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

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

Acids and bases are fundamental chemical substances that appear in laboratories, industry, and everyday life. In IGCSE Science, students must understand their properties, reactions, and the pH scale. This article covers all essential topics, from definitions to titration, helping you solidify your knowledge for the exam.

酸和碱是出现在实验室、工业及日常生活中的基本化学物质。在 IGCSE 科学中，学生需要掌握它们的性质、反应以及 pH 概念。本文涵盖从定义到滴定的所有重要主题，帮助你扎实掌握考试要点。

1. Defining Acids and Bases | 酸与碱的定义

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. Strong acids ionize completely, while weak acids ionize only partially.

酸是在水中溶解时释放氢离子（H⁺）的物质。强酸完全电离，弱酸仅部分电离。

A base is a substance that can neutralize an acid, producing a salt and water. Most bases are metal oxides or metal hydroxides. An alkali is a soluble base that releases hydroxide ions (OH⁻) in water.

碱是能中和酸、生成盐和水的物质。大多数碱是金属氧化物或金属氢氧化物。碱可溶于水时称为可溶性碱（alkali），在水中释放氢氧根离子（OH⁻）。

2. The pH Scale | pH 标度

The pH scale is a measure of the acidity or alkalinity of a solution, ranging from 0 (very acidic) to 14 (very alkaline). A pH of 7 is neutral. The scale is logarithmic: each unit change represents a tenfold change in H⁺ concentration.

pH 标度用于衡量溶液的酸性或碱性，范围从 0（强酸性）到 14（强碱性）。pH 为 7 时呈中性。该标度是对数标度：每变化一个单位，H⁺ 浓度改变十倍。

Universal indicator and pH probes can be used to measure pH. Common substances include:

可使用通用指示剂和 pH 探头测量 pH。常见物质包括：

• pH 1–2: stomach acid, battery acid | pH 1–2：胃酸、蓄电池酸液

• pH 3–4: lemon juice, vinegar | pH 3–4：柠檬汁、醋

• pH 5–6: rain water, black coffee | pH 5–6：雨水、黑咖啡

• pH 7: pure water | pH 7：纯水

• pH 8–9: baking soda, toothpaste | pH 8–9：小苏打、牙膏

• pH 10–12: soap, limewater | pH 10–12：肥皂、石灰水

• pH 13–14: oven cleaner, drain cleaner | pH 13–14：烤箱清洁剂、管道疏通剂

3. Indicators | 指示剂

Indicators show whether a solution is acidic, neutral, or alkaline by changing colour. Litmus is the simplest: red in acid, blue in alkali. Universal indicator gives a full pH colour range.

指示剂通过颜色变化显示溶液是酸性、中性还是碱性。石蕊是最简单的：酸中变红，碱中变蓝。通用指示剂可给出完整的 pH 颜色范围。

Indicator	Colour in Acid	Colour in Neutral	Colour in Alkali
Litmus	Red	Purple	Blue
Phenolphthalein	Colourless	Colourless	Pink
Methyl orange	Red	Yellow/orange	Yellow

Phenolphthalein is often used in titrations because its colour change from colourless to pink is sharp around pH 8.3–10.0.

酚酞常用于滴定，因为它在 pH 8.3–10.0 附近从无色变为粉红色的变色明显。

4. Strong vs Weak Acids and Concentrated vs Dilute | 强酸与弱酸、浓与稀

Strength refers to the degree of ionization. A strong acid fully dissociates in water (e.g., HCl, H₂SO₄, HNO₃), while a weak acid only partially dissociates (e.g., ethanoic acid CH₃COOH, citric acid). Concentration refers to the amount of acid or alkali dissolved in a given volume of water. You can have a dilute strong acid or a concentrated weak acid.

强度指的是电离程度。强酸在水中完全解离（如 HCl、H₂SO₄、HNO₃），而弱酸仅部分解离（如乙酸 CH₃COOH、柠檬酸）。浓度是指在一定体积水中溶解的酸或碱的量。可以存在稀的强酸或浓的弱酸。

For a given concentration, strong acids have a lower pH (more H⁺ ions) than weak acids. When reacting with metals or carbonates, strong acids react faster because there are more free hydrogen ions.

对于相同浓度，强酸的 pH 更低（H⁺ 离子更多）比弱酸。当与金属或碳酸盐反应时，强酸反应更快，因为有更多的游离氢离子。

5. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base to form a salt and water. The general word equation:

中和是酸与碱反应生成盐和水的过程。一般文字方程式：

Acid + Base → Salt + Water

The ionic equation for neutralisation involving a strong acid and a strong alkali is:

涉及强酸和强碱的中和离子方程式为：

H⁺(aq) + OH⁻(aq) → H₂O(l)

This explains why the reaction of any strong acid with any strong alkali releases the same amount of heat per mole of water formed (enthalpy of neutralisation ≈ –57 kJ mol⁻¹).

这解释了为何任何强酸与强碱反应，每生成一摩尔水释放的热量相同（中和焓约为 –57 kJ mol⁻¹）。

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

Acids react with many metals to produce a salt and hydrogen gas. The word equation:

酸与许多金属反应生成盐和氢气。文字方程式：

Acid + Metal → Salt + Hydrogen

For example, zinc with sulfuric acid: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g). The test for hydrogen is a lighted splint producing a ‘squeaky pop’ sound.

例如，锌与硫酸：Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)。氢气的检验是点燃的木条发出“噗”的一声。

Metals like copper, silver, and gold do not react with dilute acids because they are below hydrogen in the reactivity series. Very reactive metals (e.g., potassium, sodium) are too dangerous to add to acids in the lab.

像铜、银和金等金属不与稀酸反应，因为它们在金属活动性顺序中排在氢之后。极活泼的金属（如钾、钠）在实验室中加入酸中过于危险。

7. Reactions of Acids with Carbonates and Bases | 酸与碳酸盐及碱的反应

Acids react with metal carbonates and hydrogencarbonates to produce a salt, water, and 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：

Acid + Carbonate → Salt + Water + Carbon Dioxide

For example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g). Carbon dioxide turns limewater milky.

例如：2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)。二氧化碳能使石灰水变浑浊。

Acids react with metal oxides and hydroxides (bases) to give a salt and water only:

酸与金属氧化物和氢氧化物（碱）反应仅生成盐和水：

Acid + Metal Oxide → Salt + Water

Example: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l). This is often used to prepare soluble salts by reacting an excess of insoluble base with an acid and then filtering.

例子：CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)。此法常用于制备可溶性盐：用过量的不溶性碱与酸反应，然后过滤。

8. Naming Salts and Predicting Products | 盐的命名与产物预测

The name of the salt comes from the metal in the base/carbonate and the acid used:

盐的名称来源于碱/碳酸盐中的金属和所用的酸：

• Hydrochloric acid → chloride (e.g., sodium chloride) | 盐酸 → 氯化物（如氯化钠）

• Sulfuric acid → sulfate (e.g., copper sulfate) | 硫酸 → 硫酸盐（如硫酸铜）

• Nitric acid → nitrate (e.g., potassium nitrate) | 硝酸 → 硝酸盐（如硝酸钾）

• Ethanoic acid → ethanoate (e.g., sodium ethanoate) | 乙酸 → 乙酸盐（如乙酸钠）

To predict products, identify the positive ion from the base/metal/carbonate and the negative ion from the acid, then balance the charges correctly.

为预测产物，从碱/金属/碳酸盐中找出正离子，从酸中找出负离子，然后正确平衡电荷。

9. Solubility of Salts and Choosing Preparation Methods | 盐的溶解性与制备方法选择

All sodium, potassium, and ammonium salts are soluble. All nitrates are soluble. Most chlorides are soluble except silver chloride and lead chloride. Most sulfates are soluble except barium sulfate, lead sulfate, and calcium sulfate (slightly). Carbonates are mostly insoluble except Group 1 and ammonium carbonates.

所有钠盐、钾盐和铵盐都可溶。所有硝酸盐都可溶。大多数氯化物可溶，除氯化银和氯化铅外。大多数硫酸盐可溶，除硫酸钡、硫酸铅和硫酸钙（微溶）外。碳酸盐大多不溶，除第一主族和铵的碳酸盐外。

For soluble salts, use titration (acid + alkali) if the base is soluble; use acid + excess insoluble base/carbonate, then filter, if the base is insoluble.

对于可溶性盐，若碱可溶，使用滴定法（酸+碱）；若碱不溶，使用酸与过量不溶性碱/碳酸盐反应，然后过滤。

10. Titration Technique | 滴定技术

Titration is used to determine the exact volume of an acid needed to neutralise an alkali (or vice versa). Apparatus includes a pipette, burette, conical flask, and an indicator (e.g., phenolphthalein).

滴定用于确定中和一定量碱（或酸）所需的准确酸体积。仪器包括移液管、滴定管、锥形瓶和指示剂（如酚酞）。

Steps: Rinse pipette with the solution to be measured; use a pipette filler to transfer a fixed volume of alkali into a conical flask; add indicator; fill the burette with acid; add acid slowly while swirling until the indicator just changes colour (end-point); record the volume. Repeat for concordant results.

步骤：用待测溶液润洗移液管；使用洗耳球移取一定体积的碱液至锥形瓶；加入指示剂；用酸装填滴定管；缓慢滴加酸并摇动锥形瓶，直至指示剂恰好变色（终点）；记录体积。重复直至获得一致结果。

Titration is essential for making pure soluble salts without indicator contamination (the indicator is omitted in the final repeat to obtain a colourless pure salt solution).

滴定对于制备纯净的可溶性盐至关重要，不受指示剂污染（在最后一次重复中不加指示剂，以获得无色纯净盐溶液）。

11. Oxides: Acidic, Basic, Amphoteric, and Neutral | 氧化物：酸性、碱性、两性和中性

Oxides can be classified by their reactions with acids and alkalis:

氧化物可根据它们与酸和碱的反应进行分类：

• Acidic oxides (non-metal oxides like CO₂, SO₂, SiO₂) react with alkalis to form salts and water. They do not react with acids.

• 酸性氧化物（非金属氧化物如 CO₂、SO₂、SiO₂）与碱反应生成盐和水，不与酸反应。

• Basic oxides (metal oxides like CuO, Na₂O) react with acids to form salts and water.

• 碱性氧化物（金属氧化物如 CuO、Na₂O）与酸反应生成盐和水。

• Amphoteric oxides (ZnO, Al₂O₃) react with both acids and strong alkalis, showing both acidic and basic character.

• 两性氧化物（ZnO, Al₂O₃）既能与酸反应，也能与强碱反应，表现出酸性和碱性两种性质。

• Neutral oxides (CO, NO) show neither acidic nor basic properties.

• 中性氧化物（CO, NO）既不显酸性也不显碱性。

This pattern helps predict products and understand environmental issues like acid rain, where SO₂ and NO₂ form acidic solutions in water.

这一规律有助于预测产物并理解环境问题，如酸雨的形成：SO₂ 和 NO₂ 在水中形成酸性溶液。

12. Everyday Applications and Exam Tips | 日常应用与应试技巧

Acids and bases are not just laboratory reagents: stomach acid includes HCl for digestion; acidic soil can be treated with lime (calcium oxide/hydroxide); bee stings are acidic (treated with baking soda), while wasp stings are alkaline (treated with vinegar).

酸和碱不仅是实验室试剂：胃酸含 HCl 帮助消化；酸性土壤可用石灰（氧化钙/氢氧化钙）改良；蜜蜂叮咬呈酸性（用小苏打缓解），而黄蜂叮咬呈碱性（用醋缓解）。

Exam tip: Learn the ionic equations for neutralisation (H⁺ + OH⁻ → H₂O) and for reaction of acids with carbonates (2H⁺ + CO₃²⁻ → H₂O + CO₂). Always check salt solubility rules when planning a preparation method. Draw apparatus clearly if required and label key parts like burette and pipette.

应试技巧：熟记中和反应的离子方程式（H⁺ + OH⁻ → H₂O）以及酸与碳酸盐反应的离子方程式（2H⁺ + CO₃²⁻ → H₂O + CO₂）。在规划制备方法时，始终检查盐的溶解性规则。如需作图，应清晰画出仪器并标注关键部件如滴定管和移液管。

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