Acids, Bases and Salts | 酸、碱与盐

📚 Acids, Bases and Salts | 酸、碱与盐

Acids, bases and salts form a fundamental chapter in the Edexcel IGCSE Science (Double Award) syllabus. This topic links ideas about chemical reactions, the pH scale, ionic theory and practical salt preparation. A deep understanding of these ideas will allow you to interpret laboratory tests, predict products and explain everyday processes such as indigestion treatment or soil management.

酸、碱与盐是爱德思 IGCSE 科学（双重证书）教学大纲中的基础章节。这个主题将化学反应、pH 标度、离子理论和盐的制备联系在一起。深入理解这些概念，可以帮助你解释实验检验、预测生成物，并说明诸如消化不良治疗或土壤处理等日常生活过程。

1. What are Acids and Bases? | 什么是酸与碱？

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) dissociates to produce H⁺ and Cl⁻ ions. Acids usually have a sour taste and turn blue litmus red. Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).

酸是指溶于水时能产生氢离子（H⁺）的物质。例如，盐酸（HCl）电离产生 H⁺ 和 Cl⁻ 离子。酸通常有酸味，能使蓝色石蕊变红。常见的实验室酸包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。

A base is a substance that neutralises an acid to form water and a salt. A soluble base is called an alkali. Alkalis produce hydroxide ions (OH⁻) when dissolved in water. Common bases include metal oxides and metal hydroxides such as sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)₂).

碱是一种能中和酸生成水和盐的物质。可溶性碱称为碱溶液（alkali）。碱溶于水时产生氢氧根离子（OH⁻）。常见的碱包括金属氧化物和金属氢氧化物，如氢氧化钠（NaOH）和氢氧化钙（Ca(OH)₂）。

Acid + Base → Salt + Water

酸 + 碱 → 盐 + 水

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

The pH scale runs from 0 to 14. Values below 7 indicate acidic solutions; pH 7 is neutral; values above 7 indicate alkaline solutions. The pH measures the concentration of hydrogen ions: a lower pH means a higher concentration of H⁺ ions.

pH 标度范围从 0 到 14。低于 7 的数值表示酸性溶液；pH 7 为中性；高于 7 的数值表示碱性溶液。pH 反映了氢离子浓度：pH 越低，H⁺ 离子浓度越高。

Indicators are dyes that change colour depending on pH. Universal indicator gives a rainbow of colours, while litmus only distinguishes acids from alkalis. Methyl orange and phenolphthalein are useful in titrations.

指示剂是随 pH 变化而变色的染料。石蕊只能区分酸性与碱性，而广谱指示剂能显示一系列颜色变化。甲基橙和酚酞常用于滴定实验。

Indicator	Acid	Alkali
Litmus	Red	Blue
Methyl orange	Red	Yellow
Phenolphthalein	Colourless	Pink

3. Reactions of Acids | 酸的反应

Acids react with metals, bases and carbonates to produce characteristic products. You must learn each general word equation and be able to write balanced symbol equations.

酸能与金属、碱和碳酸盐反应，生成特征产物。你需要掌握每一个通式，并能够写出配平后的符号方程式。

Acid + Metal → Salt + Hydrogen

酸 + 金属 → 盐 + 氢气

For example, dilute hydrochloric acid reacts with magnesium:

例如，稀盐酸与镁反应：

Mg + 2HCl → MgCl₂ + H₂

Hydrogen gas is produced and can be tested with a lighted splint – a squeaky pop sound indicates hydrogen. Not all metals react at the same speed; reactivity affects how quickly bubbles are produced.

产生的氢气可用点燃的木条检验——听到“噗”的爆鸣声即可证明是氢气。并非所有金属反应速度相同；金属活动性影响气泡产生的快慢。

Acid + Metal Oxide or Hydroxide → Salt + Water

酸 + 金属氧化物或氢氧化物 → 盐 + 水

Example: copper(II) oxide with dilute sulfuric acid gives copper(II) sulfate and water:

例如：氧化铜与稀硫酸反应生成硫酸铜和水：

CuO + H₂SO₄ → CuSO₄ + H₂O

Acid + Carbonate → Salt + Water + Carbon Dioxide

酸 + 碳酸盐 → 盐 + 水 + 二氧化碳

For instance, sodium carbonate reacts with nitric acid:

例如，碳酸钠与硝酸反应：

Na₂CO₃ + 2HNO₃ → 2NaNO₃ + H₂O + CO₂

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

产生的二氧化碳气体可通过通入石灰水来检验，石灰水会变浑浊。

4. Strong and Weak Acids | 强酸与弱酸

In aqueous solution, a strong acid is fully ionised, meaning nearly every acid molecule splits into H⁺ ions and an anion. Hydrochloric acid, sulfuric acid and nitric acid are strong acids.

在水溶液中，强酸完全电离，几乎每个酸分子都分解为 H⁺ 离子和酸根离子。盐酸、硫酸和硝酸都是强酸。

A weak acid is only partially ionised. Only a small fraction of molecules release H⁺ ions. Ethanoic acid (found in vinegar) and citric acid are weak acids. At the same concentration, a weak acid has a higher pH than a strong acid because it produces fewer hydrogen ions.

弱酸仅部分电离，只有少量分子释放出 H⁺ 离子。乙酸（食醋中的酸）和柠檬酸是弱酸。在相同浓度下，弱酸的 pH 高于强酸，因为它产生的氢离子较少。

Acid	Ionisation in water	Example
Strong	Complete (≈100%)	HCl, H₂SO₄, HNO₃
Weak	Partial (small %)	CH₃COOH, citric acid

Similarly, a strong alkali fully ionises to release OH⁻ ions, whereas a weak alkali only partially ionises. Ammonia solution is a common weak alkali in the laboratory.

类似地，强碱完全电离释放 OH⁻ 离子，而弱碱仅部分电离。实验室常见弱碱是氨水。

5. Salts: Names and Formation | 盐的命名与生成

A salt is formed when the H⁺ ion of an acid is replaced by a metal ion or ammonium ion. The name contains two parts: the first comes from the metal (or ammonium), and the second comes from the acid.

盐是酸中的 H⁺ 离子被金属离子或铵离子取代后形成的化合物。盐的名称包含两部分：第一部分来自金属（或铵），第二部分来自酸根。

Common acid-to-salt relationships:

常见酸与对应盐的关系：

Hydrochloric acid → chloride salts (e.g. KCl)
盐酸 → 氯化物盐（如 KCl）
Sulfuric acid → sulfate salts (e.g. CuSO₄)
硫酸 → 硫酸盐（如 CuSO₄）
Nitric acid → nitrate salts (e.g. AgNO₃)
硝酸 → 硝酸盐（如 AgNO₃）

Ethanoic acid forms ethanoate salts (e.g. sodium ethanoate). The -ic ending of the acid changes to -ate for the salt. For acids ending in -ous (e.g. nitrous acid), the salt ends with -ite.

乙酸形成乙酸盐（如乙酸钠）。酸的结尾 -ic 变成 -ate 对应盐；以 -ous 结尾的酸（如亚硝酸）对应盐以 -ite 结尾。

6. Solubility Rules for Salts | 盐的溶解性规律

To choose a correct method for preparing a salt, you need to know whether the salt is soluble or insoluble in water. The table below summarises the key rules.

要选择正确的制盐方法，你需要知道该盐是否溶于水。下表总结了关键规则。

Soluble salts	Insoluble salts
All sodium, potassium and ammonium salts	Most carbonates (except Na, K, NH₄)
All nitrates	Most hydroxides (except Na, K, NH₄; Ca sparingly)
Most chlorides (except AgCl, PbCl₂)	Silver chloride, lead(II) chloride
Most sulfates (except BaSO₄, PbSO₄)	Barium sulfate, lead(II) sulfate

These rules are essential for planning preparation of salts and for predicting precipitation reactions.

这些规律对于设计盐的制备和预测沉淀反应至关重要。

7. Preparation of Salts | 盐的制备

There are three main laboratory methods depending on the solubility of the salt.

根据盐的溶解性，实验室制备盐主要有三种方法。

Method 1: Acid + insoluble base (neutralisation)

方法一：不溶性碱 + 酸（中和法）

To make a soluble salt from an insoluble base, you add the base to warm dilute acid until no more dissolves. Then filter the excess solid, and evaporate the filtrate to obtain crystals.

要从不溶性碱制备可溶性盐，将碱加到热稀酸中，直到不再溶解为止。然后过滤除去过量的固体，将滤液蒸发结晶。

Example: copper(II) oxide + sulfuric acid → copper(II) sulfate. Copper(II) oxide is a black solid; the resulting blue solution contains soluble CuSO₄.

例如：氧化铜 + 硫酸 → 硫酸铜。氧化铜是黑色固体，生成的蓝色溶液含有可溶性 CuSO₄。

Method 2: Acid + soluble base (titration)

方法二：可溶性碱 + 酸（滴定法）

For soluble bases (alkalis), mixing acid and alkali directly is dangerous because excess acid or alkali cannot be removed by filtering. Titration is used to add the exact volume required for complete neutralisation. An indicator is first used in a trial run, then the reaction is repeated without indicator.

对于可溶性碱，直接混合酸和碱是危险的，因为过量的酸或碱无法通过过滤除去。滴定法用于确定完全中和所需的精确体积。首先用指示剂进行试滴，然后在不加指示剂的情况下重复反应。

Method 3: Precipitation (for insoluble salts)

方法三：沉淀法（制备不溶性盐）

Two soluble solutions are mixed to produce an insoluble salt, which is recovered by filtering, washing and drying. For example, mix silver nitrate solution with sodium chloride solution to obtain silver chloride.

将两种可溶性溶液混合，生成不溶性盐，通过过滤、洗涤并干燥来收集。例如，将硝酸银溶液与氯化钠溶液混合，得到氯化银沉淀。

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

8. Titration: Step-by-Step | 滴定：分步操作

Titration is a practical technique used to measure the exact volume of one solution that reacts with a known volume of another. In Edexcel IGCSE, you may be asked to describe the method or to calculate concentrations.

滴定是一种用于测量一种溶液与已知体积另一种溶液恰好反应所需体积的实用技术。在爱德思 IGCSE 考试中，你可能会被要求描述步骤或计算浓度。

Key steps:

关键步骤：

Use a pipette to measure a fixed volume of alkali into a conical flask.
用移液管量取一定体积的碱放入锥形瓶。
Add a few drops of indicator (e.g. phenolphthalein) to the flask.
向锥形瓶中加入几滴指示剂（如酚酞）。
Fill a burette with acid and record the initial reading.
将酸装入滴定管并记录初始读数。
Add acid slowly while swirling the flask until the colour changes permanently.
边旋转锥形瓶边缓慢滴加酸，直到颜色永久改变。
Record the final burette reading; the titre is the difference.
记录最终读数；消耗体积为滴定度。

When calculating concentrations, use the mole formula. For example, if 25.0 cm³ of 0.100 mol/dm³ NaOH is neutralised by 20.0 cm³ of HCl:

计算浓度时，使用摩尔公式。例如，25.0 cm³ 的 0.100 mol/dm³ NaOH 被 20.0 cm³ HCl 中和：

NaOH + HCl → NaCl + H₂O | 1:1 mol ratio

Moles NaOH = 0.100 × (25.0/1000) = 0.00250 mol = moles HCl

Concentration HCl = 0.00250 / (20.0/1000) = 0.125 mol/dm³

9. Neutralisation in Everyday Life | 中和反应在生活中的应用

Neutralisation reactions are not only laboratory exercises; they are essential in agriculture, medicine and environmental science.

中和反应不仅是实验室内容，在农业、医学和环境科学中也非常重要。

Indigestion tablets often contain magnesium hydroxide (an alkali) to neutralise excess stomach acid (HCl).
消化不良药片常含氢氧化镁（碱），用于中和胃酸（HCl）过多。
Farmers add lime (calcium hydroxide) to neutralise acidic soil, improving crop growth.
农民使用石灰（氢氧化钙）来中和酸性土壤，促进作物生长。
Acidic industrial waste can be treated with lime or ammonia before release into rivers.
酸性工业废液在排入河流前可用石灰或氨水处理。
Bee stings are acidic, so they are treated with baking soda; wasp stings are alkaline, so they are treated with vinegar.
蜜蜂蜇伤呈酸性，可用小苏打处理；黄蜂蜇伤呈碱性，可用醋处理。

10. Common Exam Pitfalls and Revision Tips | 常见考点陷阱与复习建议

Students often lose marks because of incomplete equations or careless language. Here are common mistakes to avoid in the IGCSE exam.

学生常常因为方程式不完整或表述不严谨而失分。以下是在 IGCSE 考试中需要避免的常见错误。

Do not write “hydrogen” as “H” in equations; use H₂.
不要在方程式中把氢气写成 “H”，应写 H₂。
Remember to include state symbols in balanced equations: (s), (l), (aq), (g).
配平方程式时记住状态符号：(s)、(l)、(aq)、(g)。
Do not confuse “base” and “alkali”: an alkali is a soluble base.
不要混淆 “base” 与 “alkali”：alkali 是可溶性碱。
In salt preparation from an insoluble base, you must add excess base, not just a few lumps, to ensure all acid is used up.
用不溶性碱制备盐时，必须加入过量碱，而不是少量固体，以确保酸完全反应。
In titrations, only repeat the titration without indicator after finding the end-point with indicator.
在滴定中，找到终点后，才能不加指示剂重复实验。
Check whether the salt you are preparing is soluble before choosing the method.
制备盐之前先确认该盐是否溶解，再选择方法。

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Acids, Bases and Salts | 酸、碱与盐