📚 Chemical Reactions | 化学反应
Chemical reactions are happening all around us, from the rusting of a bicycle to the baking of a cake. In KS3 chemistry, you learn how to recognise a chemical reaction, describe what happens to the atoms, and write simple word equations. Understanding reactions helps you explain everyday changes and prepare for GCSE study.
化学反应无处不在，从自行车生锈到烤蛋糕都离不开它。在 KS3 化学中，你将学习如何识别化学反应、描述原子发生的变化，并书写简单的文字方程式。理解反应能帮助你解释日常生活中的变化，也为 GCSE 学习做好准备。
1. What is a Chemical Reaction? | 什么是化学反应？
A chemical reaction is a process in which one or more new substances are formed. The atoms in the starting substances rearrange themselves, but no atoms are created or destroyed.
化学反应是生成一种或多种新物质的过程。起始物质中的原子会重新排列，但原子既不会凭空产生，也不会凭空消失。
A chemical reaction is different from a physical change, such as melting ice or dissolving sugar. In a physical change, the substance may change state or appearance, but no new substance is made.
化学反应不同于物理变化，例如冰融化或糖溶解。在物理变化中，物质可能改变状态或外观，但没有新物质生成。
2. Signs of a Chemical Reaction | 化学反应的迹象
You can often tell that a chemical reaction has happened by looking for observable signs. These signs are useful evidence, but they do not prove a reaction on their own.
通常可以通过观察一些迹象来判断是否发生了化学反应。这些迹象是有用的证据，但不能仅凭它们就证明一定发生了反应。
The table below summarises the most common signs that a reaction may have occurred:
下表总结了可能发生反应的最常见迹象：
| Colour change | 颜色变化 |
| Gas bubbles given off | 产生气泡 |
| Formation of a precipitate | 生成沉淀 |
| Temperature change | 温度变化 |
| Light produced | 产生光 |
For example, when magnesium ribbon burns, it gives off a bright white light and forms white ash. These observations are evidence that a new substance has formed.
例如，镁条燃烧时会发出明亮的白光并生成白色灰烬。这些观察结果证明有新物质生成。
3. Reactants and Products | 反应物与生成物
In a chemical reaction, the substances you start with are called reactants. The new substances made by the reaction are called products.
在化学反应中，起始的物质叫做反应物。反应生成的新物质叫做生成物。
Reactants are always written on the left-hand side of a word equation, and products are written on the right-hand side. An arrow shows the direction of change.
在文字方程式中，反应物总是写在左边，生成物写在右边。箭头表示变化的方向。
reactants → products
反应物 → 生成物
4. Writing Word Equations | 书写文字方程式
A word equation describes a reaction using the names of substances. It is a simple way to summarise what happens without writing chemical formulas.
文字方程式用物质名称来描述化学反应。它是总结反应过程的简单方法，无需书写化学式。
For example, magnesium burns in oxygen to form magnesium oxide. The word equation is written as:
例如，镁在氧气中燃烧生成氧化镁。其文字方程式写作：
magnesium + oxygen → magnesium oxide
镁 + 氧气 → 氧化镁
Word equations must always be balanced in terms of atoms, but at KS3 you usually focus on the names rather than the numbers.
文字方程式在原子数目上必须守恒，但在 KS3 阶段你通常只需关注物质名称，而不必写出具体数目。
5. Conservation of Mass | 质量守恒
During a chemical reaction, the total mass of the reactants is equal to the total mass of the products. This is called the law of conservation of mass.
在化学反应中，反应物的总质量等于生成物的总质量。这叫做质量守恒定律。
Atoms are not created or destroyed, so the total number of each type of atom stays the same. If a reaction is carried out in a closed container, the mass before and after the reaction is identical.
原子不会凭空产生或消失，因此每种原子的总数保持不变。如果在密闭容器中进行反应，反应前后的质量完全相同。
When a gas seems to make the mass decrease, it is usually because the gas has escaped into the air. If the gas is trapped, the total mass does not change.
如果有气体逸出，看起来质量减少了，通常是因为气体跑到了空气中。如果气体被收集起来，总质量就不会改变。
6. Combustion Reactions | 燃烧反应
Combustion is a chemical reaction in which a fuel burns in oxygen, releasing energy as heat and light. Most combustion reactions are exothermic.
燃烧是燃料在氧气中燃烧、以热和光的形式释放能量的化学反应。大多数燃烧反应都是放热反应。
A common word equation for the complete combustion of a fuel such as methane is:
甲烷等燃料完全燃烧的常见文字方程式为：
methane + oxygen → carbon dioxide + water
甲烷 + 氧气 → 二氧化碳 + 水
Incomplete combustion can also produce carbon monoxide or carbon particles. This is why good ventilation is important when burning fuels indoors.
不完全燃烧还会产生一氧化碳或碳颗粒。这就是为什么在室内燃烧燃料时保持良好的通风很重要。
7. Oxidation Reactions | 氧化反应
Oxidation is a reaction in which a substance gains oxygen. The substance formed is called an oxide.
氧化是物质获得氧的反应。生成的物质叫做氧化物。
For example, when iron reacts with oxygen and water, rust forms. The word equation is:
例如，铁与氧气和水反应会生成铁锈。其文字方程式为：
iron + oxygen + water → hydrated iron(III) oxide
铁 + 氧气 + 水 → 水合氧化铁(III)
Rusting is a slow oxidation reaction. It can be prevented by painting, oiling, or galvanising the metal to keep oxygen and water away.
生锈是一种缓慢的氧化反应。可以通过刷漆、涂油或镀锌来隔绝氧气和水分，从而防止生锈。
8. Acids and Metals | 酸与金属的反应
Dilute acids react with many metals to produce a salt and hydrogen gas. The general word equation is:
稀酸能与许多金属反应，生成盐和氢气。通用的文字方程式为：
metal + acid → salt + hydrogen
金属 + 酸 → 盐 + 氢气
For example, magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen:
例如，镁与盐酸反应生成氯化镁和氢气：
magnesium + hydrochloric acid → magnesium chloride + hydrogen
镁 + 盐酸 → 氯化镁 + 氢气
You can test for hydrogen gas using a burning splint. Hydrogen gives a squeaky pop sound.
可以使用点燃的木条检验氢气。氢气会发出 '砰' 的爆鸣声。
9. Acids and Carbonates | 酸与碳酸盐的反应
Acids react with carbonates to produce a salt, water, and carbon dioxide gas. The general word equation is:
酸与碳酸盐反应会生成盐、水和二氧化碳气体。通用的文字方程式
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