Acids, Bases and Salts | 酸、碱和盐

📚 Acids, Bases and Salts | 酸、碱和盐

Acids and bases are fundamental classes of chemical substances that play essential roles in industry, biology and everyday life. They are defined by their properties, behaviour in water, and the reactions they undergo. This article covers the core IGCSE Edexcel Science concepts: properties, pH, neutralisation, salt preparation, and the distinction between strong and weak acids.

酸和碱是化学物质中最基础的类别，在工业、生物和日常生活中都起着至关重要的作用。它们通过自身性质、在水中的行为以及所参与的反应来定义。本文覆盖爱德思IGCSE科学的核心概念：性质、pH值、中和反应、盐的制备，以及强酸与弱酸的区别。

1. What Are Acids? | 什么是酸？

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) dissociates to give H⁺(aq) and Cl⁻(aq). Acids have a sour taste, turn blue litmus red, and have a pH less than 7 at 25 °C.

酸是溶解在水中时能产生氢离子（H⁺）的物质。例如，盐酸（HCl）离解生成H⁺(aq)和Cl⁻(aq)。酸有酸味，能使蓝色石蕊变红，在25°C时pH值小于7。

Common laboratory acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). The formula of an acid reflects the number of hydrogen atoms that can be released as ions.

实验室常见的酸包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。酸的化学式反映了能作为离子释放的氢原子数目。

2. Properties of Acids | 酸的性质

Acids show a characteristic set of chemical properties. They react with metals to produce hydrogen gas and a salt, with bases to produce salt and water, and with carbonates to produce salt, water and carbon dioxide. These reactions are used to identify acids and to prepare salts.

酸表现出一组特征性的化学性质。它们与金属反应生成氢气和盐，与碱反应生成盐和水，与碳酸盐反应生成盐、水和二氧化碳。这些反应用于鉴别酸和制备盐。

Acids have a sour taste. (酸性物质有酸味。)
Acids turn blue litmus paper red. (酸使蓝色石蕊试纸变红。)
Acids have a pH below 7. (酸的pH值低于7。)
Acids conduct electricity in aqueous solution because of mobile ions. (酸的水溶液因含有可移动离子而能导电。)

It is important to note that concentrated acids are corrosive and need careful handling. Dilute acids are still irritants and should not be tasted or touched without appropriate safety equipment.

需要注意的是，浓酸具有腐蚀性，需要小心处理。稀酸仍然是刺激物，不应用嘴尝或用手触摸，除非有适当的安全设备。

3. Bases and Alkalis | 碱和碱液

A base is a substance that neutralises an acid to form salt and water only. Many bases are metal oxides and metal hydroxides. An alkali is a base that dissolves in water, releasing hydroxide ions (OH⁻) into the solution. For example, sodium hydroxide (NaOH) dissolves to give Na⁺(aq) and OH⁻(aq).

碱是仅与酸反应生成盐和水的物质。许多碱是金属氧化物和金属氢氧化物。碱液是能溶于水并释放氢氧根离子（OH⁻）的碱。例如，氢氧化钠（NaOH）溶解生成Na⁺(aq)和OH⁻(aq)。

Insoluble bases, such as copper(II) oxide (CuO), still react with acids, but they do not dissolve in water. Soluble bases are called alkalis; common examples include sodium hydroxide, potassium hydroxide and ammonia solution.

不溶性碱（如氧化铜CuO）仍能与酸反应，但不溶于水。可溶的碱称为碱液；常见例子包括氢氧化钠、氢氧化钾和氨水。

Bases have a bitter taste and feel slippery to the skin. They turn red litmus blue and have a pH greater than 7 if soluble.

碱有苦味，皮肤触感滑腻。它们使红色石蕊变蓝，若可溶则pH值大于7。

4. The pH Scale and Indicators | pH标度与指示剂

The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. The pH can be measured using universal indicator or a pH meter.

pH标度是衡量溶液酸碱性的量度，范围从0到14。pH为7时呈中性，低于7为酸性，高于7为碱性。pH可用广谱指示剂或pH计来测量。

Universal indicator changes to a continuous rainbow colour range: red/orange for strong acids, yellow/green for weak acids, blue/purple for strong alkalis. A pH meter gives a more precise numerical value.

广谱指示剂会呈现连续的彩虹色变化：强酸显示红/橙色，弱酸显示黄/绿色，强碱显示蓝/紫色。pH计能给出更精确的数值。

pH	0–2	3–6	7	8–11	12–14
Description	Strong acid	Weak acid	Neutral	Weak alkali	Strong alkali

Litmus paper is a simpler indicator: it only tells whether a substance is acid (red) or alkaline (blue), not the strength.

石蕊试纸是一种更简单的指示剂：只能判断物质是酸性（变红）还是碱性（变蓝），无法显示酸碱强度。

5. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base to form a salt and water. The ionic equation is:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The reaction is exothermic: it releases heat energy. For example, hydrochloric acid reacts with sodium hydroxide:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

中和反应是酸与碱生成盐和水的反应。离子方程式为：

H⁺(aq) + OH⁻(aq) → H₂O(l)

该反应为放热反应，会释放热能。例如，盐酸与氢氧化钠反应：

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

When the amount of acid and alkali are chosen so that the solution is exactly neutral, the resulting salt solution has a pH of 7. This is called the equivalence point. Titration is a technique used to determine the exact volume needed.

当酸和碱的量刚好使溶液完全中和时，所得盐溶液的pH为7，这称为等当点。滴定法就是用来测定所需精确体积的技术。

6. Acids Reacting with Metals | 酸与金属的反应

Acids react with most metals to produce a salt and hydrogen gas. The general word equation is:

acid + metal → salt + hydrogen

For example, zinc reacts with sulfuric acid:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

酸与大多数金属反应生成盐和氢气。一般的文字方程式为：

酸 + 金属 → 盐 + 氢气

例如，锌与硫酸反应：

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

The hydrogen gas can be tested by holding a lit splint near the mouth of the test tube; a squeaky pop sound confirms hydrogen. However, copper and silver do not react with dilute acids because they are below hydrogen in the reactivity series.

氢气可用带火星的木条靠近试管口检验，发出“噗”的爆鸣声即可确认。但铜和银不与稀酸反应，因为它们在金属活动性顺序中位于氢之后。

Only metals above hydrogen in the reactivity series react with dilute acids. (只有活动性顺序在氢之前的金属才与稀酸反应。)
Reactions are faster with more reactive metals. (金属越活泼，反应越快。)
Hydrogen gas is produced as bubbles. (产生的氢气以气泡形式逸出。)

7. Acids Reacting with Carbonates | 酸与碳酸盐的反应

Acids react with carbonates and hydrogencarbonates to produce a salt, water and carbon dioxide gas. The general equation is:

acid + carbonate → salt + water + carbon dioxide

For example, calcium carbonate reacts with hydrochloric acid:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

酸与碳酸盐和碳酸氢盐反应生成盐、水和二氧化碳气体。一般方程式为：

酸 + 碳酸盐 → 盐 + 水 + 二氧化碳

例如，碳酸钙与盐酸反应：

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

The carbon dioxide can be tested by bubbling the gas through limewater; it turns milky (cloudy) if present. This reaction explains why acid rain damages limestone buildings: the acid dissolves the calcium carbonate.

可用石灰水检验二氧化碳：将气体通入石灰水中，若变浑浊则确认存在。这个反应解释了酸雨会侵蚀石灰岩建筑的原因：酸会溶解碳酸钙。

8. Strong and Weak Acids | 强酸与弱酸

Acids can be classified as strong or weak depending on how completely they ionise in water. Strong acids, such as hydrochloric acid, nitric acid and sulfuric acid, fully dissociate into ions. Weak acids, such as ethanoic acid and citric acid, only partially ionise.

酸可按在水中电离的程度分为强酸和弱酸。强酸（如盐酸、硝酸和硫酸）完全离解成离子；弱酸（如乙酸和柠檬酸）仅部分电离。

Strong: HCl → H⁺ + Cl⁻ (complete)

Weak: CH₃COOH ⇌ H⁺ + CH₃COO⁻ (reversible)

The symbol ⇌ shows that weak acids establish an equilibrium between the un-ionised acid and its ions. A weak acid has a higher pH than a strong acid of the same concentration because it releases fewer H⁺ ions.

符号⇌表示弱酸在未电离的酸分子与其离子之间建立平衡。浓度相同时，弱酸的pH值比强酸高，因为它释放的H⁺离子较少。

Strong acids are strong electrolytes. (强酸是强电解质。)
Weak acids have lower electrical conductivity than strong acids at equal concentration. (等浓度下，弱酸的电导率低于强酸。)
All acids still react with metals, bases and carbonates, but weak acids react more slowly. (所有酸都能与金属、碱和碳酸盐反应，但弱酸反应更慢。)

9. Preparing Soluble Salts | 制备可溶性盐

Soluble salts can be prepared by reacting an acid with a suitable insoluble substance, such as a metal, an insoluble base or an insoluble carbonate. The insoluble reactant is added in excess, then filtered to remove unreacted solid; the filtrate is evaporated and crystallised.

可溶性盐可通过酸与合适的可溶性前提下的不溶物（如金属、不溶性碱或不溶性碳酸盐）反应来制备。将不溶反应物过量加入，然后过滤除去未反应的固体；滤液经蒸发和结晶得到盐。

Example: To prepare copper(II) sulfate from copper(II) oxide and sulfuric acid:

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

例子：由氧化铜和硫酸制备硫酸铜：

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Steps:

Warm the acid and add copper(II) oxide until it no longer dissolves (excess). (加热酸并加入氧化铜直到不再溶解（过量）。)
Filter to remove the excess black solid. (过滤除去多余的黑色固体。)
Heat the blue filtrate to concentrate it. (加热蓝色滤液使之浓缩。)
Allow the solution to cool and crystallise. (让溶液冷却并结晶。)
Dry the crystals between filter papers. (用滤纸干燥晶体。)

This method works for salts whose reactants are insoluble in water, like copper(II) oxide, because the acid is the only reactant in solution, and the desired salt is soluble.

该方法适用于反应物（如氧化铜）不溶于水的盐，因为酸是溶液中唯一的反应物，而目标盐可溶。

10. Preparing Insoluble Salts (Precipitation) | 制备不溶性盐（沉淀法）

Insoluble salts are prepared by precipitation: two soluble salts are mixed so that the desired salt forms as an insoluble solid. The precipitate is collected by filtration, washed with distilled water and dried.

不溶性盐通过沉淀反应制备：将两种可溶盐混合，使目标盐以不溶固体沉淀出来。沉淀经过滤收集、用蒸馏水洗涤并干燥。

For example, silver chloride is insoluble and is made by mixing silver nitrate and sodium chloride:

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

例如，氯化银不溶于水，可通过混合硝酸银和氯化钠制备：

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

The ionic equation is:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

离子方程式为：

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Precipitation reactions are also used to remove harmful ions from wastewater, such as lead(II) ions, by precipitating them as insoluble hydroxides.

沉淀反应用于去除废水中有害离子，例如将铅(II)离子沉淀为难溶的氢氧化物。

11. Salts and the Salt Name Table | 盐及其命名规律

The name of a salt depends on the acid used and the metal in the base or carbonate. For example, hydrochloric acid forms chlorides, nitric acid forms nitrates, and sulfuric acid forms sulfates. The metal replaces the hydrogen in the acid.

盐的名称取决于所用的酸以及碱或碳酸盐中的金属。例如，盐酸生成氯化物，硝酸生成硝酸盐，硫酸生成硫酸盐。金属取代了酸中的氢。

Acid	Salt type	Example
Hydrochloric acid (HCl)	Chloride	NaCl
Nitric acid (HNO₃)	Nitrate	KNO₃
Sulfuric acid (H₂SO₄)	Sulfate	CuSO₄

In the Edexcel IGCSE course, you are expected to write word and balanced equations for these reactions and to recognise common salts from their names.

在爱德思IGCSE课程中，你需要会写这些反应的文字方程式和配平的化学方程式，并能从名称识别常见盐。

12. Applications and Everyday Examples | 应用与日常实例

Acids, bases and salts are everywhere. Antacids like magnesium hydroxide neutralise excess stomach acid. Fertilisers often contain ammonium salts made from ammonia solution and nitric acid. Baking powder contains sodium hydrogencarbonate, which reacts with acid in batter to release carbon dioxide and make cakes rise.

酸、碱和盐无处不在。抗酸药（如氢氧化镁）中和胃酸过多。肥料中常含由氨水和硝酸制成的铵盐。泡打粉含有碳酸氢钠，与面糊中的酸反应释放二氧化碳，使蛋糕膨胀。

Soil acidity is controlled by adding lime (calcium oxide or calcium hydroxide), which is a base. Industrial processes such as the manufacture of paper and textiles rely on alkalis.

土壤酸性可通过施加石灰（氧化钙或氢氧化钙）来控制，因为石灰是碱。造纸和纺织等工业过程也依赖碱。

Knowing the properties and reactions of acids and bases helps us understand and manage these real-world systems. It also forms a foundation for further study in chemistry and biology.

了解酸和碱的性质与反应，有助于我们理解并管理这些现实系统，也为未来在化学和生物学领域的学习打下基础。

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Acids, Bases and Salts | 酸、碱和盐