📚 Making Salts – Acids and Alkalis | 制盐——酸与碱
Making salts is one of the most practical topics in KS3 chemistry. When an acid reacts with an alkali, a metal or a carbonate, a salt is formed. Understanding how to choose the right starting materials, control the reaction and collect a pure dry salt is essential for both the laboratory and the exam.
制盐是 KS3 化学中最具实操性的主题之一。当酸与碱、金属或碳酸盐反应时，会生成盐。理解如何选择合适的反应物、控制反应并收集纯净干燥的盐，对实验操作和考试都至关重要。
1. Acids and Alkalis in the Lab | 实验室中的酸和碱
Acids are substances with a pH below 7. They release hydrogen ions, H⁺, when dissolved in water. Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Alkalis are soluble bases with a pH above 7. They release hydroxide ions, OH⁻, in water. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are typical alkalis.
酸是 pH 低于 7 的物质。它们溶于水时会释放氢离子 H⁺。实验室常见酸包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。碱是可溶性的碱，pH 高于 7，在水中释放氢氧根离子 OH⁻。氢氧化钠（NaOH）和氢氧化钾（KOH）是常见的碱。
Both concentrated acids and concentrated alkalis are corrosive. In school experiments, dilute solutions are used to reduce risk. A corrosive substance can burn skin and damage eyes, so safety goggles must always be worn.
浓酸和浓碱都具有腐蚀性。在学校实验中，使用稀溶液以降低风险。腐蚀性物质会灼伤皮肤并损伤眼睛，因此必须始终佩戴护目镜。
- Hydrochloric acid (HCl) — found in stomach acid, used in labs. 盐酸（HCl）——存在于胃酸中，实验室常用。
- Sulfuric acid (H₂SO₄) — used in car batteries and fertilisers. 硫酸（H₂SO₄）——用于汽车电池和化肥。
- Nitric acid (HNO₃) — used to make fertilisers and explosives. 硝酸（HNO₃）——用于制造化肥和炸药。
- Sodium hydroxide (NaOH) — used in soap and drain cleaners. 氢氧化钠（NaOH）——用于肥皂和下水道清洁剂。
2. pH Scale and Indicators | pH 标度与指示剂
The pH scale runs from 0 to 14. Values below 7 indicate an acidic solution, 7 is neutral, and values above 7 indicate an alkaline solution. The lower the pH, the more acidic the solution; the higher the pH, the more alkaline it is.
pH 标度范围是 0 到 14。数值低于 7 表示酸性溶液，7 为中性，高于 7 表示碱性溶液。pH 越低，溶液酸性越强；pH 越高，碱性越强。
Indicators are chemicals that change colour in acids and alkalis. Litmus turns red in acid and blue in alkali. Universal indicator is a mixture of indicators that shows a range of colours depending on pH, making it more useful for estimating pH than litmus.
指示剂是在酸和碱中会变色的化学物质。石蕊在酸中变红，在碱中变蓝。通用指示剂是多种指示剂的混合物，会根据 pH 显示不同颜色，因此比石蕊更适合估计 pH。
| Indicator | Colour in acid | Colour in neutral | Colour in alkali | 中文对照 |
|---|---|---|---|---|
| Litmus | Red | Purple | Blue | 石蕊：酸中红、中性紫、碱中蓝 |
| Phenolphthalein | Colourless | Colourless | Pink | 酚酞：酸中无色、碱中粉红 |
| Universal indicator | Red, orange or yellow | Green | Blue or purple | 通用指示剂：酸中红/橙/黄，中性绿，碱中蓝/紫 |
Universal indicator is often used on pH paper. Matching the colour to a pH chart allows us to record an approximate pH value, which is important when controlling neutralisation.
通用指示剂常用于 pH 试纸。将颜色与 pH 色卡对照，可以记录近似的 pH 值，这在控制中和反应时非常重要。
3. Neutralisation Reaction | 中和反应
Neutralisation is the reaction between an acid and a base or alkali. It is an exothermic reaction, meaning it releases heat. The products of a neutralisation between an acid and an alkali are always a salt and water.
中和是酸与碱或碱性物质之间的反应。它是放热反应，会释放热量。酸与碱中和的产物总是盐和水。
acid + alkali → salt + water
酸 + 碱 → 盐 + 水
At the ionic level, the hydrogen ions from the acid react with hydroxide ions from the alkali to form water. The remaining ions form the salt.
在离子层面上，酸中的氢离子与碱中的氢氧根离子反应生成水。剩余的离子形成盐。
H⁺ + OH⁻ → H₂O
氢离子 + 氢氧根离子 → 水
A common example is the reaction between hydrochloric acid and sodium hydroxide, which produces sodium chloride, also known as table salt.
一个常见的例子是盐酸与氢氧化钠反应，生成氯化钠，也就是食盐。
hydrochloric acid + sodium hydroxide → sodium chloride + water
盐酸 + 氢氧化钠 → 氯化钠 + 水
HCl + NaOH → NaCl + H₂O
HCl + NaOH → NaCl + H₂O
4. What Is a Salt? | 什么是盐？
In chemistry, a salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion. Salts are not only used in food; many are industrial chemicals, fertilisers and medicines.
在化学中，盐是酸中的氢离子被金属离子或铵根离子取代后形成的离子化合物。盐不仅用于食品，许多盐还是工业化学品、化肥和药物。
Sodium chloride (NaCl) is the most familiar salt, but there are many others, such as copper(II) sulfate (CuSO₄), magnesium chloride (MgCl₂) and potassium nitrate (KNO₃).
氯化钠（NaCl）是最熟悉的盐，但还有很多其他盐，例如硫酸铜（CuSO₄）、氯化镁（MgCl₂）和硝酸钾（KNO₃）。
A salt contains a positive ion from the base or metal and a negative ion from the acid. For example, in sodium chloride, the positive ion is Na⁺ and the negative ion is Cl⁻.
盐含有来自碱或金属的正离子和来自酸的负离子。例如，在氯化钠中，正离子是 Na⁺，负离子是 Cl⁻。
Salts can be soluble or insoluble. Most salts made from common acids are soluble in water, which allows them to be separated from solid impurities by filtration and then recovered by evaporation or crystallisation.
盐可以是可溶的或不可溶的。大多数由常见酸制得的盐可溶于水，因此可以通过过滤与固体杂质分离，再通过蒸发或结晶回收。
5. Naming Salts | 盐的命名
The name of a salt has two parts. The first part comes from the metal or base used, and the second part comes from the acid. This rule makes it easy to predict the name of the salt formed in a reaction.
盐的名称由两部分组成。第一部分来自所用的金属或碱，第二部分来自所用的酸。这个规则使预测反应中生成的盐的名称变得容易。
- Hydrochloric acid produces chloride salts. 盐酸生成氯化物盐。
- Sulfuric acid produces sulfate salts. 硫酸生成硫酸盐。
- Nitric acid produces nitrate salts. 硝酸生成硝酸盐。
For example, sodium hydroxide reacting with sulfuric acid gives sodium sulfate, not sodium sulfide or sodium sulfite. The ending must match the acid.
例如，氢氧化钠与硫酸反应生成硫酸钠，而不是硫化钠或亚硫酸钠。后缀必须与酸匹配。
sodium hydroxide + sulfuric acid → sodium sulfate + water
氢氧化钠 + 硫酸 → 硫酸钠 + 水
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
When a metal is used instead of an alkali, the metal still gives the first part of the salt name. Magnesium with hydrochloric acid makes magnesium chloride.
当使用金属代替碱时，金属仍然提供盐名称的第一部分。镁与盐酸反应生成氯化镁。
6. Making Salts: Acid + Alkali | 制盐：酸与碱
The reaction between an acid and an alkali is a neutralisation reaction. It can be used to make soluble salts of reactive metals, especially sodium, potassium and ammonium salts. However, because an alkali is soluble, an excess cannot be filtered off easily.
酸与碱之间的反应是中和反应。它可用于制备活泼金属的可溶性盐，特别是钠盐、钾盐和铵盐。但是，由于碱是可溶的，过量的碱不容易通过过滤除去。
The titration method is used to mix exactly the right amounts of acid and alkali. First, a known volume of acid is placed in a flask. A few drops of indicator are added. Alkali is then added slowly until the indicator changes colour, showing that neutralisation is complete.
滴定法用于精确混合适量的酸和碱。首先，将已知体积的酸放入锥形瓶中。加入几滴指示剂。然后缓慢加入碱，直到指示剂变色，表明中和完成。
The volume of alkali needed is recorded. The experiment is repeated without the indicator to avoid contaminating the salt. The neutral solution is then heated gently to evaporate most of the water, and the salt is left to crystallise.
记录所需碱的体积。然后在不加指示剂的情况下重复实验，以免污染盐。将中性溶液缓慢加热，蒸发掉大部分水，让盐结晶析出。
hydrochloric acid + potassium hydroxide → potassium chloride + water
盐酸 + 氢氧化钾 → 氯化钾 + 水
HCl + KOH → KCl + H₂O
HCl + KOH → KCl + H₂O
7. Making Salts: Acid + Metal | 制盐：酸与金属
Dilute acids react with certain metals to form a salt and hydrogen gas. The reaction is vigorous with reactive metals such as magnesium and zinc, but it does not occur with unreactive metals such as copper when using hydrochloric or sulfuric acid.
稀酸与某些金属反应生成盐和氢气。镁、锌等活泼金属反应剧烈，但铜等不活泼金属与盐酸或硫酸不发生反应。
acid + metal → salt + hydrogen
酸 + 金属 →
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