📚 Acids, Bases and Salts | 酸、碱和盐
In Edexcel IGCSE Science (Chemistry), understanding acids, bases and salts is fundamental. This topic explains the properties, reactions and methods of preparing these important compounds. Mastering the content helps students solve quantitative problems and apply knowledge in practical contexts.
在Edexcel IGCSE科学（化学）课程中，理解酸、碱和盐是基础。这一主题解释了这些重要化合物的性质、反应以及制备方法。掌握这些内容有助于学生解决定量问题，并在实际情境中应用知识。
1. Introduction to Acids and Bases | 酸和碱简介
Acids are substances that release hydrogen ions (H⁺) when dissolved in water. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Bases are substances that can neutralise acids to form salts and water. Metal oxides and metal hydroxides are typical bases. An alkali is a soluble base that releases hydroxide ions (OH⁻) in water, such as sodium hydroxide (NaOH).
酸是溶于水后释放氢离子（H⁺）的物质。常见例子包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。碱是可以中和酸生成盐和水的物质。金属氧化物和金属氢氧化物是典型的碱。碱是一种可溶性碱，在水中释放氢氧根离子（OH⁻），例如氢氧化钠（NaOH）。
2. The pH Scale | pH值等级
The pH scale measures the acidity or alkalinity of a solution. It ranges from 0 to 14. A pH less than 7 indicates an acidic solution (lower pH means higher H⁺ concentration). A pH of 7 is neutral (pure water). A pH greater than 7 indicates an alkaline solution. Universal indicator or pH meters can be used to measure pH.
pH值等级用于衡量溶液的酸碱性，范围从0到14。pH值小于7表示酸性溶液（pH越低，H⁺浓度越高）。pH为7是中性（纯水）。pH大于7表示碱性溶液。通用指示剂或pH计可用于测定pH值。
3. Reactions of Acids | 酸的反应
r + carbon dioxide. These reactions are characteristic and allow identification of acids and production of salts.
酸能与许多物质反应。IGCSE水平重点学习的三种反应是：(i) 酸 + 金属 → 盐 + 氢气；(ii) 酸 + 碱 → 盐 + 水；(iii) 酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。这些反应具有特征性，可用于鉴别酸并制备盐类。
SO₄(aq) → ZnSO₄(aq) + H₂(g). The hydrogen gas can be tested with a lit splint, producing a 'pop' sound.
例如，锌与稀硫酸反应生成硫酸锌和氢气： Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)。氢气可以用点燃的木条检验，会产生'噗'的一声。
4. Neutralisation and Salt Formation | 中和反应与盐的形成
lkali to form water: H⁺(aq) + OH⁻(aq) → H₂O(l). The remaining ions form the salt. The salt' drochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.
中和反应是酸和碱反应生成盐和水的过程。酸中的H⁺离子与碱中的OH⁻离子结合生成水： H⁺(aq) + OH⁻(aq) → H₂O(l)。其余离子形成盐。盐的名称来源于金属（或铵）部分及所用的酸：盐酸产生氯化物，硫酸产生硫酸盐，硝酸产生硝酸盐。
5. Making Soluble Salts | 可溶性盐的制备
Soluble salts can be prepared by reacting an acid with a suitable insoluble base, metal or carbonate. A common method is to add excess solid (e.g. copper(II) oxide) to warm dilute acid until no more reacts, filter off the excess, and then crystallise the salt by evaporating the water. This produces pure dry crystals of the soluble salt.
可溶性盐可以通过酸与合适的不溶性碱、金属或碳酸盐反应制备。常见方法是向温热的稀酸中加入过量固体（如氧化铜）直至不再反应，过滤除去过量固体，然后通过蒸发水分使盐结晶。这样可以得到纯净干燥的可溶性盐晶体。
SO₄(aq) + H₂O(l). The blue copper(II) sulfate solution is filtered and evaporated to obtain blue crystals.
例如，制备硫酸铜时，使用温热的稀硫酸与过量氧化铜反应。反应式为：CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)。蓝色硫酸铜溶液经过滤、蒸发后可获得蓝色晶体。
6. Making Insoluble Salts | 不溶性盐的制备
and sodium sulfate: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). The white precipitate is barium sulfate.
不溶性盐通过沉淀反应制备。将含有所需离子的两种水溶液混合，不溶性盐以沉淀形式生成，可经过滤、洗涤和干燥得到。例如，制备硫酸钡（一种不溶性盐），混合氯化钡和硫酸钠溶液： BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)。白色沉淀即为硫酸钡。
To choose the correct reagents, one must know the solubility rules. All sodium, potassium and ammonium salts are soluble, all nitrates are soluble, most chlorides are soluble (except silver and lead chlorides), most sulfates are soluble (except barium, lead and calcium sulfates), and most carbonates and hydroxides are insoluble unless they are from Group 1 or ammonium.
选择合适的试剂必须了解溶解度规律。所有钠盐、钾盐和铵盐均可溶，所有硝酸盐可溶，大多数氯化物可溶（银和铅的氯化物除外），大多数硫酸盐可溶（钡、铅和钙的硫酸盐除外），而大多数碳酸盐和氢氧化物不溶（除非是第1族金属或铵的化合物）。
7. Bases and Alkalis | 碱与碱性物质
Bases are substances that neutralise acids. All metal oxides and metal hydroxides are bases. Copper(II) oxide, iron(III) oxide and magnesium hydroxide are bases. Alkalis are a subset of bases that dissolve in water to produce OH⁻ ions. Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)₂), although calcium hydroxide is only slightly soluble. Alkalis feel soapy and turn red litmus blue.
碱是能中和酸的物质。所有金属氧化物和金属氢氧化物都是碱。氧化铜、氧化铁和氢氧化镁都是碱。碱是碱的一个子类，它们溶于水产生OH⁻离子。常见的碱包括氢氧化钠（NaOH）、氢氧化钾（KOH）和氢氧化钙（Ca(OH)₂），尽管氢氧化钙仅微溶。碱具有滑腻感，并使红色石蕊试纸变蓝。
8. Indicators | 指示剂
Indicators are substances that change colour depending on the pH of the solution. Litmus is red in acidic solutions and blue in alkaline solutions. Universal indicator gives a range of colours from red (strong acid) to green (neutral) to purple (strong alkali). Methyl orange turns red in acid and yellow in alkali. Phenolphthalein is colourless in acid and pink in alkali. Choosing the correct indicator is essential for titrations.
指示剂是根据溶液pH值改变颜色的物质。石蕊在酸性溶液中呈红色，在碱性溶液中呈蓝色。通用指示剂显示从红（强酸）到绿（中性）到紫（强碱）的颜色范围。甲基橙在酸中呈红色，在碱中呈黄色。酚酞在酸中无色，在碱中呈粉红色。选择正确的指示剂对于滴定至关重要。
9. Acid-Base Titrations | 酸碱滴定
Titration is a quantitative technique used to determine the concentration of an acid or an alkali. A known volume of one solution is measured with a pipette, and the other solution is added from a burette until the end-point is reached, shown by an indicator colour change. The volume used is recorded to calculate the concentration using the mole concept and the balanced equation.
滴定是一种定量技术，用于测定酸或碱的浓度。先用移液管量取一定体积的一种溶液，再从滴定管中滴加另一种溶液，直到指示剂变色显示终点。记录所用的体积，利用摩尔概念和配平方程式计算浓度。
For a strong acid – ion calculation often uses the formula: moles of solute = concentration × volume (in dm³).
对于强酸-强碱滴定，例如HCl和NaOH，终点pH为7，可使用在pH 7附近变色的任何指示剂，如酚酞或甲基橙。滴定计算通常使用公式：溶质的物质的量 = 浓度 × 体积（单位为dm³）。
10. Everyday Examples and Uses | 日常实例和用途
Acids and alkalis are widely used. Hydrochloric acid in the stomach aids digestion. Citric acid is found in citrus fruits. Vinegar contains ethanoic acid. Alkalis like sodium hydroxide are used in soap making and drain cleaners. Calcium hydroxide is used in agriculture to neutralise acidic soils. Ammonium salts are used as fertilisers, produced by reacting ammonia with acids.
酸和碱广泛使用。胃里的盐酸帮助消化。柠檬酸存在于柑橘类水果中。醋含有乙酸。碱如氢氧化钠用于制皂和疏通剂。氢氧化钙用于农业中和酸性土壤。铵盐通过氨与酸反应制成，用作肥料。
11. Strong and Weak Acids | 强酸和弱酸
A strong acid completely ionises in water, giving a high concentration of H⁺ ions. Examples: HCl, H₂SO₄, HNO₃. A weak acid partially ionises, so at the same concentration it has a lower concentration of H⁺ ions and a higher pH than a strong acid. Examples: ethanoic acid (CH₃COOH), carbonic acid (H₂CO₃). This concept explains differences in reactivity and electrical conductivity.
强酸在水中完全电离，给出高浓度的H⁺离子。例子：HCl，H₂SO₄，HNO₃。弱酸仅部分电离，因此在相同浓度下，H⁺离子浓度较低，pH值比强酸高。例子：乙酸（CH₃COOH），碳酸（H₂CO₃）。这一概念解释了反应活性和导电性的差异。
12. Summary and Key Points | 总结与要点
To excel in this topic, students must memorise the general equations, solubility rules, and the preparation methods for salts. Practical skills such as filtration, crystallisation, and titration are also assessed. Regular practice of balancing and writing ionic equations is recommended.
要在本主题中取得优异成绩，学生必须记住一般反应方程式、溶解性规律以及盐的制备方法。过滤、结晶和滴定等实验技能也会被评估。建议定期练习配平方程式和书写离子方程式。
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