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 fundamental topics in IGCSE Science. This topic explores how substances are classified by their chemical behaviour, how pH is measured, and how salts are prepared through neutralization and precipitation reactions. Understanding these concepts is essential for achieving top marks in the Edexcel IGCSE Science examinations.

酸、碱和盐是 IGCSE 科学中最基础也是最重要的主题之一。本主题探讨如何根据化学性质对物质进行分类、如何测量 pH 值，以及如何通过中和反应和沉淀反应制备盐。理解这些概念对于在 Edexcel IGCSE 科学考试中取得高分至关重要。

1. What Are Acids and Bases? | 什么是酸和碱？

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and ethanoic acid (CH₃COOH). Acids have a pH less than 7, turn blue litmus paper red, and typically have a sour taste.

酸是溶于水时能产生氢离子（H⁺）的物质。常见的例子包括盐酸（HCl）、硫酸（H₂SO₄）和乙酸（CH₃COOH）。酸的 pH 值小于 7，能使蓝色石蕊试纸变红，通常有酸味。

A base is a substance that neutralizes an acid. A soluble base is called an alkali, which produces hydroxide ions (OH⁻) when dissolved in water. Common bases include metal oxides, metal hydroxides and metal carbonates, such as copper(II) oxide (CuO), sodium hydroxide (NaOH) and calcium carbonate (CaCO₃).

碱是能中和酸的物质。可溶性的碱称为碱液（alkali），溶于水时会产生氢氧根离子（OH⁻）。常见的碱包括金属氧化物、金属氢氧化物和金属碳酸盐，例如氧化铜（CuO）、氢氧化钠（NaOH）和碳酸钙（CaCO₃）。

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2. The pH Scale and Indicators | pH 刻度与指示剂

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. A pH of 7 is neutral. pH values below 7 indicate an acidic solution, while pH values above 7 indicate an alkaline solution. The pH scale is logarithmic, meaning a difference of one pH unit represents a tenfold change in hydrogen ion concentration.

pH 刻度范围从 0 到 14，用于衡量溶液的酸性或碱性程度。pH 值为 7 表示中性。pH 值小于 7 表示酸性溶液，大于 7 表示碱性溶液。pH 刻度是对数刻度，意味着 pH 每变化 1 个单位，氢离子浓度就变化 10 倍。

Universal indicator is a mixture of dyes that changes colour across a wide pH range. In acidic solutions it appears red or orange, in neutral solutions it is green, and in alkaline solutions it turns blue or purple. This allows the pH of an unknown solution to be estimated simply by comparing its colour to a standard chart.

万能指示剂是多种染料的混合物，能在很宽的 pH 范围内发生颜色变化。在酸性溶液中呈红色或橙色，在中性溶液中呈绿色，在碱性溶液中变成蓝色或紫色。这样只需将颜色与标准比色卡进行比较，就能估算出未知溶液的 pH 值。

Other common indicators include litmus and methyl orange. Litmus is red under acidic conditions and blue under alkaline conditions, but it cannot give an approximate pH value. Methyl orange, on the other hand, is red in acid and yellow in alkali, functioning effectively within a relatively narrow pH range of 3.1 to 4.4.

其他常见指示剂包括石蕊试纸和甲基橙。石蕊在酸性条件下呈红色，在碱性条件下呈蓝色，但它无法给出近似的 pH 值。甲基橙在酸中呈红色，在碱中呈黄色，有效变色范围较窄，大约在 pH 3.1 至 4.4 之间。

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3. Reactions of Acids | 酸的反应

Acids react with several types of substances in predictable ways. When an acid reacts with a metal, a salt and hydrogen gas are produced. For example:

酸能与多种类型的物质发生反应，其规律清晰且可预测。当酸与金属反应时，生成盐和氢气。例如：

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Not all metals react with acids. Only metals that are more reactive than hydrogen, such as magnesium, zinc and iron, will react. Copper and silver, being less reactive than hydrogen, do not react with dilute acids.

并非所有金属都能与酸反应。只有比氢更活泼的金属，如镁、锌和铁，才会发生反应。铜和银的活泼性低于氢，因此不会与稀酸反应。

Acids also react with metal carbonates to produce a salt, water and carbon dioxide gas. This reaction can be tested by bubbling the gas produced through limewater, which turns milky or cloudy in the presence of carbon dioxide.

酸还与金属碳酸盐反应生成盐、水和二氧化碳气体。可以通过将产生的气体通入石灰水来检验；石灰水遇到二氧化碳时会变浑浊（乳白色）。

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

The reaction between an acid and a metal oxide or metal hydroxide is called neutralization. It produces a salt and water only. For example, when hydrochloric acid is added to sodium hydroxide:

酸与金属氧化物或金属氢氧化物之间的反应称为中和反应，只生成盐和水。例如，当盐酸与氢氧化钠反应时：

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

In ionic terms, the neutralization reaction can be represented simply as:

从离子角度来说，中和反应可以简洁地表示为：

H⁺(aq) + OH⁻(aq) → H₂O(l)

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4. Naming Salts | 盐的命名

A salt is the compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion. The name of the salt has two parts: the first part comes from the metal in the base, and the second part comes from the acid used.

盐是酸中的氢离子被金属离子或铵根离子取代后形成的化合物。盐的名称由两部分组成：第一部分来自碱中的金属，第二部分来自所用的酸。

The table below shows the common acid–salt relationships:

下表展示了常见的酸–盐对应关系：

Acid 酸	Salt formed 生成的盐
Hydrochloric acid 盐酸	Chloride 氯化物
Sulfuric acid 硫酸	Sulfate 硫酸盐
Nitric acid 硝酸	Nitrate 硝酸盐
Ethanoic acid 乙酸	Ethanoate 乙酸盐

For example, zinc oxide reacting with sulfuric acid produces zinc sulfate and water. Sodium hydroxide reacting with nitric acid produces sodium nitrate and water.

例如，氧化锌与硫酸反应生成硫酸锌和水；氢氧化钠与硝酸反应生成硝酸钠和水。

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5. Preparing Soluble Salts by Titration | 通过滴定制备可溶性盐

There are two main laboratory methods for preparing salts. The first method is used to prepare soluble salts and involves the reaction between an acid and an insoluble base. This is known as the titration method when a soluble alkali is used.

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