📚 626: The pH Scale and Neutralisation | 626：pH标度与中和反应
The pH scale is a universal measure used in chemistry to describe how acidic or alkaline a solution is. Neutralisation is the reaction between an acid and a base, producing salt and water. These concepts are central to the Edexcel IGCSE Science curriculum and appear frequently in exam questions.
pH标度是化学中用于描述溶液酸碱性的通用量度。中和反应是酸与碱之间生成盐和水的反应。这些概念是Edexcel IGCSE科学课程的核心，也是考试题目中的常客。
1. Defining Acids and Bases | 酸和碱的定义
An acid is a substance that donates hydrogen ions (H⁺) when dissolved in water. A base is a substance that accepts hydrogen ions, often producing hydroxide ions (OH⁻) in solution. For example, hydrochloric acid produces H⁺ and Cl⁻ ions, while sodium hydroxide produces Na⁺ and OH⁻ ions.
酸是溶于水时能提供氢离子（H⁺）的物质。碱是能接受氢离子的物质，在水中常产生氢氧根离子（OH⁻）。例如，盐酸产生H⁺和Cl⁻离子，而氢氧化钠产生Na⁺和OH⁻离子。
- Acids taste sour, turn blue litmus red, and have pH values below 7.
- 酸碱起来发酸，使蓝色石蕊变红，pH值低于7。
- Bases taste bitter, feel slippery, turn red litmus blue, and have pH values above 7.
- 碱尝起来发苦，摸起来滑腻，使红色石蕊变蓝，pH值高于7。
- Alkalis are bases that dissolve in water to form a solution containing OH⁻ ions.
- 可溶性碱是能溶于水并产生OH⁻离子的碱。
2. The pH Scale | pH标度
The pH scale runs from 0 to 14. A pH of 7 is neutral. Values below 7 indicate increasing acidity, and values above 7 indicate increasing alkalinity. The scale is logarithmic: each whole number change represents a ten-fold change in hydrogen ion concentration.
pH标度的范围是0到14。pH为7时呈中性。低于7表示酸性增强，高于7表示碱性增强。该标度是对数的：每改变一个整数，氢离子浓度变化十倍。
pH = −log₁₀[H⁺]
For example, a solution with pH 2 has ten times more H⁺ ions than a solution with pH 3, and one hundred times more than a solution with pH 4.
例如，pH为2的溶液的H⁺离子浓度是pH为3的溶液的十倍，是pH为4的溶液的一百倍。
3. Indicators | 指示剂
Indicators are substances that change colour depending on the pH of their environment. Common indicators include litmus, methyl orange, phenolphthalein, and universal indicator. Universal indicator is a mixture of indicators that produces a continuous colour range, allowing an approximate pH value to be read.
指示剂是根据环境pH改变颜色的物质。常见指示剂包括石蕊、甲基橙、酚酞和万能指示剂。万能指示剂是多种指示剂的混合物，能产生连续的颜色变化，从而读出近似的pH值。
| Indicator | Colour in acid | Colour in alkali |
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
In exams, students must be able to match the indicator colour change to a pH range and explain why a mixture is used in universal indicator.
在考试中，学生需要能将指示剂颜色变化与pH范围对应，并解释为什么万能指示剂使用混合物。
4. Strong and Weak Acids | 强酸与弱酸
Strong acids, such as hydrochloric acid, sulfuric acid, and nitric acid, ionise completely in water. Weak acids, such as ethanoic acid, citric acid, and carbonic acid, ionise only partially. The pH of a solution depends on the concentration of H⁺ ions, not on the total amount of acid molecules.
强酸如盐酸、硫酸和硝酸在水中完全电离。弱酸如乙酸、柠檬酸和碳酸仅部分电离。溶液的pH取决于H⁺离子的浓度，而不是酸分子的总量。
Strong acid: HA(aq) → H⁺(aq) + A⁻(aq) (complete ionisation)
Weak acid: HA(aq) ⇌ H⁺(aq) + A⁻(aq) (partial ionisation)
For the same concentration, a strong acid has a lower pH than a weak acid. However, a very dilute strong acid can have a higher pH than a concentrated weak acid.
在相同浓度下，强酸的pH低于弱酸。但极稀的强酸可能比浓的弱酸具有更高的pH。
5. Neutralisation Reactions | 中和反应
Neutralisation occurs when an acid reacts with a base to form a salt and water. The ionic equation is simple: H⁺(aq) + OH⁻(aq) → H₂O(l). Acid plus alkali always produces salt plus water.
中和反应是酸与碱反应生成盐和水的过程。离子方程式很简单：H⁺(aq) + OH⁻(aq) → H₂O(l)。酸加可溶性碱总是生成盐和水。
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
In this equation, the salt is sodium chloride. Different acids and bases produce different salts. For example, sulfuric acid with potassium hydroxide produces potassium sulfate.
在这个方程中，生成的盐是氯化钠。不同的酸和碱生成不同的盐。例如，硫酸与氢氧化钾反应生成硫酸钾。
- Hydrochloric acid forms chlorides.
- 盐酸生成氯化物。
- Nitric acid forms nitrates.
- 硝酸生成硝酸盐。
- Sulfuric acid forms sulfates.
- 硫酸生成硫酸盐。
6. Reactions of Acids with Metals | 酸与金属的反应
Acids react with certain metals to produce a salt and hydrogen gas. For example, magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen. The test for hydrogen gas is that it burns with a squeaky pop.
酸与某些金属反应生成盐和氢气。例如，镁与稀盐酸反应生成氯化镁和氢气。氢气的检验方法是使其燃烧并听到“爆鸣”声。
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Only metals above hydrogen in the reactivity series can react with acids. Copper and silver do not react with dilute acids because they are less reactive than hydrogen.
只有活动性顺序中位于氢之前的金属才能与酸反应。铜和银不如氢活泼，因此不与稀酸反应。
Reaction speed increases with metal reactivity. Potassium and sodium are dangerously violent, while zinc reacts moderately. This explains why acids are stored in glass containers and handled with care.
反应速率随金属活动性增加而加快。钾和钠反应剧烈危险，锌反应适中。这解释了为什么酸需存放在玻璃容器中并小心处理。
7. Reactions of Acids with Carbonates | 酸与碳酸盐的反应
Acids react with carbonates and hydrogencarbonates to produce a salt, water, and carbon dioxide. The carbon dioxide gas can be tested by bubbling it through limewater, which turns cloudy.
酸与碳酸盐和碳酸氢盐反应生成盐、水和二氧化碳。二氧化碳气体可通过通入石灰水来检验，石灰水会变浑浊。
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)
These reactions are often used in the laboratory to prepare carbon dioxide and to remove acid spills on limestone surfaces. Fizzing or effervescence is a clear sign that a carbonate is reacting.
这些反应常用于实验室制备二氧化碳，也用于处理石灰岩表面的酸溢出。冒泡或嘶嘶声是碳酸盐正在反应的明显证据。
8. Acid–Base Titration | 酸碱滴定
Titration is a technique used to determine the exact volume of an acid needed to neutralise a known volume of alkali, or vice versa. A burette delivers the acid, a pipette measures the alkali, and an indicator shows the end point.
滴定是用于确定中和已知体积碱所需的酸的确切体积（或反过来）的技术。酸式滴定管用于滴加酸，移液管用于量取碱，指示剂指示终点。
- Rinse the burette with the acid solution.
- 用酸溶液润洗滴定管。
- Use a pipette to transfer 25.0 cm³ of alkali into a conical flask.
- 用移液管量取25.0 cm³碱液注入锥形瓶。
- Add 2–3 drops of a suitable indicator.
- 加入2–3滴合适的指示剂。
- Add acid from the burette until the colour changes permanently.
- 从滴定管中滴加酸，直至颜色持续改变。
- Record the initial and final burette readings to find the titre.
- 记录初始和最终读数以得到滴定体积。
Phenolphthalein is commonly used because its colourless-to-pink change is sharp. Methyl orange is also suitable, changing from yellow to red.
酚酞常用，因其从无色到粉色的变化非常敏锐。甲基橙也适用，从黄色变为红色。
9. Everyday Applications | 日常应用
Neutralisation is vital in agriculture and medicine. Farmers add lime (calcium oxide or calcium hydroxide) to acidic soil to increase its pH and improve crop growth. Indigestion remedies contain weak bases such as magnesium hydroxide to neutralise excess stomach acid.
中和在农业和医学中至关重要。农民向酸性土壤中添加石灰（氧化钙或氢氧化钙）来提高pH并促进作物生长。胃药含有弱碱如氢氧化镁，用于中和多余的胃酸。
Other applications include treating bee stings with vinegar (acid) or wasp stings with baking soda (alkali). It is important to identify the type of sting first: bee venom is acidic, while wasp venom is alkaline.
其他应用包括用醋（酸）处理蜜蜂蜇伤，或用小苏打（碱）处理黄蜂蜇伤。关键是先确定蜇伤类型：蜂毒是酸性的，而黄蜂毒是碱性的。
Industrial effluents are also neutralised before release into rivers. Wastewater containing acids is treated with alkalis to prevent environmental damage and protect aquatic life.
工业废水排放前也要进行中和。含酸废水用碱处理，以防止环境破坏并保护水生生物。
10. Common Exam Mistakes | 常见考试错误
Many students lose marks by confusing terms such as “strong” and “concentrated”. A strong acid is fully ionised, while a concentrated acid has many moles of acid per litre. These are independent properties.
许多学生因混淆“强酸”和“浓酸”而失分。强酸是完全电离的，而浓酸是每升溶液中酸的总物质的量很大。这是两个独立的性质。
Another frequent error is writing the ionic equation for neutralisation incorrectly. Remember that water is the only product besides the salt in the overall reaction, and the net ionic equation does not include spectator ions.
另一个常见错误是写出错误的中和离子方程式。请记住，在总反应中除盐外只生成水，净离子方程不包括旁观离子。
Finally, students often forget to state the physical states in balanced equations. Always include (s), (aq), (l), or (g) when asked for full chemical equations.
最后，学生经常忘记在平衡方程中注明物理状态。当要求写完整化学方程式时，务必标注(s)、(aq)、(l)或(g)。
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