📚 Acids and Bases: IGCSE CIE Science Essentials | 酸与碱：IGCSE CIE 科学考点精讲
Acids and bases are core concepts in IGCSE Co-ordinated Sciences, appearing across Chemistry, Biology, and even aspects of Physics such as electrolysis. This article distils the must-know ideas, from definitions and the pH scale to indicators, neutralisation, and everyday applications, ensuring you can confidently tackle Paper 2, 4, and 6 questions.
酸与碱是 IGCSE 科学的核心概念，横跨化学、生物，甚至涉及物理的电解部分。本文提炼了必备知识点——从定义、pH 标度、指示剂到中和反应和日常应用，帮助你自信应对 Paper 2、4 和 6。
1. What are Acids and Bases? | 什么是酸和碱？
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) ionises to form H⁺ and Cl⁻. Acids have a sour taste (do not taste them in the lab!) and turn blue litmus red.
酸是溶于水时产生氢离子（H⁺）的物质。例如，盐酸（HCl）电离形成 H⁺ 和 Cl⁻。酸有酸味（实验室里不要尝！），可使蓝色石蕊试纸变红。
ons (OH⁻) in water, such as NaOH → Na⁺ + OH⁻. Alkalis feel soapy and turn red litmus blue.
碱是与酸反应生成盐和水的物质。可溶的碱称为碱液；它们溶于水产生氢氧根离子（OH⁻），如 NaOH → Na⁺ + OH⁻。碱液触摸有滑腻感，可使红色石蕊试纸变蓝。
ion used at IGCSE level is the Arrhenius model: acid → H⁺ in water, alkali → OH⁻ in water.
IGCSE 层次使用的最简单定义是阿伦尼乌斯模型：酸在水中产生 H⁺，碱液在水中产生 OH⁻。
2. The pH Scale and Universal Indicator | pH 标度与通用指示剂
l, e.g., pure water. A pH greater than 7 indicates an alkaline solution (higher pH = stronger alkali, higher concentration of OH⁻). Each unit change in pH represents a tenfold change in H⁺ concentration.
pH 标度范围是 0 到 14。pH 小于 7 为酸性溶液（pH 越低，酸性越强，H⁺ 浓度越高）。pH 等于 7 为中性，如纯水。pH 大于 7 为碱性溶液（pH 越高，碱性越强，OH⁻ 浓度越高）。pH 每变化一个单位，H⁺ 浓度变化十倍。
Universal indicator (UI) is a mixture of dyes showing a range of colours: red (pH 1-2), orange/yellow (pH 3-6), green (pH 7), blue (pH 8-11), violet/purple (pH 12-14). It can be used as a solution or on paper.
通用指示剂（UI）是混合染料，显示一系列颜色：红（pH 1-2）、橙/黄（pH 3-6）、绿（pH 7）、蓝（pH 8-11）、紫（pH 12-14）。可制成溶液或试纸使用。
3. Common Indicators and Their Colour Changes | 常见指示剂及其颜色变化
Litmus: red in acid, blue in alkali. Methyl orange: red in acid, yellow in alkali. Phenolphthalein: colourless in acid, pink in alkali. These are used in titrations to show the endpoint.
石蕊：酸中呈红色，碱中呈蓝色。甲基橙：酸中呈红色，碱中呈黄色。酚酞：酸中无色，碱中呈粉红色。这些指示剂用于滴定中以显示终点。
| Indicator | Colour in acid | Colour in neutral | Colour in alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Methyl orange | Red | Orange | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
4. Reactions of Acids | 酸的反应
le: Mg + 2HCl → MgCl₂ + H₂. The test for hydrogen is a lighted splint gives a squeaky pop.
酸与金属（活动性顺序位于氢之前的金属）反应生成盐和氢气。例：Mg + 2HCl → MgCl₂ + H₂。检验氢气的方法是：点燃的木条放在气体中发出 squeaky pop 爆鸣声。
(neutralisation). E.g., CuO + H₂SO₄ → CuSO₄ + H₂O. This is used to prepare soluble salts.
酸与金属氧化物和氢氧化物反应仅生成盐和水（中和反应）。如 CuO + H₂SO₄ → CuSO₄ + H₂O。这可用于制备可溶性盐。
+ 2HCl → CaCl₂ + H₂O + CO₂. Bubbling (effervescence) observed; CO₂ turns limewater milky.
酸与碳酸盐反应生成盐、水和二氧化碳。例：CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂。观察到气泡（冒泡）；CO₂ 使石灰水变浑浊。
5. Reactions of Bases and Alkalis | 碱与碱液的反应
a neutralisation reaction: H⁺(aq) + OH⁻(aq) → H₂O(l). This is the ionic equation for acid–alkali neutralisation. The cation from the base and the anion from the acid form the salt.
碱液与酸发生中和反应：H⁺(aq) + OH⁻(aq) → H₂O(l)。这是酸碱中和的离子方程式。来自碱的阳离子和来自酸的阴离子形成盐。
OH + NH₄Cl → NaCl + H₂O + NH₃. Ammonia turns damp red litmus blue and has a pungent smell.
碱液与铵盐加热反应释放氨气。如 NaOH + NH₄Cl → NaCl + H₂O + NH₃。氨气使湿润的红色石蕊试纸变蓝，并有刺激性气味。
6. Neutralisation and Salt Formation | 中和反应与盐的生成
ochloric acid → chloride, sulfuric acid → sulfate, nitric acid → nitrate, phosphoric acid → phosphate.
当酸中的氢被金属或铵离子取代时便形成盐。盐的名称来自金属（或碱）与酸：盐酸→氯化物，硫酸→硫酸盐，硝酸→硝酸盐，磷酸→磷酸盐。
Preparing a soluble salt: add excess insoluble base (e.g., CuO) to warm acid, filter off excess, evaporate the filtrate to crystallisation point, leave to cool, dry the crystals.
制备可溶性盐：将过量不溶性碱（如氧化铜）加入温热酸中，过滤掉过量物，将滤液蒸发至结晶点，冷却、干燥晶体。
7. Acid–Base Titration | 酸碱滴定
Titration is a technique to find the exact volume of an acid needed to neutralise an alkali (or vice versa). A burette delivers the acid, and a pipette measures the alkali. An indicator (e.g., phenolphthalein or methyl orange) shows the endpoint.
滴定是一种确定中和一定量碱（或酸）所需准确体积的技术。酸从滴定管中滴出，碱液用移液管量取。指示剂（如酚酞或甲基橙）指示终点。
/ volume (in dm³) or using the formula c₁V₁ / n₁ = c₂V₂ / n₂ for complete neutralisation.
结果用于计算浓度，公式：浓度 = 物质的量（mol）/ 体积（dm³）；完全中和时可用 c₁V₁/n₁ = c₂V₂/n₂。
8. pH and Biology: Enzymes and Digestion | pH 与生物学：酶与消化
The stomach produces hydrochloric acid (pH ~1-2), which kills bacteria and provides the optimum pH for the protease enzyme pepsin. The small intestine requires an alkaline pH (around 8-9) for pancreatic enzymes, achieved by bile from the liver.
胃分泌盐酸（pH ~1-2），可杀菌并为蛋白酶——胃蛋白酶提供最适 pH。小肠需要碱性环境（pH 约 8-9）以利于胰酶作用，这由肝脏分泌的胆汁实现。
Enzyme activity is highly sensitive to pH changes; extreme acidity or alkalinity denatures enzymes, altering their active site permanently.
酶活性对 pH 变化高度敏感；过酸或过碱会使酶变性，永久改变其活性位点。
9. Acid Rain and Its Environmental Impact | 酸雨及其环境影响
Burning fossil fuels releases sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which dissolve in water in the atmosphere to form sulfuric acid and nitric acid, making rainwater more acidic (pH <5.2).
燃烧化石燃料释放二氧化硫（SO₂）和氮氧化物（NOₓ），它们溶于大气中的水形成硫酸和硝酸，使雨水酸化（pH <5.2）。
Acid rain damages forests, leaches toxic aluminium from soil into lakes, kills aquatic life, and corrodes limestone buildings and marble statues (CaCO₃ reacts with acid).
酸雨危害森林，将土壤中的有毒铝渗入湖泊，杀死水生生物，并腐蚀石灰岩建筑和大理石雕像（碳酸钙与酸反应）。
10. Acids and Bases in Everyday Life | 日常生活中的酸与碱
Common acids: citric acid in lemons, ethanoic acid in vinegar, lactic acid in sour milk, carbonic acid in fizzy drinks. Common alkalis: sodium hydroxide in drain cleaners, calcium hydroxide in lime for agriculture, magnesium hydroxide in antacids to relieve indigestion.
常见酸：柠檬中的柠檬酸、醋中的乙酸、酸奶中的乳酸、碳酸饮料中的碳酸。常见碱：管道疏通剂中的氢氧化钠、农业用石灰中的氢氧化钙、用于缓解胃灼热的抗酸剂氢氧化镁。
Bees and wasps stings: bee sting is acidic, treated with baking soda (basic); wasp sting is alkaline, treated with vinegar (acidic). This is a simple neutralisation application.
蜜蜂和胡蜂叮咬：蜜蜂叮咬呈酸性，用小苏打（碱性）处理；胡蜂叮咬呈碱性，用醋（酸性）处理。这是简单的中和应用。
11. Key Equations and Formulas to Remember | 必记关键方程式与公式
Here are the essential general equations:
以下是必需的一般方程式：
- Acid + metal → salt + hydrogen
- 酸 + 金属 → 盐 + 氢气
- Acid + metal oxide/hydroxide → salt + water
- 酸 + 金属氧化物/氢氧化物 → 盐 + 水
- Acid + carbonate → salt + water + carbon dioxide
- 酸 + 碳酸盐 → 盐 + 水 + 二氧化碳
- Alkali + ammonium salt → salt + water + ammonia
- 碱液 + 铵盐 → 盐 + 水 + 氨气
Ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
中和离子方程式：H⁺(aq) + OH⁻(aq) → H₂O(l)
Moles calculation: moles (mol) = concentration (mol/dm³) × volume (dm³)
摩尔计算：物质的量 (mol) = 浓度 (mol/dm³) × 体积 (dm³)
12. Exam Tips and Common Pitfalls | 考试技巧与常见错误
Always name salts correctly: sulfuric acid gives sulfates, not sulfites (SO₃²⁻ comes from sulfurous acid). Use state symbols in equations when required. In titration calculations, convert cm³ to dm³ by dividing by 1000. Watch out for the difference between 'strong' acid (fully ionised) and 'concentrated' acid (lot of solute per volume). For the IGCSE practical, refine crystallisation: heat until saturated, then let cool slowly.
务必正确命名盐：硫酸产生硫酸盐（sulfate），而不是亚硫酸盐（SO₃²⁻ 来自亚硫酸）。在方程式中按要求标上状态符号。在滴定计算中，将 cm³ 转换为 dm³ 需除以 1000。注意 '强' 酸（完全电离）和 '浓' 酸（单位体积内溶质多）的区别。IGCSE 实验要求完善结晶：加热至饱和，然后缓慢冷却。
In multiple-choice items, indicators often trick students with colours; memorise the table. In acid rain questions, link to calcium carbonate erosion and effects on living organisms. In Biology, stomach pH and enzyme denaturation often appear together.
在选择题中，指示剂常以颜色迷惑学生，记住颜色表格。酸雨问题可关联到碳酸钙侵蚀及对生物的影响。在生物部分，胃的 pH 和酶变性经常一起考查。
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