Acids, Bases and Salts: Core Reactions and Salt Preparation | 酸、碱和盐：核心反应与盐的制备

📚 Acids, Bases and Salts: Core Reactions and Salt Preparation | 酸、碱和盐：核心反应与盐的制备

This revision guide covers the essential Cambridge IGCSE Science topic of acids, bases and salts. It focuses on definitions, pH, neutralisation, salt preparation methods, precipitation, ion tests and mole calculations. The content is matched to the Cambridge Coordinated Sciences and Chemistry specifications and is designed to help you revise effectively.

本复习指南涵盖剑桥 IGCSE 科学中酸、碱和盐的核心主题。内容集中于定义、pH、中和反应、盐的制备方法、沉淀反应、离子检验以及摩尔计算。知识点与剑桥协调科学和化学大纲相匹配，旨在帮助你高效复习。

1. Defining Acids and Bases | 定义酸和碱

An acid is a substance that produces hydrogen ions, H⁺, when dissolved in water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, which ionises completely.

酸是一种溶于水时产生氢离子 H⁺ 的物质。例如，氯化氢气体溶于水形成盐酸，并完全电离。

A base is a substance that can neutralise an acid to form a salt and water. An alkali is a soluble base that releases hydroxide ions, OH⁻, in aqueous solution.

碱是一种能中和酸生成盐和水的物质。可溶性碱在水溶液中释放氢氧根离子 OH⁻，称为碱溶液。

Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Common bases include sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂) and ammonia solution (NH₃).

常见实验室酸包括盐酸 HCl、硫酸 H₂SO₄ 和硝酸 HNO₃。常见碱包括氢氧化钠 NaOH、氢氧化钙 Ca(OH)₂ 和氨水 NH₃。

HCl → H⁺ + Cl⁻

In the Bronsted-Lowry definition, an acid is a proton donor and a base is a proton acceptor. This broader definition is useful when ammonia accepts a proton to form NH₄⁺.

在布朗斯特-劳里定义中，酸是质子给体，碱是质子受体。当氨接受一个质子形成 NH₄⁺ 时，这个更广泛的定义非常有用。

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

The pH scale runs from 0 to 14. Solutions with pH less than 7 are acidic, pH 7 is neutral, and pH greater than 7 is alkaline. The scale is logarithmic, so a change of one pH unit represents a tenfold change in hydrogen ion concentration.

pH 标度范围从 0 到 14。pH 小于 7 的溶液为酸性，pH 等于 7 为中性，pH 大于 7 为碱性。该标度是对数标度，因此 pH 每变化 1 个单位，氢离子浓度变化十倍。

Indicators are substances that change colour depending on the pH of the solution. Litmus is red in acid and blue in alkali. Phenolphthalein is colourless in acid and pink in alkali. Methyl orange is red in acid and yellow in alkali.

指示剂是随溶液 pH 变化而改变颜色的物质。石蕊在酸中呈红色，在碱中呈蓝色。酚酞在酸中无色，在碱中呈粉红色。甲基橙在酸中呈红色，在碱中呈黄色。

Indicator	Colour in acid	Colour in alkali
Litmus	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow

Universal indicator is a mixture of indicators that shows a range of colours from red for strong acid to violet for strong alkali. It can be used to estimate pH more precisely than a single indicator.

通用指示剂是多种指示剂的混合物，显示从强酸的红色到强碱的紫色的一系列颜色。与单一指示剂相比，它能更精确地估计 pH 值。

3. Strong and Weak Acids | 强酸与弱酸

A strong acid is one that fully ionises in aqueous solution. Hydrochloric acid, sulfuric acid and nitric acid are strong acids. In a 0.1 mol dm⁻³ solution of HCl, virtually all HCl molecules split into H⁺ and Cl⁻ ions.

强酸是在水溶液中完全电离的酸。盐酸、硫酸和硝酸都是强酸。在 0.1 mol dm⁻³ 的 HCl 溶液中，几乎所有的 HCl 分子都分解为 H⁺ 和 Cl⁻ 离子。

A weak acid is one that only partially ionises in aqueous solution. Ethanoic acid (CH₃COOH) and carbonic acid (H₂CO₃) are weak acids. The ionisation is reversible, so an equilibrium is established.

弱酸是在水溶液中仅部分电离的酸。乙酸 CH₃COOH 和碳酸 H₂CO₃ 都是弱酸。其电离是可逆的，因此会建立平衡。

CH₃COOH ⇌ CH₃COO⁻ + H⁺

At the same concentration, a strong acid has a lower pH and a higher electrical conductivity than a weak acid because it produces more free ions. Do not confuse the terms strong and concentrated: concentrated refers to the amount of acid dissolved in water, while strong refers to the degree of ionisation.

在相同浓度下，强酸比弱酸具有更低的 pH 和更高的导电性，因为它产生更多的自由离子。不要混淆强和浓这两个术语：浓是指酸溶解在水中的量，而强是指电离程度。

4. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base to form a salt and water. The essential ionic equation for the reaction between an acid and an alkali is:

中和是酸和碱反应生成盐和水的过程。酸和碱溶液反应的核心离子方程式为：

H⁺ + OH⁻ → H₂O

When an acid reacts with a metal, a salt and hydrogen gas are produced. For example, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen.

当酸与金属反应时，生成盐和氢气。例如，锌与盐酸反应生成氯化锌和氢气。

Zn + 2HCl → ZnCl₂ + H₂

When an acid reacts with a carbonate, a salt, water and carbon dioxide are produced. For example, calcium carbonate reacts with hydrochloric acid to form calcium chloride, water and carbon dioxide.

当酸与碳酸盐反应时，生成盐、水和二氧化碳。例如，碳酸钙与盐酸反应生成氯化钙、水和二氧化碳。

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

5. Making Salts from Insoluble Bases | 用不溶性碱制备盐

One common method of preparing a soluble salt is to react an acid with an excess insoluble base, such as a metal oxide or metal carbonate. The excess solid ensures that all the acid is used up.

制备可溶性盐的一种常用方法是让酸与过量的不溶性碱（如金属氧化物或金属碳酸盐）反应。过量的固体可确保所有的酸都被消耗完。

The steps are: add the insoluble base to warm acid until no more dissolves, filter to remove the excess solid, and then evaporate the filtrate to obtain salt crystals.

步骤为：将不溶性碱加入温热的酸中直到不再溶解，过滤除去多余的固体，然后蒸发滤液得到盐晶体。

For example, copper(II) oxide reacts with sulfuric acid to form copper(II) sulfate, a blue solution. After filtration and evaporation, blue copper(II) sulfate crystals are obtained.

例如，氧化铜与硫酸反应生成蓝色的硫酸铜溶液。经过过滤和蒸发后，得到蓝色的硫酸铜晶体。

CuO + H₂SO₄ → CuSO₄ + H₂O

This method works well for preparing salts from insoluble bases such as MgO, ZnO and Fe₂O₃. It is not suitable for alkalis because they dissolve completely and cannot be removed by filtration.

该方法适用于用 MgO、ZnO 和 Fe₂O₃ 等不溶性碱制备盐。它不适用于碱溶液，因为它们完全溶解，无法通过过滤除去。

6. Making Salts by Titration | 用滴定法制备盐

When preparing a salt from an acid and an alkali, both reactants are soluble, so the exact volume of each must be known. Titration is used to determine this volume accurately.

用酸和碱溶液制备盐时，两种反应物都是可溶的，因此必须知道各自的准确体积。滴定法用于准确测定该体积。

First, a titration is carried out using an indicator to find the volume of alkali that exactly neutralises a known volume of acid. The volumes are recorded and the process is repeated without the indicator to keep the salt pure.

首先，使用指示剂进行滴定，找到恰好中和已知体积酸所需的碱的体积。记录体积，然后在不加指示剂的情况下重复实验，以保持盐的纯度。

For example, sodium hydroxide solution is titrated with hydrochloric acid to form sodium chloride. The indicator is omitted in the final run, and the solution is evaporated to obtain white sodium chloride crystals.

例如，氢氧化钠溶液与盐酸滴定生成氯化钠。最终实验省略指示剂，蒸发溶液得到白色氯化钠晶体。

NaOH + HCl → NaCl + H₂O

Titration is more accurate than using excess solid because it gives the exact end point. It is essential when preparing salts of sodium, potassium or ammonium, as all their bases are soluble.

滴定比使用过量固体更精确，因为它能给出准确的终点。在制备钠盐、钾盐或铵盐时，滴定是必不可少的，因为这些碱都是可溶的。

7. Precipitation Reactions | 沉淀反应

A precipitation reaction occurs when two soluble salts are mixed and an insoluble salt, called a precipitate, forms. The precipitate can be filtered, washed and dried.

当两种可溶性盐混合并生成一种不溶性盐（称为沉淀）时，就会发生沉淀反应。沉淀可以过滤、洗涤和干燥。

For example, mixing solutions of barium chloride and sodium sulfate produces a white precipitate of barium sulfate.

例如，将氯化钡溶液与硫酸钠溶液混合会产生白色的硫酸钡沉淀。

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Precipitation is also used in qualitative analysis to identify ions. The colour and solubility of the precipitate often indicate which ion is present.

沉淀反应也用于定性分析中鉴定离子。沉淀的颜色和溶解性通常能指示存在哪种离子。

Test	Positive result
Add dilute nitric acid and silver nitrate solution	White precipitate of AgCl (chloride)
Add dilute hydrochloric acid and barium chloride solution	White precipitate of BaSO₄ (sulfate)
Add dilute acid to solid or solution	Effervescence, CO₂ turns limewater milky (carbonate)

These tests are important practical skills. You must be able to describe the reagent, the condition and the observation for each ion test.

这些检验是重要的实验技能。你必须能够描述每种离子检验所用的试剂、条件和观察结果。

8. Testing for Gases and Ions | 气体与离子的检验

Several gases produced in acid-base reactions can be identified by simple tests. Hydrogen burns with a squeaky pop when a lighted splint is held at the mouth of the test tube.

酸碱反应中产生的几种气体可以通过简单的实验来鉴别。在试管口放一根点燃的木条，氢气会发出轻微的爆鸣声。

Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Ammonia turns damp red litmus paper blue and has a pungent smell.

氧气能使带火星的木条复燃。二氧化碳能使石灰水变浑浊。氨气能使湿润的红色石蕊试纸变蓝，并有刺激性气味。

Flame tests are used to identify metal ions. Lithium gives a red flame, sodium gives a yellow flame, potassium gives a lilac flame, calcium gives a brick-red flame, and copper gives a blue-green flame.

焰色试验用于鉴定金属离子。锂产生红色火焰，钠产生黄色火焰，钾产生淡紫色火焰，钙产生砖红色火焰，铜产生蓝绿色火焰。

Cation tests with sodium hydroxide can also identify metal ions. For example, copper(II) ions form a blue precipitate, iron(II) forms a green precipitate, and iron(III) forms a brown precipitate.

用氢氧化钠进行的阳离子检验也能鉴定金属离子。例如，铜(II)离子形成蓝色沉淀，铁(II)形成绿色沉淀，铁(III)形成棕色沉淀。

9. Key Equations and Mole Calculations | 关键方程式与摩尔计算

Balanced chemical equations show the mole ratio between reactants and products. You can use these ratios to calculate the mass or volume of a substance produced or required.

配平的化学方程式表示了反应物和生成物之间的摩尔比。你可以利用这些比率计算生成或所需物质的质量或体积。

The key formula linking moles, concentration and volume is: n = c × V, where n is moles in mol, c is concentration in mol dm⁻³ and V is volume in dm³.

连接摩尔、浓度和体积的关键公式为：n = c × V，其中 n 为摩尔数（mol），c 为浓度（mol dm⁻³），V 为体积（dm³）。

n = c × V

To convert between mass and moles, use n = mass ÷ Mᵣ, where Mᵣ is the relative formula mass. At room temperature and pressure, one mole of any gas occupies 24 dm³.

质量与摩尔之间的换算使用 n = 质量 ÷ Mᵣ，其中 Mᵣ 是相对式量。在室温和常压下，1 摩尔任何气体的体积为 24 dm³。

For example, if 25.0 cm³ of 0.100 mol dm⁻³ NaOH neutralises 20.0 cm³ of HCl, the moles of NaOH are 0.00250 mol, so the moles of HCl are also 0.00250 mol, giving an HCl concentration of 0.125 mol dm⁻³.

例如，若 25.0 cm³ 的 0.100 mol dm⁻³ NaOH 中和 20.0 cm³ 的 HCl，则 NaOH 的摩尔数为 0.00250 mol，因此 HCl 的摩尔数也为 0.00250 mol，得出 HCl 浓度为 0.125 mol dm⁻³。

10. Exam Tips and Common Errors | 考试技巧与常见错误

Always balance chemical equations and include state symbols if the question asks for them. State symbols are (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution.

务必配平化学方程式，如果题目要求，还要标注状态符号。状态符号为：(s) 固体，(l) 液体，(g) 气体，(aq) 水溶液。

Do not confuse a strong acid with a concentrated acid. A strong acid refers to complete ionisation, while concentration refers to the amount of acid dissolved in a given volume of water.

不要把强酸和浓酸混为一谈。强酸指的是完全电离，而浓度指的是在一定体积水中溶解的酸的量。

A common error in salt preparation is using excess alkali instead of excess insoluble base. If the base is soluble, it cannot be removed by filtration, and the salt will be contaminated.

制备盐时的一个常见错误是使用过量的碱溶液而不是过量的不溶性碱。如果碱可溶，则无法通过过滤除去，盐会被污染。

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Acids, Bases and Salts: Core Reactions and Salt Preparation | 酸、碱和盐：核心反应与盐的制备