📚 Acids and Bases | 酸与碱
Acids and bases are fundamental chemical substances that you encounter every day, from the citric acid in lemons to the alkaline cleaning products in your home. In OCR Science, understanding their properties, reactions, and the concept of pH is essential for describing a wide range of chemical processes. This revision guide covers all the key points you need to know, from definitions and indicators to neutralisation and salt preparation, ensuring you are fully prepared for your examination.
酸与碱是日常接触的基础化学物质，从柠檬中的柠檬酸到家中的碱性清洁产品。在 OCR 科学中，理解它们的性质、反应以及 pH 概念对于描述众多化学过程至关重要。本复习指南涵盖你需要掌握的全部关键考点，从定义和指示剂到中和反应和盐的制备，确保你为考试做好充分准备。
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. For example, hydrochloric acid (HCl) ionises completely in water: HCl → H⁺ + Cl⁻. A base is a substance that can neutralise an acid, producing a salt and water. Many bases are metal oxides or hydroxides that release hydroxide ions (OH⁻) in water, such as sodium hydroxide (NaOH). Alkalis are soluble bases that release OH⁻ ions in solution.
酸是溶于水时释放氢离子（H⁺）的物质。例如，盐酸（HCl）在水中完全电离：HCl → H⁺ + Cl⁻。碱是能中和酸并生成盐和水的物质。许多碱是金属氧化物或氢氧化物，它们在水中释放氢氧根离子（OH⁻），例如氢氧化钠（NaOH）。可溶性碱在溶液中释放 OH⁻ 离子，被称为碱溶液。
In the OCR specification, it is important to remember that the term ‘alkali’ refers specifically to a soluble base, whereas ‘base’ is a broader term. Common alkalis include sodium hydroxide, potassium hydroxide, and calcium hydroxide (limewater).
在 OCR 大纲中，重要的是记住“alkali”特指可溶性碱，而“base”是更广义的术语。常见的碱溶液包括氢氧化钠、氢氧化钾和氢氧化钙（石灰水）。
2. The pH Scale and Its Measurement | pH 标度及其测量
The pH scale ranges from 0 to 14 and measures the acidity or alkalinity of a solution. A pH less than 7 indicates an acidic solution; the lower the pH, the higher the concentration of H⁺ ions. A pH greater than 7 indicates an alkaline solution, meaning there are more OH⁻ ions relative to H⁺ ions. A pH of 7 is neutral, where the concentrations of H⁺ and OH⁻ are equal, as in pure water.
pH 标度范围从 0 到 14，用于测量溶液的酸碱度。pH 小于 7 表示酸性溶液；pH 越低，H⁺ 离子浓度越高。pH 大于 7 表示碱性溶液，即相对于 H⁺ 离子，OH⁻ 离子更多。pH 为 7 是中性，此时 H⁺ 和 OH⁻ 浓度相等，例如纯水。
The pH of a solution can be measured using universal indicator or a pH probe. A pH probe connected to a data logger provides a precise numerical value, while universal indicator gives a colour change that can be matched to a pH colour chart. Litmus paper only distinguishes acid (red) from alkali (blue), but does not give a pH value.
溶液的 pH 可使用通用指示剂或 pH 探头测量。连接到数据记录器的 pH 探头提供精确的数值，而通用指示剂则呈现颜色变化，可与 pH 色卡比对。石蕊试纸只能区分酸（红）和碱（蓝），但不能给出具体的 pH 值。
| Substance | Approximate pH | Classification |
|---|---|---|
| Stomach acid | 1.0 – 2.0 | Strong acid |
| Lemon juice | 2.2 – 2.4 | Weak acid |
| Vinegar | 2.4 – 3.4 | Weak acid |
| Pure water | 7 | Neutral |
| Blood | 7.35 – 7.45 | Weakly alkaline |
| Baking soda solution | 8.3 | Weak alkali |
| Household ammonia | 11 – 12 | Strong alkali |
| Sodium hydroxide (1 mol/dm³) | 14 | Strong alkali |
Note that the pH scale is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value. Thus a solution of pH 3 is ten times more acidic than pH 4, and a hundred times more acidic than pH 5.
注意 pH 标度是对数尺度，这意味着 pH 值每降低 1，酸性增强 10 倍。因此 pH 3 的溶液比 pH 4 的酸性强 10 倍，比 pH 5 的酸性强 100 倍。
3. Indicators and Their Colour Changes | 指示剂及其颜色变化
Indicators are substances that change colour depending on the pH of the solution. Litmus, phenolphthalein, and methyl orange are commonly used in titrations. Universal indicator is a mixture of several indicators that produces a range of colours across the pH scale, making it ideal for estimating pH values.
指示剂是根据溶液 pH 改变颜色的物质。石蕊、酚酞和甲基橙常用于滴定。通用指示剂是多种指示剂的混合物，在 pH 标度上呈现一系列颜色，适合用于估计溶液 pH 值。
| Indicator | Colour in acid | Colour in neutral | Colour in alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
Phenolphthalein is especially useful for strong acid–strong base titrations because it changes colour sharply at the equivalence point (around pH 8–10). Methyl orange is better for strong acid–weak base titrations, changing colour between pH 3.1 and 4.4.
酚酞特别适合强酸–强碱滴定，因为它在等当点附近（pH 约 8–10）发生敏锐的颜色变化。甲基橙更适合强酸–弱碱滴定，在 pH 3.1 至 4.4 之间变色。
4. Hydrogen Ions and Hydroxide Ions | 氢离子与氢氧根离子
Acidity is directly related to the concentration of hydrogen ions, H⁺(aq). In water, H⁺ ions are usually represented as H₃O⁺ (hydronium ions), but for simplicity the OCR specification uses H⁺. A higher concentration of H⁺ ions results in a lower pH. Conversely, alkalinity is due to the presence of hydroxide ions, OH⁻. The higher the concentration of OH⁻, the higher the pH.
酸性直接与氢离子 H⁺(aq) 的浓度相关。在水中，H⁺ 离子通常表示为 H₃O⁺（水合氢离子），但为简化起见，OCR 大纲使用 H⁺。H⁺ 离子浓度越高，pH 越低。相反，碱性则由氢氧根离子 OH⁻ 引起。OH⁻ 浓度越高，pH 越高。
In pure water, a tiny proportion of water molecules dissociate: H₂O ⇌ H⁺(aq) + OH⁻(aq). At 25°C, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol/dm³. This equilibrium is the basis of the neutral pH of 7. Adding an acid increases [H⁺] and reduces [OH⁻] through the shift of equilibrium, thus lowering the pH.
在纯水中，极少量水分子发生电离：H₂O ⇌ H⁺(aq) + OH⁻(aq)。在 25°C 下，[H⁺] = [OH⁻] = 1.0 × 10⁻⁷ mol/dm³。这一平衡是中性 pH 为 7 的基础。加入酸会增加 [H⁺]，并通过平衡移动降低 [OH⁻]，从而降低 pH。
5. Strong and Weak Acids | 强酸与弱酸
A strong acid is one that completely ionises in aqueous solution, releasing all its hydrogen ions. Examples are hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). The reaction HCl → H⁺ + Cl⁻ goes to completion. Consequently, a 1 mol/dm³ solution of a strong acid has a very high concentration of H⁺ ions and a low pH (typically 0–1).
强酸是在水溶液中完全电离、释放全部氢离子的酸。例子包括盐酸 (HCl)、硫酸 (H₂SO₄) 和硝酸 (HNO₃)。反应 HCl → H⁺ + Cl⁻ 进行完全。因此，1 mol/dm³ 的强酸溶液中 H⁺ 浓度很高，pH 很低（通常为 0–1）。
A weak acid only partially ionises in solution. For example, ethanoic acid (CH₃COOH) establishes an equilibrium: CH₃COOH ⇌ H⁺ + CH₃COO⁻. At any moment, only a small fraction of the acid molecules have donated protons. This results in a lower concentration of H⁺ ions than a strong acid of the same concentration, giving a higher pH (e.g., 1 mol/dm³ ethanoic acid has a pH around 2.4).
弱酸在溶液中仅部分电离。例如，乙酸（CH₃COOH）建立平衡：CH₃COOH ⇌ H⁺ + CH₃COO⁻。在任何时刻，只有一小部分酸分子释放了质子。这就使得 H⁺ 浓度低于相同浓度的强酸，从而具有较高的 pH（例如，1 mol/dm³ 乙酸的 pH 约为 2.4）。
OCR exams often ask you to compare the reaction rates of strong and weak acids with metals or carbonates. A strong acid reacts faster because of its higher concentration of H⁺ ions, even if the acid concentration is the same. The concept of ‘strength’ refers to degree of ionisation, not concentration. It is possible to have a concentrated weak acid and a dilute strong acid.
OCR 考试经常要求比较强酸和弱酸与金属或碳酸盐的反应速率。强酸反应更快，因为其 H⁺ 离子浓度更高，即使酸的标称浓度相同。“强度”概念指电离程度，而非浓度。完全可能有浓的弱酸和稀的强酸。
6. Bases and Alkalis | 碱与可溶性碱
A base is any substance that reacts with an acid to form a salt and water. Metal oxides (e.g., CuO, CaO) and metal hydroxides (e.g., NaOH, KOH) are typical bases. An alkali is a soluble base that releases OH⁻ ions in water. All alkalis are bases, but not all bases are alkalis. For instance, copper(II) oxide is a base but insoluble in water, so it is not an alkali.
碱是任何能与酸反应生成盐和水的物质。金属氧化物（如 CuO、CaO）和金属氢氧化物（如 NaOH、KOH）是典型的碱。可溶性碱是在水中释放 OH⁻ 离子的碱。所有可溶性碱都是碱，但并非所有碱都是可溶性碱。例如，氧化铜是一种碱，但不溶于水，因此不是可溶性碱。
Common alkalis you will meet include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂). Ammonia solution (NH₃(aq)) is also an alkali because it reacts with water to produce OH⁻ ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
你将遇到的常见可溶性碱包括氢氧化钠 (NaOH)、氢氧化钾 (KOH) 和氢氧化钙 (Ca(OH)₂)。氨水 (NH₃(aq)) 也是一种可溶性碱，因为它与水反应产生 OH⁻ 离子：NH₃ + H₂O ⇌ NH₄⁺ + OH⁻。
Just like acids, bases can be strong or weak. A strong base (e.g., NaOH) dissociates completely in water, while a weak base (e.g., NH₃) only partially ionises. Strong alkalis produce very high concentrations of OH⁻, leading to pH values close to 14.
和酸一样，碱也有强碱和弱碱之分。强碱（如 NaOH）在水中完全解离，而弱碱（如 NH₃）仅部分电离。强碱溶液产生很高的 OH⁻ 浓度，导致 pH 接近 14。
7. Neutralisation Reactions | 中和反应
Neutralisation is the reaction between an acid and a base to produce a salt and water. The essential ionic equation for neutralisation is:
中和是酸与碱反应生成盐和水的过程。中和的基本离子方程式为：
H⁺(aq) + OH⁻(aq) → H₂O(l)
This equation applies to any reaction between a strong acid and a strong alkali, where the spectator ions can be omitted. For example, the reaction between hydrochloric acid and sodium hydroxide:
此方程式适用于任何强酸与强碱之间的反应，其中旁观离子可被省略。例如，盐酸与氢氧化钠的反应：
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
However, if the base is a metal oxide or an insoluble base, the reaction requires heating. For instance, copper(II) oxide reacts with sulfuric acid:
然而，如果碱是金属氧化物或不溶性碱，则反应需要加热。例如，氧化铜与硫酸反应：
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
In neutralisation reactions with carbonates, a salt, water, and carbon dioxide are produced. The general equation is: acid + carbonate → salt + water + carbon dioxide. For example:
在与碳酸盐的中和反应中，会生成盐、水和二氧化碳。通式为：酸 + 碳酸盐 → 盐 + 水 + 二氧化碳。例如：
2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Remember to use the correct state symbols (s), (l), (g), and (aq) in equations, as they are often required in OCR examinations.
记住在方程式中使用正确的状态符号 (s)、(l)、(g) 和 (aq)，因为这在 OCR 考试中经常被要求。
8. Salts and Their Preparation | 盐及其制备方法
A salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion (NH₄⁺). The name of the salt comes from the acid and the base used: hydrochloric acid produces chlorides, sulfuric acid produces sulfates, and nitric acid produces nitrates.
盐是酸的氢离子被金属离子或铵离子 (NH₄⁺) 取代时形成的离子化合物。盐的名称来源于所用的酸和碱：盐酸生成氯化物，硫酸生成硫酸盐，硝酸生成硝酸盐。
There are four main methods for preparing salts in the laboratory, depending on whether the salt is soluble or insoluble and whether the base is soluble. The methods are:
依据盐是可溶还是不可溶以及碱是否可溶，实验室制备盐主要有四种方法：
- Soluble salt from a soluble base (alkali): Titration is used. Exact volumes of acid and alkali are mixed to avoid excess, and the solution is evaporated to crystallise the salt.
可溶碱制备可溶盐：使用滴定法。精确混合酸与碱的体积以避免过量，然后将溶液蒸发结晶得到盐。 - Soluble salt from an insoluble base or metal: Add excess base/metal to acid, filter to remove the excess solid, and then evaporate the filtrate. For example, making copper(II) sulfate from CuO and H₂SO₄.
不溶性碱或金属制备可溶盐：向酸中加入过量的碱/金属，过滤除去过量固体，然后蒸发滤液。例如，由 CuO 和 H₂SO₄ 制备硫酸铜。 - Insoluble salt by precipitation: Mix solutions of two soluble salts that contain the required ions. The insoluble salt precipitates and is separated by filtration, washed, and dried. E.g., BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
沉淀法制备不溶性盐：将含有所需离子的两种可溶性盐溶液混合。不溶性盐沉淀后通过过滤分离、洗涤、干燥。例如，BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)。 - Soluble ammonium salts: React ammonia solution with an acid via titration, as ammonium hydroxide is a weak alkali.
可溶性铵盐：使用滴定法使氨水与酸反应，因为氢氧化铵是弱碱。
During crystallisation, the solution is heated to evaporate some of the water and then left to cool slowly. This allows large, well-defined crystals to form. Rapid cooling or over-evaporation can lead to small crystals or anhydrous powder, which may lose marks in a practical examination.
结晶时，将溶液加热蒸发掉一部分水，然后让其缓慢冷却。这样可形成大而完整的晶体。快速冷却或过度蒸发会导致小晶体或无水晶末，在实验考试中可能会失分。
9. Everyday Applications of Acids and Bases | 酸与碱的日常应用
Acids and bases play a crucial role in daily life and industry. Dilute acids are found in foodstuffs (citric acid, acetic acid) and in car batteries (sulfuric acid). Alkalis are used in cleaning products (ammonia, sodium hydroxide) and in agriculture (lime to neutralise acidic soil). Understanding pH control is also vital in biology, for example, in maintaining blood pH around 7.4.
酸和碱在日常生活中和工业中起着至关重要的作用。稀酸存在于食物（柠檬酸、醋酸）和汽车电池（硫酸）中。可溶性碱用于清洁产品（氨水、氢氧化钠）和农业（石灰用于中和酸性土壤）。理解 pH 控制对生物学也至关重要，例如维持血液 pH 在 7.4 左右。
Indigestion remedies often contain bases such as magnesium hydroxide or calcium carbonate to neutralise excess stomach acid (HCl). The neutralisation reaction between antacid tablets and stomach acid relieves discomfort and is a classic example of a base–acid reaction in the human body.
消化不良药物常含有碱，如氢氧化镁或碳酸钙，以中和多余胃酸（HCl）。抗酸药片与胃酸之间的中和反应能缓解不适，是人体内碱–酸反应的典型例子。
In the environment, acid rain caused by dissolved sulfur dioxide and nitrogen oxides can damage buildings and harm aquatic life. Limestone (calcium carbonate) is used to neutralise acidic lakes and soils, demonstrating large-scale application of neutralisation.
在环境中，由溶解的二氧化硫和氮氧化物引起的酸雨会损坏建筑物并危害水生生物。石灰石（碳酸钙）被用来中和酸性湖泊和土壤，展示了中和反应的大规模应用。
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
When answering questions about acids and bases in OCR exams, precision with terminology is essential. Many students confuse ‘strong’ with ‘concentrated’. Remember: ‘strong’ means complete ionisation; ‘concentrated’ means containing a large amount of solute in a given volume. A dilute strong acid still ionises fully but contains fewer H⁺ ions per unit volume.
在 OCR 考试中回答有关酸与碱的问题时，术语的准确性至关重要。许多学生混淆“强”和“浓”。记住：“强”表示完全电离；“浓”表示在一定体积内含有大量溶质。稀的强酸仍然完全电离，但单位体积内含有的 H⁺ 离子较少。
Another common mistake is failing to use the correct ionic equation for neutralisation. In a reaction between a strong acid and a strong base, the ionic equation is simply H⁺ + OH⁻ → H₂O. Do not include spectator ions like Na⁺ or Cl⁻ unless specifically asked for a full equation.
另一个常见错误是未使用正确的中和离子方程式。在强酸与强碱的反应中，离子方程式简化为 H⁺ + OH⁻ → H₂O。除非特别要求写完整方程式，否则不要加入 Na⁺ 或 Cl⁻ 等旁观离子。
In practical-based questions, be specific about the steps for making a salt. Stating ‘mix acid and alkali’ is insufficient. You should mention using a pipette and burette for titration, adding indicator, swirling, and then repeating without indicator after rough volumes are known. Crystallisation details (gentle heating, cooling) can earn extra marks.
在实践类问题中，要具体说明制备盐的步骤。仅说“混合酸碱”是不够的。你应该提到使用移液管和滴定管进行滴定，加入指示剂，旋摇混合，在粗测体积后不加指示剂重复滴定。结晶细节（缓慢加热、冷却）可为你赢得额外分数。
Finally, when explaining pH curves or indicator choice, link the indicator’s pH range to the titration type. For a strong acid–strong base titration, phenolphthalein is suitable because the pH change at equivalence is steep and covers its colour change range (8.2–10.0). Methyl orange is unsuitable because its colour change occurs at a pH below 7, well before the equivalence point.
最后，在解释 pH 曲线或指示剂选择时，要将指示剂的 pH 范围与滴定类型联系起来。对于强酸–强碱滴定，酚酞是合适的，因为等当点处的 pH 变化陡峭，且覆盖了其变色范围（8.2–10.0）。甲基橙不合适，因为它变色的 pH 低于 7，远早于等当点。
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