Acids, Bases and Salts | 酸、碱和盐

📚 Acids, Bases and Salts | 酸、碱和盐

Acids, bases and salts form the backbone of many chemical reactions that you will meet in your IGCSE Edexcel Science course. Understanding their definitions, properties and reactions is essential for tackling exam questions with confidence.

酸、碱和盐构成了你在 IGCSE Edexcel 科学课程中遇到的许多化学反应的基础。理解它们的定义、性质和反应，对于自信地回答考试问题至关重要。

1. What is an Acid? | 什么是酸？

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid dissociates to form H⁺ and Cl⁻ ions in aqueous solution.

酸是一种溶于水时产生氢离子（H⁺）的物质。例如，盐酸在水溶液中解离形成 H⁺ 和 Cl⁻ 离子。

Acids have a sour taste and can be corrosive.
酸性物质有酸味，并且可能具有腐蚀性。
Common examples include citric acid in lemons and ethanoic acid in vinegar.
常见例子包括柠檬中的柠檬酸和醋中的乙酸。

Strong acids, such as sulfuric acid (H₂SO₄), ionise completely in water, while weak acids like ethanoic acid only ionise partially.

强酸如硫酸（H₂SO₄）在水中完全电离，而弱酸如乙酸仅部分电离。

2. Properties of Acids | 酸的性质

Acids show characteristic properties that you can test in the laboratory:

酸表现出一些可以在实验室中检验的特征性质：

They turn blue litmus paper red.
它们使蓝色石蕊试纸变红。
They react with metals to produce hydrogen gas and a salt.
它们与金属反应生成氢气和盐。
They react with carbonates to produce carbon dioxide, water and a salt.
它们与碳酸盐反应生成二氧化碳、水和盐。
They conduct electricity in aqueous solution due to the presence of ions.
它们在水溶液中因存在离子而能导电。

When reacting with metals, the general equation is:

与金属反应的通式为：

Acid + Metal → Salt + Hydrogen

酸 + 金属 → 盐 + 氢气

The hydrogen gas can be tested using a burning splint – it produces a squeaky pop.

氢气可以用燃烧的木条检验——会产生“噗”的爆鸣声。

3. Bases and Alkalis | 碱与可溶性碱

A base is a substance that neutralises an acid to form water and a salt. Bases are usually metal oxides or metal hydroxides.

碱是能中和酸生成水和盐的物质。碱通常是金属氧化物或金属氢氧化物。

An alkali is a base that dissolves in water, producing hydroxide ions (OH⁻) in solution. Examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH).

可溶性碱是能溶于水并在溶液中产生氢氧根离子（OH⁻）的碱。例如氢氧化钠（NaOH）和氢氧化钾（KOH）。

Alkalis turn red litmus paper blue.
可溶性碱使红色石蕊试纸变蓝。
They are soapy to touch and are also corrosive.
它们摸起来有滑腻感，同样具有腐蚀性。

Insoluble bases, like copper(II) oxide, do not dissolve in water but still neutralise acids.

不溶性碱如氧化铜不溶于水，但同样可以中和酸。

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

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.

pH 标度范围从 0 到 14，用于衡量溶液的酸碱性。

pH	0–6	7	8–14
Nature	Acidic	Neutral	Alkaline
性质	酸性	中性	碱性

Universal indicator gives a colour change across the whole pH range, while litmus only distinguishes acid from alkali.

万能指示剂在全部 pH 范围内显示颜色变化，而石蕊只能区分酸和碱。

Methyl orange changes from red in acid to yellow in alkali; thymolphthalein goes from colourless to blue.

甲基橙在酸性中呈红色，在碱性中呈黄色；百里酚酞由无色变为蓝色。

5. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base to produce a salt and water only.

中和反应是酸与碱反应仅生成盐和水的过程。

H⁺(aq) + OH⁻(aq) → H₂O(l)

氢离子（aq）+ 氢氧根离子（aq）→ 水（l）

The ionic equation shows the essential change: hydrogen ions combine with hydroxide ions to form water. The other ions are spectator ions.

离子方程式显示了本质变化：氢离子与氢氧根离子结合生成水，其他离子为旁观离子。

Neutralisation is used in treating acid indigestion (antacids) and in agriculture to correct acidic soils using lime (calcium oxide).

中和反应可用于治疗胃酸过多（抗酸药），以及农业中用石灰（氧化钙）改良酸性土壤。

6. Salts and Their Formation | 盐及其形成

A salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion (or ammonium ion).

盐是酸中的氢离子被金属离子（或铵根离子）取代后形成的离子化合物。

The name of a salt has two parts: the metal (or ammonium) comes first, followed by the acid root.

盐的名称包含两部分：金属（或铵根）在前，酸根在后。

Hydrochloric acid → chloride salts
盐酸 → 氯化物盐
Sulfuric acid → sulfate salts
硫酸 → 硫酸盐
Nitric acid → nitrate salts
硝酸 → 硝酸盐

For example, zinc reacts with sulfuric acid to form zinc sulfate and hydrogen.

例如，锌与硫酸反应生成硫酸锌和氢气。

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

锌（s）+ 硫酸（aq）→ 硫酸锌（aq）+ 氢气（g）

7. Acid + Metal Reactions | 酸与金属反应

Only metals above hydrogen in the reactivity series react with dilute acids.

只有位于金属活动性顺序中氢之前的金属才能与稀酸反应。

For example, magnesium reacts vigorously with hydrochloric acid:

例如，镁与盐酸反应剧烈：

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

镁（s）+ 2 盐酸（aq）→ 氯化镁（aq）+ 氢气（g）

Copper does not react with dilute acids because it is below hydrogen in the reactivity series.
铜不与稀酸反应，因为它在活动性顺序中位于氢之后。
The rate of effervescence indicates the reactivity of the metal.
气泡冒出的速率可以指示金属的活泼性。

8. Acid + Carbonate Reactions | 酸与碳酸盐反应

Acids react with carbonates and hydrogencarbonates to produce carbon dioxide, water and a salt.

酸与碳酸盐和碳酸氢盐反应生成二氧化碳、水和盐。

Acid + Carbonate → Salt + Water + Carbon dioxide

酸 + 碳酸盐 → 盐 + 水 + 二氧化碳

For example, calcium carbonate reacts with nitric acid:

例如，碳酸钙与硝酸反应：

CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)

碳酸钙（s）+ 2 硝酸（aq）→ 硝酸钙（aq）+ 水（l）+ 二氧化碳（g）

The carbon dioxide gas can be tested by bubbling it through limewater, which turns milky.

二氧化碳气体可以通过通入石灰水来检验，石灰水会变浑浊。

9. Preparing Soluble Salts | 可溶性盐的制备

There are two main methods to prepare a soluble salt: using an insoluble base or using a titration.

制备可溶性盐主要有两种方法：使用不溶性碱或使用滴定法。

Method 1: Insoluble base – Add excess insoluble base (e.g. copper(II) oxide) to warm dilute acid until no more reacts. Filter to remove the excess base, then evaporate the filtrate to obtain the salt.

方法一：不溶性碱 – 向温热的稀酸中加入过量的不溶性碱（如氧化铜），直到不再反应。过滤除去过量碱，然后蒸发滤液得到盐。

Method 2: Titration – Used with soluble bases (alkalis). Carefully add acid to alkali using a burette with an indicator. When neutralised, repeat without indicator to obtain a pure salt solution.

方法二：滴定 – 用于可溶性碱（碱液）。用滴定管小心地将酸加入碱中，并加指示剂。当恰好中和时，不加指示剂重复实验以获得纯净盐溶液。

Insoluble salts can be prepared by precipitation.
不溶性盐可以通过沉淀反应制备。
For example, silver nitrate and sodium chloride react to form silver chloride precipitate.
例如，硝酸银与氯化钠反应生成氯化银沉淀。

10. Titration Technique | 滴定技术

Titration is a precise method to determine the exact volume of acid needed to neutralise a known volume of alkali.

滴定是确定中和已知体积碱液所需酸液精确体积的精确方法。

Measure a known volume of alkali with a pipette and place it in a conical flask.
用移液管量取已知体积的碱液，放入锥形瓶中。
Add a few drops of indicator (e.g. phenolphthalein).
加入几滴指示剂（如酚酞）。
Fill a burette with acid of known concentration.
用已知浓度的酸液充满滴定管。
Add acid slowly until the indicator just changes colour.
缓慢加入酸液，直到指示剂刚好变色。
Record the volume of acid used.
记录所用酸的体积。

This is a core practical for Edexcel IGCSE Science, so remember the key steps and apparatus names.

这是 Edexcel IGCSE 科学的核心实验，请记住关键步骤和仪器名称。

11. Everyday Applications | 日常应用

Acids and bases are everywhere in daily life and industry.

酸和碱在日常生活中和工业中无处不在。

Vinegar (acetic acid) is used in food preservation.
醋（乙酸）用于食品保存。
Lime (calcium oxide) is used to reduce soil acidity.
石灰（氧化钙）用于降低土壤酸性。
Hydrochloric acid is used in cleaning metals.
盐酸用于清洁金属。
Magnesium hydroxide is used in antacids to relieve indigestion.
氢氧化镁用于抗酸药以缓解消化不良。

Modern farming relies on pH control to optimise crop growth, and waste water treatment uses neutralisation to adjust pH before release.

现代农业依赖 pH 控制来优化作物生长，废水处理在排放前使用中和反应调节 pH。

12. Common Exam Pitfalls | 常见考试陷阱

Students often lose marks on this topic for avoidable reasons.

学生常常因为可以避免的原因在这部分失分。

Writing “salt” instead of a specific salt formula – always name the salt correctly.
把“盐”写成通式而不是具体的盐——一定要正确命名盐。
Forgetting to include state symbols in balanced equations.
在配平方程式中忘记写状态符号。
Confusing strong acids with concentrated acids – they are not the same.
混淆强酸和浓酸——它们不是一回事。
Not remembering that insoluble bases do not change litmus colour.
忘记不溶性碱不会改变石蕊的颜色。

To avoid these, always practise writing balanced ionic equations and check the solubility of salts before choosing a preparation method.

为了避免这些错误，要经常练习书写配平的离子方程式，并在选择制备方法前检查盐的溶解性。

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Acids, Bases and Salts | 酸、碱和盐