edexcel Science: Acids, Bases and Salts | IGCSE Edexcel 科学：酸、碱和盐

Acids, bases and salts form one of the most important foundation topics in the Edexcel IGCSE Science syllabus. This revision guide explains the key definitions, the pH scale, neutralisation, the reactions of acids, and the preparation of salts, with exam-style tips throughout to help you secure top marks.

酸、碱和盐是 Edexcel IGCSE 科学教学大纲中最重要基础主题之一。本复习指南详细讲解了关键定义、pH 标度、中和反应、酸的反应以及盐的制备，并贯穿考试技巧提示，助你斩获高分。

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1. What Are Acids? | 什么是酸？

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. In the model used by Edexcel, an acid can also be described as a proton donor because a hydrogen atom without its electron is simply a proton.

酸是溶于水时能产生氢离子（H⁺）的物质。在 Edexcel 采用的模型里，酸也可被描述为质子供体，因为失去电子的氢原子本质上就是一个质子。

For example, when hydrogen chloride gas dissolves in water, it dissociates completely:

例如，氯化氢气体溶于水时完全解离：

HCl(g) → H⁺(aq) + Cl⁻(aq)

The characteristic properties of acids include a sour taste, the ability to turn blue litmus paper red, and a pH value below 7. Acids also react with reactive metals, carbonates and bases to produce characteristic products that you must be able to recall.

酸的典型性质包括酸味、能使蓝色石蕊试纸变红，以及 pH 值低于 7。酸还能与活泼金属、碳酸盐和碱反应生成特定的产物，这些反应你是必须牢记的。

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2. What Are Bases and Alkalis? | 什么是碱和可溶性碱？

A base is any substance that neutralises an acid to form a salt and water. Common examples include insoluble metal oxides such as copper(II) oxide (CuO) and insoluble metal hydroxides such as iron(III) hydroxide (Fe(OH)₃).

碱是能与酸中和生成盐和水的任何物质。常见例子包括不溶性金属氧化物（如氧化铜 CuO）和不溶性金属氢氧化物（如氢氧化铁 Fe(OH)₃）。

An alkali is a base that dissolves in water. Soluble bases produce hydroxide ions (OH⁻) in aqueous solution. For example, sodium hydroxide dissolves to give:

可溶性碱（alkali）是指能溶于水的碱。可溶性碱在水溶液中会产生氢氧根离子（OH⁻）。例如，氢氧化钠溶解时：

NaOH(s) → Na⁺(aq) + OH⁻(aq)

Alkalis have a bitter taste, a slippery feel, turn red litmus paper blue, and have a pH greater than 7. Examples of alkalis include NaOH, KOH, Ca(OH)₂ and aqueous ammonia (NH₃).

可溶性碱有苦味、滑腻感，能使红色石蕊试纸变蓝，pH 大于 7。常见的可溶性碱包括 NaOH、KOH、Ca(OH)₂ 和氨水（NH₃）。

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3. The pH Scale and Indicators | pH 标度与指示剂

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. A pH of 7 is neutral; values below 7 are acidic, with lower numbers indicating stronger acidity; values above 7 are alkaline, with higher numbers indicating stronger alkalinity.

pH 标度范围是 0 到 14，用于衡量溶液的酸碱性。pH 为 7 是中性；低于 7 为酸性，数字越小酸性越强；高于 7 为碱性，数字越大碱性越强。

Universal indicator is a mixture of dyes that shows a continuous range of colours across the pH scale, allowing an approximate pH value to be determined.

万能指示剂是多种染料的混合物，能在 pH 标度上显示连续的颜色变化，从而估算出近似的 pH 值。

pH range	Universal indicator colour	Classification
1–2	Red	Strongly acidic
3–6	Orange / yellow	Weakly acidic
7	Green	Neutral
8–11	Blue	Weakly alkaline
12–14	Purple	Strongly alkaline

Litmus paper is simpler and only distinguishes acids from alkalis: blue litmus turns red in acid, while red litmus turns blue in alkali. Methyl orange and phenolphthalein are used in titrations because they give sharp colour changes at the endpoint.

石蕊试纸更简单，只能区分酸和碱：蓝色石蕊遇酸变红，红色石蕊遇碱变蓝。甲基橙和酚酞用于滴定实验，因为它们在终点时产生明显的颜色变化。

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4. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base to produce a salt and water. This is a core concept that appears in many exam questions.

中和反应是酸与碱之间生成盐和水的反应。这是核心概念，在考试中频繁出现。

acid + base → salt + water

The simplified ionic equation for neutralisation shows exactly what happens at the particle level:

中和反应的简化离子方程式揭示了粒子层面的实际变化：

H⁺(aq) + OH⁻(aq) → H₂O(l)

The hydrogen ions from the acid combine with hydroxide ions from the alkali to form water. The remaining ions stay in solution; when the water evaporates, the salt crystallises out.

酸中的氢离子与可溶性碱中的氢氧根离子结合生成水。其余离子留在溶液中；当水蒸发后，盐便结晶析出。

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5. Reactions of Acids with Metals | 酸与金属的反应

When a reactive metal is added to an acid, a salt and hydrogen gas are produced. This is a redox reaction in which the metal loses electrons and hydrogen ions gain electrons.

当活泼金属加入酸中时，会生成盐和氢气。这是一个氧化还原反应：金属失去电子，氢离子获得电子。

acid + metal → salt + hydrogen

For example, zinc reacts with dilute hydrochloric acid:

例如，锌与稀盐酸反应：

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

The hydrogen gas can be tested with a lighted splint; it burns with a squeaky pop. Only

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