IGCSE OCR Science: Acids and Bases – Key Concepts Explained | IGCSE OCR 科学：酸与碱考点精讲

📚 IGCSE OCR Science: Acids and Bases – Key Concepts Explained | IGCSE OCR 科学：酸与碱考点精讲

Acids and bases are fundamental to chemistry and play a vital role in both laboratory reactions and everyday life, from digestion to cleaning products. This revision guide covers all the essential concepts for the IGCSE OCR Science syllabus, including the nature of H⁺ and OH⁻ ions, the pH scale, indicators, typical reactions of acids, neutralisation, salt formation, strong vs weak acids and even acid rain.

酸和碱是化学的基础，在实验室反应以及从消化到清洁产品的日常生活中都起着至关重要的作用。这份复习指南涵盖了 IGCSE OCR 科学大纲中所有重要概念，包括 H⁺ 和 OH⁻ 离子的本质、pH 标度、指示剂、酸的典型反应、中和反应、盐的形成、强酸与弱酸乃至酸雨。

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

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. For example, hydrogen chloride gas (HCl) dissolves in water to form hydrochloric acid, which exists as H⁺ and Cl⁻ ions. The more H⁺ ions present, the more acidic the solution. Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).

酸是一种溶于水时会释放出氢离子 (H⁺) 的物质。例如，氯化氢气体 (HCl) 溶于水形成盐酸，以 H⁺ 和 Cl⁻ 离子的形式存在。溶液中存在的 H⁺ 离子越多，酸性就越强。常见的实验室酸包括盐酸 (HCl)、硫酸 (H₂SO₄) 和硝酸 (HNO₃)。

A base is any substance that can neutralise an acid to form a salt and water. Bases that are soluble in water are called alkalis, and they release hydroxide ions (OH⁻) in solution. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are typical alkalis. Insoluble bases, such as copper(II) oxide (CuO), still act as bases by reacting with acids but do not give alkaline solutions.

碱是指任何能中和酸生成盐和水的物质。可溶于水的碱称为碱（alkali），它们在溶液中释放出氢氧根离子 (OH⁻)。氢氧化钠 (NaOH) 和氢氧化钾 (KOH) 是典型的碱。不溶性碱，如氧化铜 (CuO)，仍然通过与酸反应而起碱的作用，但不会产生碱性溶液。

2. The Role of H⁺ and OH⁻ Ions | H⁺ 与 OH⁻ 离子的作用

The acidity of a solution is entirely due to the presence of H⁺ ions, while alkalinity arises from OH⁻ ions. When an acid dissolves in water, it essentially donates protons (H⁺) to water molecules, forming H₃O⁺ (hydronium) ions, but for IGCSE purposes we simply talk in terms of H⁺. The greater the concentration of H⁺, the lower the pH.

溶液的酸性完全来源于 H⁺ 离子的存在，而碱性则来源于 OH⁻ 离子。当酸溶于水时，它实质上将质子 (H⁺) 提供给水分子，形成 H₃O⁺（水合氢离子），但对于 IGCSE 而言，我们只需按 H⁺ 来讨论。H⁺ 浓度越大，pH 值越低。

Alkalis contain high concentrations of OH⁻ ions. The OH⁻ ions are what react with H⁺ ions during neutralisation, combining to form water: H⁺ + OH⁻ → H₂O. This simple equation is the key to understanding how acidity is neutralised.

碱含有高浓度的 OH⁻ 离子。在中和反应中，正是 OH⁻ 离子与 H⁺ 离子发生反应，结合生成水：H⁺ + OH⁻ → H₂O。这个简单的方程式是理解酸性如何被中和的关键。

3. The pH Scale | pH 标度

The pH scale ranges from 0 to 14 and measures how acidic or alkaline a solution is. A pH of 7 is neutral (pure water). Values below 7 indicate an acidic solution; the lower the number, the higher the H⁺ concentration and the stronger the acid. Values above 7 indicate an alkaline solution; higher numbers mean greater OH⁻ concentration.

pH 标度范围从 0 到 14，用于衡量溶液的酸性或碱性程度。pH 值为 7 时呈中性（纯水）。低于 7 的值表示酸性溶液；数值越低，H⁺ 浓度越高，酸性越强。高于 7 的值表示碱性溶液；数值越高，OH⁻ 浓度越大。

Universal indicator is a mixture of dyes that turns different colours across the pH range: red for strongly acidic, orange for weakly acidic, green for neutral, blue for weakly alkaline, and purple for strongly alkaline. pH can also be measured using a pH probe connected to a data logger for precise readings.

通用指示剂是一种混合染料，在 pH 范围内会呈现不同颜色：强酸为红色，弱酸为橙色，中性为绿色，弱碱为蓝色，强碱为紫色。pH 值也可以使用连接至数据记录仪的 pH 探头来精确测量。

4. Indicators: Universal Indicator and Litmus | 指示剂：通用指示剂与石蕊试纸

Several chemical indicators are used to determine whether a substance is an acid or an alkali. The most common ones for IGCSE are litmus paper, phenolphthalein and methyl orange. Their colour changes are summarised below.

有几种化学指示剂可用于确定某物质是酸还是碱。IGCSE 最常用的是石蕊试纸、酚酞和甲基橙。它们的颜色变化总结如下。

Indicator	Colour in acid	Colour in neutral	Colour in alkali
Litmus paper	Red	Purple (no change)	Blue
Phenolphthalein	Colourless	Colourless	Pink
Methyl orange	Red	Orange	Yellow

Litmus paper is often used as a quick qualitative test. Blue litmus turns red in acid; red litmus turns blue in alkali. Universal indicator provides a much more detailed picture of pH by showing a spectrum of colours.

石蕊试纸常被用作快速的定性测试。蓝色石蕊试纸遇酸变红；红色石蕊试纸遇碱变蓝。通用指示剂通过显示一系列颜色，能更细致地反映 pH 情况。

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

Dilute acids react with many metals to produce a salt and hydrogen gas. The general word equation is: metal + acid → salt + hydrogen. The salt formed depends on the acid used; hydrochloric acid produces chlorides, sulfuric acid produces sulfates, and nitric acid produces nitrates.

稀酸与许多金属反应生成盐和氢气。通用的文字方程式是：金属 + 酸 → 盐 + 氢气。生成的盐取决于所用的酸：盐酸生成氯化物，硫酸生成硫酸盐，硝酸生成硝酸盐。

Not all metals react—potassium, sodium and calcium are too reactive for safe classroom use, while copper does not react with dilute acids because it is below hydrogen in the reactivity series. Typical reactive metals used in the lab include magnesium, zinc and iron. For example:

Mg + 2HCl → MgCl₂ + H₂

并非所有金属都会反应——钾、钠和钙过于活泼，在课堂上使用不安全，而铜由于在金属活动性顺序中排在氢之后，不与稀酸反应。实验室中常用的活泼金属包括镁、锌和铁。例如：

Mg + 2HCl → MgCl₂ + H₂

The hydrogen gas can be tested using a burning splint held at the mouth of the test tube: it burns with a squeaky pop. This is a classic IGCSE observation.

氢气可以通过在试管口处用燃着的木条进行检验：氢气体燃烧时会发出尖锐的爆鸣声。这是经典的 IGCSE 实验现象。

6. Reactions of Acids with Metal Oxides and Hydroxides | 酸与金属氧化物和氢氧化物的反应

Acids react with metal oxides and metal hydroxides to produce a salt and water only. This is a type of neutralisation reaction, as the base cancels out the acidic properties. The general pattern is: acid + metal oxide → salt + water, and acid + metal hydroxide → salt + water.

酸与金属氧化物和金属氢氧化物反应，只生成盐和水。这是一种中和反应，因为碱消除了酸的特性。一般规律为：酸 + 金属氧化物 → 盐 + 水，以及酸 + 金属氢氧化物 → 盐 + 水。

For instance, black copper(II) oxide reacts with warm dilute sulfuric acid to give a blue solution of copper(II) sulfate:

CuO + H₂SO₄ → CuSO₄ + H₂O

例如，黑色的氧化铜与温热的稀硫酸反应，生成蓝色的硫酸铜溶液：

CuO + H₂SO₄ → CuSO₄ + H₂O

Similarly, sodium hydroxide (an alkali) neutralises hydrochloric acid:

NaOH + HCl → NaCl + H₂O

同样地，氢氧化钠（一种碱）中和盐酸：

NaOH + HCl → NaCl + H₂O

These reactions are carried out in the lab to prepare soluble salts. The excess solid base is added to the acid and then filtered off, leaving the salt solution to be crystallised.

这些反应可在实验室中用于制备可溶性盐。将过量的固体碱加入酸中，然后过滤掉剩余固体，得到的盐溶液再经过结晶处理。

7. Reactions of Acids with Carbonates | 酸与碳酸盐的反应

Acids react with metal carbonates and hydrogencarbonates to produce a salt, water and carbon dioxide gas. Effervescence (fizzing) is clearly observed as CO₂ is released. The general word equation is: acid + carbonate → salt + water + carbon dioxide.

酸与金属碳酸盐和碳酸氢盐反应，生成盐、水和二氧化碳气体。随着 CO₂ 的释放，可以明显观察到冒泡现象（嘶嘶作响）。通用的文字方程式为：酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。

A well-known example is the reaction between hydrochloric acid and calcium carbonate (limestone or marble chips):

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

一个众所周知的例子是盐酸与碳酸钙（石灰石或大理石屑）的反应：

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Carbon dioxide gas turns limewater milky (cloudy) due to the formation of insoluble calcium carbonate. This is the standard test for CO₂ and a frequent exam question.

二氧化碳气体会使石灰水变浑浊（呈乳白色），这是由于生成了不溶性碳酸钙。这是检验 CO₂ 的标准方法，也是常见的考题。

8. Neutralisation and Salt Formation | 中和反应与盐的形成

Neutralisation occurs when an acid and a base are mixed in exactly the right proportions so that the H⁺ ions from the acid and the OH⁻ ions from the alkali combine to form water, leaving the other ions to form a salt. The ionic equation is simply H⁺ + OH⁻ → H₂O. A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal ion (or ammonium).

当酸和碱恰好以适当的比例混合时，就会发生中和反应，酸中的 H⁺ 离子与碱中的 OH⁻ 离子结合生成水，而其他离子则形成盐。离子方程式就是 H⁺ + OH⁻ → H₂O。盐是酸的氢被金属（或铵）离子取代后形成的离子化合物。

Salts are named according to the acid and the metal. Hydrochloric acid gives chlorides (e.g. sodium chloride, NaCl), sulfuric acid gives sulfates (e.g. copper(II) sulfate, CuSO₄), and nitric acid gives nitrates (e.g. potassium nitrate, KNO₃). The state symbols (aq) are often used for solutions.

盐的名称取决于所用的酸和金属。盐酸生成氯化物（如氯化钠 NaCl），硫酸生成硫酸盐（如硫酸铜 CuSO₄），硝酸生成硝酸盐（如硝酸钾 KNO₃）。通常会使用状态符号 (aq) 表示溶液。

Neutralisation has many practical applications: treating acid indigestion with antacid tablets (containing bases like calcium carbonate), treating acidic soil with lime (calcium oxide/hydroxide), and controlling pH in swimming pools.

中和反应有许多实际应用：用抗酸片（含有碳酸钙等碱性物质）治疗胃酸过多、用石灰（氧化钙/氢氧化钙）改良酸性土壤，以及控制游泳池水的 pH 值。

9. Strong and Weak Acids | 强酸与弱酸

A strong acid is one that fully dissociates (ionises) in water, releasing all its H⁺ ions. Hydrochloric acid is a typical strong acid, so a 0.1 mol/dm³ solution of HCl has a high concentration of H⁺ and a low pH (around 1). Other strong acids include sulfuric acid and nitric acid.

强酸是指在水中完全解离（电离），释放出所有 H⁺ 离子的酸。盐酸是典型的强酸，因此 0.1 mol/dm³ 的 HCl 溶液 H⁺ 浓度高，pH 值低（约为 1）。其他强酸包括硫酸和硝酸。

A weak acid only partially dissociates, so the equilibrium lies far to the left. Ethanoic acid (CH₃COOH), found in vinegar, is a common weak acid. A solution of the same concentration as HCl will have a much higher pH (e.g. around 3) because fewer H⁺ ions are released. Despite being weak, the acid still has all the typical acidic properties because it can still supply H⁺ ions during a reaction.

弱酸只发生部分解离，平衡位置非常偏左。醋中的乙酸 (CH₃COOH) 是常见的弱酸。与 HCl 相同浓度的溶液，其 pH 值会高得多（例如约为 3），因为释放的 H⁺ 离子较少。虽然酸性弱，但这种酸仍具有所有典型的酸性，因为在反应过程中它仍能提供 H⁺ 离子。

The terms ‘strong’ and ‘weak’ refer to the degree of dissociation, not the concentration. You can have a concentrated weak acid or a dilute strong acid.

“强”和“弱”指的是解离的程度，而不是浓度。可以有浓的弱酸，也可以有稀的强酸。

10. Acid Rain | 酸雨

Acid rain is caused by the dissolution of acidic gases in rainwater. The main culprits are sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), released from burning fossil fuels and vehicle exhausts. In the atmosphere, SO₂ oxidises and reacts with water to form sulfuric acid, while nitrogen dioxide forms nitric acid.

酸雨是由酸性气体溶解在雨水中引起的。主要元凶是燃烧化石燃料和汽车尾气释放的二氧化硫 (SO₂) 和氮氧化物 (NOₓ)。在大气中，SO₂ 被氧化并与水反应生成硫酸，而二氧化氮则生成硝酸。

The reactions can be simplified as:

SO₂ + H₂O → H₂SO₃ (sulfurous acid), which further oxidises to H₂SO₄

这些反应可以简化为：

SO₂ + H₂O → H₂SO₃（亚硫酸），进一步氧化成 H₂SO₄

Acid rain harms ecosystems by lowering the pH of soil and water bodies, damaging plant leaves, leaching aluminium from soil which is toxic to fish, and eroding limestone buildings and marble statues. Reducing acid rain involves catalytic converters in cars, scrubbing flue gases in power stations, and switching to renewable energy sources.

酸雨降低土壤和水体的 pH 值，损害植物叶片，从土壤中滤出对鱼类有毒的铝，并侵蚀石灰石建筑和大理石雕像，从而危害生态系统。减少酸雨的措施包括在汽车上安装催化转化器、在发电站对烟道气进行洗涤，以及改用可再生能源。

11. Titration: A Quantitative Neutralisation | 滴定：一种定量中和方法

Titration is a technique used to find the exact volume of acid needed to neutralise a given volume of alkali (or vice versa), enabling the concentration of an unknown solution to be determined. It makes use of a burette, a pipette and a suitable indicator, typically phenolphthalein or methyl orange.

滴定法是用于找出中和一定体积碱（或反之）所需酸的准确体积的一种技术，从而可以测定未知溶液的浓度。滴定需要使用滴定管、移液管和合适的指示剂，通常是酚酞或甲基橙。

The procedure: a known volume of alkali is measured into a conical flask using a pipette, and a few drops of indicator are added. The acid is placed in the burette and slowly released while swirling the flask until the indicator changes colour at the end point. The volume of acid used is recorded. The experiment is repeated until concordant results are obtained (within 0.1 cm³).

操作步骤：用移液管量取一定体积的碱，移至锥形瓶中，并加入几滴指示剂。将酸装入滴定管，缓缓释放酸液，同时摇动锥形瓶，直至指示剂在终点变色。记录所用酸的体积。重复实验，直至获得符合要求的数据（偏差在 0.1 cm³ 以内）。

If the concentration of the acid is known, the concentration of the alkali can be calculated using the mole concept. Even for IGCSE Science, understanding that the titration gives the exact reacting volumes is important.

如果酸的浓度已知，则可以使用摩尔概念计算出碱的浓度。即使对于 IGCSE 科学课程，理解滴定能提供精确的反应体积也是非常重要的。

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IGCSE OCR Science: Acids and Bases – Key Concepts Explained | IGCSE OCR 科学：酸与碱 考点精讲