📚 Acids, Bases and Salts | 酸、碱和盐
Acids, bases and salts are fundamental to understanding chemical reactions in IGCSE Edexcel Science. Their behaviour, reactions and applications appear throughout the syllabus, linking laboratory chemistry to the world around us. From the sour taste of citrus fruits to the effectiveness of indigestion remedies, these substances explain a vast range of phenomena.
酸、碱和盐是理解IGCSE Edexcel科学中化学反应的基础。它们的性质、反应和应用贯穿整个课程大纲，将实验室化学与周围世界联系起来。从柑橘类水果的酸味到消化不良药物的有效性，这些物质解释了大量现象。
1. Defining Acids and Bases | 定义酸和碱
An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. The more H⁺ ions produced in solution, the more acidic the solution. Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).
酸是一种溶于水时释放氢离子（H⁺）的物质。溶液中产生的氢离子越多，酸性越强。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. A soluble base is called an alkali, which releases hydroxide ions (OH⁻) in water. Examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH). Metal oxides and metal carbonates are also bases, though many are insoluble.
碱是任何能中和酸生成盐和水的物质。可溶性碱称为碱，在水中释放氢氧根离子（OH⁻）。例子包括氢氧化钠（NaOH）和氢氧化钾（KOH）。金属氧化物和金属碳酸盐也是碱，尽管许多不溶于水。
2. The pH Scale | pH标度
The pH scale measures the acidity or alkalinity of an aqueous solution. It ranges from 0 (strongly acidic) to 14 (strongly alkaline), with 7 being neutral. Universal indicator or a pH meter can be used to determine the pH value.
pH标度用于测量水溶液的酸碱度，范围从0（强酸性）到14（强碱性），7为中性。通用指示剂或pH计可用于测定pH值。
A solution with pH less than 7 has a higher concentration of H⁺ ions than OH⁻ ions. A solution with pH greater than 7 has more OH⁻ ions. Pure water at 25 °C has a pH of exactly 7, as H⁺ and OH⁻ concentrations are equal.
pH小于7的溶液中H⁺浓度高于OH⁻浓度。pH大于7的溶液含有更多OH⁻离子。25°C时纯水的pH值为精确的7，因为H⁺和OH⁻浓度相等。
| pH range / pH范围 | Description / 描述 | Example / 示例 |
|---|---|---|
| 0–2 | Strongly acidic / 强酸性 | Battery acid, stomach acid / 电池酸液、胃酸 |
| 3–4 | Weakly acidic / 弱酸性 | Vinegar, lemon juice / 醋、柠檬汁 |
| 5–6 | Very weakly acidic / 极弱酸性 | Rainwater (unpolluted) / 雨水（未污染） |
| 7 | Neutral / 中性 | Pure water / 纯水 |
| 8–9 | Very weakly alkaline / 极弱碱性 | Baking soda solution / 小苏打溶液 |
| 10–11 | Weakly alkaline / 弱碱性 | Soap / 肥皂 |
| 12–14 | Strongly alkaline / 强碱性 | Oven cleaner, bleach / 烤箱清洁剂、漂白剂 |
3. Strong and Weak Acids | 强酸与弱酸
tric acid (HNO₃) are typical strong acids. For example, HCl → H⁺ + Cl⁻ goes to completion.
强酸是在水中完全电离、释放全部H⁺离子的酸。盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）是典型的强酸。例如，HCl → H⁺ + Cl⁻ 反应进行到底。
A weak acid only partially dissociates in water, existing as an equilibrium mixture of molecules and ions. Ethanoic acid (CH₃COOH) in vinegar is a weak acid. Its dissociation is reversible: CH₃COOH ⇌ H⁺ + CH₃COO⁻.
弱酸仅部分电离，以分子和离子的平衡混合物存在。醋中的乙酸（CH₃COOH）是一种弱酸。其电离是可逆的：CH₃COOH ⇌ H⁺ + CH₃COO⁻。
The strength of an acid is different from its concentration. Concentration refers to the amount of acid dissolved, while strength refers to the degree of dissociation. A dilute strong acid may have a higher pH than a concentrated weak acid, but the strong acid will still have a lower pH for the same concentration.
酸的强度不同于浓度。浓度指溶解的酸的量，而强度指电离程度。稀释的强酸可能比浓的弱酸pH更高，但对于相同浓度，强酸仍具有更低的pH。
4. Characteristic Reactions of Acids | 酸的典型反应
2HCl(aq) → MgCl₂(aq) + H₂(g). Bubbling hydrogen can be tested with a lit splint causing a 'squeaky pop'.
酸与金属活动顺序中氢以上的金属反应，生成盐和氢气。一般方程式为：酸 + 金属 → 盐 + 氢气。例如，Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)。氢气可用点燃的木条检验，会发出“爆鸣声”。
uSO₄(aq) + H₂O(l). Metal carbonates react with acids to give a salt, water and carbon dioxide. Carbon dioxide turns limewater cloudy.
酸与金属氧化物和金属氢氧化物中和，仅生成盐和水：酸 + 金属氧化物 → 盐 + 水。例如，CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)。金属碳酸盐与酸反应生成盐、水和二氧化碳。二氧化碳使石灰水变浑浊。
- Metal carbonate + acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
- 金属碳酸盐 + 酸: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
5. Bases and Alkalis | 碱与碱
All alkalis are bases, but not all bases are alkalis. A base is any proton (H⁺) acceptor, whereas an alkali is a specific type of base that dissolves in water to produce OH⁻ ions. Common alkalis include NaOH, KOH and aqueous ammonia (NH₃ solution).
所有碱溶液都是碱，但并非所有碱都是碱溶液。碱是任何质子（H⁺）接受体，而碱是一种特殊的碱，溶于水产生OH⁻离子。常见的碱包括NaOH、KOH和氨水（NH₃溶液）。
Insoluble bases like copper(II) oxide (CuO) and zinc oxide (ZnO) do not produce OH⁻ ions directly but can still neutralise acids. This distinction is important when choosing reagents for salt preparation.
不溶性碱如氧化铜（CuO）和氧化锌（ZnO）不会直接产生OH⁻离子，但仍能中和酸。在选择制备盐的试剂时，这种区别很重要。
6. Neutralisation as an Ionic Equation | 中和反应的离子方程式
O(l). Spectator ions, such as Na⁺ and Cl⁻ from NaOH and HCl, remain unchanged in solution and are omitted from the ionic equation.
酸和碱之间的中和反应可用净离子方程式表示：H⁺(aq) + OH⁻(aq) → H₂O(l)。旁观离子，如NaOH和HCl中的Na⁺和Cl⁻，在溶液中保持不变，在离子方程式中被省略。
This equation highlights that the essential chemical change is the formation of water from hydrogen ions and hydroxide ions. The same ionic equation holds for any strong acid–strong alkali neutralisation, making it a universal description.
该方程式强调，关键的化学变化是氢离子和氢氧根离子结合生成水。任何强酸-强碱的中和反应都适用相同的离子方程式，使其成为一个普遍描述。
7. Preparation of Soluble Salts | 可溶性盐的制备
Salts are ionic compounds produced when the hydrogen of an acid is replaced by a metal or ammonium ion. Soluble salts can be prepared by reacting an acid with a suitable metal, metal oxide, hydroxide or carbonate, then crystallising the product.
盐是酸中的氢被金属或铵离子取代时产生的离子化合物。可溶性盐可通过酸与合适的金属、金属氧化物、氢氧化物或碳酸盐反应，然后结晶产物来制备。
₂O(l). Excess black CuO is filtered off, leaving a blue solution that yields blue crystals upon cooling.
一般方法涉及加入过量固体碱（确保酸完全反应），加热，过滤掉过量固体，蒸发滤液得到晶体。例如，制备硫酸铜晶体：CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)。过量的黑色CuO被滤掉，留下蓝色溶液，冷却后析出蓝色晶体。
8. Solubility Rules and Precipitation | 溶解度规则与沉淀反应
Not all salts are soluble. Key solubility rules are essential for predicting salt preparation methods. All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Most chlorides are soluble except silver chloride (AgCl) and lead(II) chloride (PbCl₂). Most sulfates are soluble except barium sulfate (BaSO₄), lead(II) sulfate (PbSO₄) and calcium sulfate (CaSO₄ is slightly soluble). Most carbonates are insoluble except those of Group 1 and ammonium.
并非所有盐都可溶。关键溶解度规则对于预测盐的制备方法至关重要。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，除了氯化银（AgCl）和氯化铅（PbCl₂）。大多数硫酸盐可溶，除了硫酸钡（BaSO₄）、硫酸铅（PbSO₄）和硫酸钙（CaSO₄微溶）。大多数碳酸盐不溶，除了第1族和铵的碳酸盐。
+ Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). The white precipitate is filtered, washed and dried.
不溶性盐通过沉淀法制备，将含有所需离子的两种可溶盐溶液混合。例如，硫酸钡（用于X射线成像）通过混合氯化钡溶液和硫酸钠溶液制得：BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)。白色沉淀经过滤、洗涤和干燥。
9. Titration for Soluble Salts of Alkalis | 碱溶液制备可溶盐的滴定法
When both acid and alkali are soluble and the salt does not contain a solid base residue, titration is the preferred method. For example, sodium chloride can be prepared from hydrochloric acid and sodium hydroxide solution. Because neither reactant can be used as an insoluble excess, we need a precise technique to reach the endpoint.
当酸和碱都可溶且盐中不含固体碱残留时，滴定是首选方法。例如，氯化钠可由盐酸和氢氧化钠溶液制备。由于两种反应物都不能作为不溶性过量物使用，我们需要精确的技术达到终点。
a pure salt solution. Evaporate to crystallise. The ionic equation remains H⁺ + OH⁻ → H₂O.
步骤：将酸装入滴定管，向烧瓶中加入一定量的碱和指示剂（如酚酞）。滴加酸直至指示剂刚变色。记录所用体积。重复实验，不加指示剂，使用相同体积以获得纯盐溶液。蒸发结晶。离子方程式仍为H⁺ + OH⁻ → H₂O。
10. Acids and Bases in Everyday Life | 日常生活中的酸和碱
Acids are present in many foods: citric acid in citrus fruits, lactic acid in sour milk, and ethanoic acid in vinegar. Hydrochloric acid in the stomach aids digestion by breaking down proteins and killing bacteria.
许多食物中含有酸：柑橘类水果中的柠檬酸、酸牛奶中的乳酸以及醋中的乙酸。胃中的盐酸通过分解蛋白质和杀死细菌来帮助消化。
Alkalis are commonly used in cleaning products. Sodium hydroxide is a powerful oven cleaner, while ammonia solution is found in household glass and floor cleaners. Antacid tablets contain bases such as magnesium hydroxide or calcium carbonate to neutralise excess stomach acid and relieve indigestion.
碱常用于清洁产品中。氢氧化钠是强效烤箱清洁剂，而氨水存在于家用玻璃和地板清洁剂中。抗酸片含有氢氧化镁或碳酸钙等碱，用于中和过多的胃酸、缓解消化不良。
11. Acid Rain and Environmental Impact | 酸雨及其环境影响
Acid rain is caused by the dissolution of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) in rainwater. These gases come from the combustion of fossil fuels in power stations and vehicle engines. The resulting rain can have a pH below 5.6, harming aquatic life, vegetation and even buildings.
酸雨是由二氧化硫（SO₂）和氮氧化物（NOₓ）溶解在雨水中引起的。这些气体来自发电站和车辆发动机中化石燃料的燃烧。形成的雨水pH值可低于5.6，危害水生生物、植物甚至建筑物。
id), then oxidised to H₂SO₄. Similarly, NO₂ forms nitric acid (HNO₃). Reducing SO₂ emissions via flue gas desulfurisation and using catalytic converters in cars can mitigate acid rain.
形成过程涉及进一步氧化：SO₂ + H₂O → H₂SO₃（亚硫酸），然后被氧化为H₂SO₄。同样，NO₂形成硝酸（HNO₃）。通过烟气脱硫减少SO₂排放以及在汽车上使用催化转化器可以减轻酸雨。
12. Summary of Key Principles | 关键原理总结
Salt preparation depends on solubility rules and whether reactants can provide an insoluble excess. Real-world applications span from industrial processes to biological systems and environmental chemistry.
酸是质子给体，产生H⁺离子；碱是可溶性质子受体，产生OH⁻离子。中和反应生成水和盐，核心反应为H⁺ + OH⁻ → H₂O。盐的制备取决于溶解度规则以及反应物是否能提供不溶性过量物。实际应用涵盖工业过程、生物系统和环境化学。
Mastering the language of acids, bases and salts allows you to interpret laboratory procedures, predict reactions, and explain familiar phenomena—forming a crucial foundation for further study in chemistry.
掌握酸、碱和盐的语言使你能够解读实验室操作，预测反应，并解释熟悉的现象——这为进一步学习化学奠定了关键基础。
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