IGCSE化学 Acids 酸知识点全解析
Acids are a core topic in IGCSE CAIE Chemistry. This bilingual guide covers everything you need for Papers 2, 4, and 6 — from definitions to reactions to exam tips.
酸是IGCSE CAIE化学的核心主题。这篇中英双语指南涵盖Paper 2、4、6所需的一切——从定义到反应到考试技巧。
一、What Is an Acid? 什么是酸？
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. Acids are proton donors. Key point: all acids contain hydrogen, but not all hydrogen compounds are acids — the substance must release H⁺ in water.
酸是溶于水时产生氢离子（H⁺）的物质。酸是质子给予体。关键点：所有酸都含氢，但并非所有含氢化合物都是酸——物质必须在水中释放H⁺。
二、Common Acids 常见酸
Mineral acids 无机酸： Hydrochloric acid 盐酸 (HCl) — stomach acid, produces chlorides. Sulfuric acid 硫酸 (H₂SO₄) — car batteries, fertilisers, produces sulfates. Nitric acid 硝酸 (HNO₃) — fertilisers, produces nitrates; reactions often yield NO₂ not H₂.
Organic acids 有机酸： Ethanoic acid 乙酸 (CH₃COOH) — vinegar, weak acid, food preservative. Citric acid 柠檬酸 — citrus fruits, food flavouring.
三、Strong vs Weak Acids 强酸与弱酸
Strong acids fully ionise in water — every molecule releases H⁺ (e.g., HCl, H₂SO₄, HNO₃). Weak acids partially ionise — only a small fraction releases H⁺ (e.g., CH₃COOH, citric acid).
强酸在水中完全电离——每个分子都释放H⁺（如HCl、H₂SO₄、HNO₃）。弱酸部分电离——只有少量分子释放H⁺（如CH₃COOH、柠檬酸）。
Exam tip: At the same concentration, strong acids have lower pH than weak acids because they produce more H⁺ ions. This is a classic CAIE question.
考试提示：相同浓度下，强酸比弱酸pH更低，因为产生更多H⁺离子。这是CAIE经典考题。
四、The pH Scale pH标度
pH 0-6: acidic | pH 7: neutral | pH 8-14: alkaline. Universal indicator colours: red (strong acid), orange/yellow (weak acid), green (neutral), blue (weak alkali), purple (strong alkali). Each pH unit = tenfold change in H⁺ concentration.
pH 0-6：酸性 | pH 7：中性 | pH 8-14：碱性。通用指示剂颜色：红（强酸）、橙/黄（弱酸）、绿（中性）、蓝（弱碱）、紫（强碱）。pH每变化1 = H⁺浓度变化10倍。
五、Reactions of Acids 酸的反应
1. Acid + Metal → Salt + H₂ 酸+金属→盐+氢气： Metals above hydrogen in the reactivity series. Test for H₂: lighted splint gives squeaky pop. Example: 2HCl + Mg → MgCl₂ + H₂.
2. Acid + Base → Salt + H₂O 酸+碱→盐+水： Neutralisation reaction. Example: H₂SO₄ + CuO → CuSO₄ + H₂O. Ionic equation for all neutralisations: H⁺ + OH⁻ → H₂O.
3. Acid + Carbonate → Salt + H₂O + CO₂ 酸+碳酸盐→盐+水+CO₂： Test for CO₂: bubble through limewater — turns milky. Example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂.
4. Acid + Ammonia → Ammonium Salt 酸+氨→铵盐： HCl + NH₃ → NH₄Cl. Important for fertiliser production.
六、Making Salts 制盐方法
Excess solid method 过量固体法： Add excess insoluble base (metal oxide/carbonate) to warm acid → filter → evaporate → crystallise. For salts like CuSO₄.
Titration 滴定法： When both reactants are soluble (acid + alkali). Neutralise with indicator → repeat without indicator → crystallise. For salts like NaCl, KNO₃.
七、Key Definitions 必背定义
Acid: produces H⁺ in aqueous solution / 在水溶液中产生H⁺
Base: reacts with acid to form salt + water only / 与酸反应只生成盐和水
Alkali: soluble base producing OH⁻ in water / 可溶性碱，在水中产生OH⁻
Neutralisation: acid + base → salt + water / 酸+碱→盐+水
Strong acid: fully ionises / 完全电离
Weak acid: partially ionises / 部分电离
八、Common Exam Mistakes 常见考试错误
Mistake 1: Confusing strong/weak with concentrated/dilute. "Strong" = degree of ionisation, not concentration. 混淆强弱酸与浓稀酸。
Mistake 2: Using H₂ instead of H⁺ in ionic equations. 离子方程式中错用H₂而非H⁺。
Mistake 3: Forgetting state symbols (s, l, g, aq). 忘记标状态符号。
Mistake 4: Assuming all metal-acid reactions produce H₂. Nitric acid is an exception. 认为所有金属-酸反应都产氢。硝酸例外。
九、Paper 6 Practical Tips 实验考试技巧
Use 2-3 drops of indicator only. Heat gently — never boil vigorously. Stop heating when crystals appear at the solution edge, then cool slowly for large crystals. Repeat titrations for concordant results (within 0.2 cm³).
指示剂只用2-3滴。温和加热——切勿剧烈沸腾。溶液边缘出现晶体时停止加热，缓慢冷却得大晶体。重复滴定至结果一致（相差0.2 cm³以内）。
Master the acids topic through understanding — all acid reactions follow predictable patterns. Regular past paper practice is the key to confidence in the exam.
理解酸的规律——所有酸反应都有可预测的模式。定期练习历年真题是考试自信的关键。
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