📚 Chemical Reactions | 化学反应
Chemical reactions are at the heart of chemistry. They change one set of substances into new substances, and they happen all around us – from a burning candle to the digestion of food. In KS3 CIE Science, you need to recognise reactions, write simple word equations, and explain what happens to mass and energy.
化学反应是化学的核心。它们将一组物质转变为新物质，并且在我们周围随处可见——从燃烧的蜡烛到食物的消化。在 KS3 CIE 科学课程中，你需要识别反应、书写简单的文字方程式，并解释质量和能量发生了什么变化。
1. What Is a Chemical Reaction? | 什么是化学反应？
A chemical reaction occurs when atoms are rearranged to make new substances with different properties. The starting substances are called reactants, and the new substances formed are called products.
当原子重新排列生成性质不同的新物质时，就发生了化学反应。起始物质称为反应物，生成的新物质称为生成物。
During a reaction, chemical bonds between atoms break and new bonds form. This is why the products usually look, smell or behave differently from the reactants.
在反应过程中，原子之间的化学键断裂并形成新键。这就是为什么生成物通常在外观、气味或性质上与反应物不同。
Unlike a physical change, which only alters appearance or state, a chemical change creates substances that did not exist before. For example, when wood burns, ash and gases are produced.
与只改变外观或状态的物理变化不同，化学变化会生成以前不存在的新物质。例如，木材燃烧时会产生灰烬和气体。
A useful way to think about it is that in a chemical reaction you cannot easily get the original reactants back just by cooling or filtering.
一个有用的思考方式是：在化学反应中，你通常很难仅通过冷却或过滤就得到原来的反应物。
2. Signs of a Chemical Reaction | 化学反应的迹象
We can often tell that a chemical reaction has taken place by observing certain signs: colour change, gas production (bubbles), temperature change, precipitate formation, or light being given off.
我们通常可以通过观察某些迹象来判断是否发生了化学反应：颜色变化、产生气体（气泡）、温度变化、生成沉淀或发出光。
A colour change is a strong clue. For example, when blue copper sulfate solution reacts with sodium hydroxide, a pale blue precipitate forms.
颜色变化是一个重要线索。例如，蓝色硫酸铜溶液与氢氧化钠反应时，会生成淡蓝色沉淀。
Gas production is another common sign. When vinegar is added to sodium hydrogen carbonate, bubbles of carbon dioxide form quickly.
产生气体是另一个常见迹象。将醋加入碳酸氢钠时，会迅速产生二氧化碳气泡。
Some reactions release heat and light, such as burning magnesium ribbon. Others, like the reaction between citric acid and sodium hydrogen carbonate, absorb heat and feel colder.
有些反应会释放热和光，例如燃烧镁条。另一些反应，如柠檬酸与碳酸氢钠的反应，会吸收热量，摸起来更冷。
One sign alone may not prove a chemical change, but seeing two or more signs together is good evidence that a reaction has occurred.
仅凭一个迹象可能不足以证明化学变化，但若同时观察到两个或多个迹象，就能较好地证明发生了反应。
3. Reactants and Products | 反应物与生成物
In every reaction, reactants are written on the left-hand side of the arrow and products on the right. The arrow means ‘changes into’ or ‘produces’.
在每个反应中，反应物写在箭头左侧，生成物写在右侧。箭头表示“变成”或“生成”。
Reactants are the substances you start with, and products are the new substances made by the reaction. The total number of atoms does not change, but they are rearranged.
反应物是起始物质，生成物是反应生成的新物质。原子总数不变，但发生了重新排列。
For example, copper carbonate decomposes on heating to produce copper oxide and carbon dioxide. Here, copper carbonate is the reactant; copper oxide and carbon dioxide are the products.
例如，碳酸铜受热分解生成氧化铜和二氧化碳。这里，碳酸铜是反应物；氧化铜和二氧化碳是生成物。
Being able to identify reactants and products from an observation is an important skill. If a gas is collected, it is a product; if a solid disappears, it is usually a reactant.
能够根据观察识别反应物和生成物是一项重要技能。如果收集到气体，它通常是生成物；如果固体消失，它通常是反应物。
4. Word Equations | 文字方程式
A word equation uses the names of substances to summarise a reaction. It follows the general pattern: reactant + reactant → product + product.
文字方程式使用物质名称来概括反应。其一般模式为：反应物 + 反应物 → 生成物 + 生成物。
For example:
例如：
magnesium + oxygen → magnesium oxide
This tells us that magnesium reacts with oxygen from the air to form the compound magnesium oxide.
这告诉我们镁与空气中的氧气反应生成化合物氧化镁。
Another example is the neutralisation reaction between hydrochloric acid and sodium hydroxide:
另一个例子是盐酸与氢氧化钠的中和反应：
hydrochloric acid + sodium hydroxide → sodium chloride + water
Word equations do not show numbers of atoms, but they are an excellent way to practise identifying what reacts and what is made.
文字方程式不显示原子数目，但它们是练习识别什么参加反应、什么被生成的好方法。
5. Chemical Change vs Physical Change | 化学变化与物理变化
Physical changes include melting, freezing, boiling, condensing and dissolving. In these changes, no new substance is made; the material is the same chemically, even if its state or appearance changes.
物理变化包括熔化、凝固、沸腾、凝结和溶解。在这些变化中，没有生成新物质；即使状态或外观改变，材料在化学上仍是同一种物质。
For example, ice melting into water is a physical change. You can freeze the water again to get ice, and the chemical identity remains H₂O.
例如，冰融化成水是物理变化。你可以再次将水冷冻成冰，其化学式仍是 H₂O。
Chemical changes produce at least one new substance and are usually difficult to reverse. Frying an egg, rusting iron, burning paper and baking a cake are all chemical changes.
化学变化至少生成一种新物质，通常很难逆转。煎鸡蛋、铁生锈、烧纸和烤蛋糕都是化学变化。
When deciding whether a change is physical or chemical, ask: has a new substance been made? If yes, it is chemical; if no, it is physical.
判断一个变化是物理变化还是化学变化时，可以问：是否生成了新物质？如果是，则为化学变化；如果不是，则为物理变化。
6. Conservation of Mass | 质量守恒
In a chemical reaction, atoms are not created or destroyed; they are only rearranged. Therefore, the total mass of reactants equals the total mass of products.
在化学反应中，原子既不会凭空产生也不会凭空消失，它们只是重新排列。因此，反应物的总质量等于生成物的总质量。
This is called the law of conservation of mass. If you carry out a reaction in a sealed container and measure carefully, the mass before and after stays the same.
这称为质量守恒定律。如果在密封容器中进行反应并仔细测量，反应前后的质量保持不变。
total mass of reactants = total mass of products
If the mass appears to decrease in an open container, it is often because a gas has escaped into the air. If the mass appears to increase, a gas from the air, such as oxygen, may have combined with a reactant.
如果在开放容器中质量似乎减少了，通常是因为有气体逸散到空气中。如果质量似乎增加了，可能是空气中的气体（如氧气）与反应物结合了。
For example, when magnesium burns in air, the product magnesium oxide has a greater mass than the original magnesium because oxygen atoms from the air have joined the reaction.
例如，镁在空气中燃烧时，生成物氧化镁的质量比原来的镁大，因为空气中的氧原子参与了反应。
7. Common Types of Reaction | 常见反应类型
There are several important reaction types at KS3. Combustion is burning in oxygen, producing heat and light. Oxidation is the addition of oxygen to a substance.
KS3 阶段有几种重要的反应类型。燃烧是在氧气中燃烧，产生热和光。氧化是物质与氧结合。
A common combustion reaction is methane burning in air:
一个常见的燃烧反应是甲烷在空气中燃烧：
methane + oxygen → carbon dioxide + water
Thermal decomposition breaks down a compound using heat. For example, copper carbonate decomposes into copper oxide and carbon dioxide.
热分解是利用热使化合物分解。例如，碳酸铜分解为氧化铜和二氧化碳。
copper carbonate → copper oxide + carbon dioxide
Neutralisation is the reaction between an acid and a base, making salt and water. Displacement occurs when a more reactive element takes the place of a less reactive element in a compound.
中和是酸与碱反应生成盐和水。置换是指较活泼的元素取代化合物中较不活泼的元素。
Example equations include:
示例方程式包括：
hydrochloric acid + sodium hydroxide → sodium chloride + water
zinc + copper sulfate → zinc sulfate + copper
8. Exothermic and Endothermic Reactions | 放热与吸热反应
Reactions that give out heat energy to the surroundings are called exothermic. Combustion and neutralisation are common examples; the temperature of the surroundings rises.
向周围环境放出热能的反应称为放热反应。燃烧和中和是常见的例子；周围温度会上升。
Burning fuels, such as methane or petrol, is exothermic. This is why we use fuels to heat homes and power vehicles.
燃烧燃料（如甲烷或汽油）是放热反应。这就是我们使用燃料供暖和驱动车辆的原因。
Reactions that absorb heat energy from the surroundings are called endothermic. Photosynthesis and the reaction between citric acid and sodium hydrogen carbonate are examples; the surroundings feel cooler.
从周围环境吸收热能的反应称为吸热反应。光合作用和柠檬酸与碳酸氢钠的反应是例子；周围环境会变冷。
You can measure temperature change with a thermometer to decide whether a reaction is exothermic or endothermic. If the temperature rises, it is exothermic; if it falls, it is endothermic.
你可以用温度计测量温度变化，以判断反应是放热还是吸热。如果温度升高，则为放热；如果降低，则为吸热。
9. Rates of Reaction | 反应速率
The rate of a reaction is how quickly reactants are used up or products are made. Faster reactions show more rapid bubbles, colour change or mass decrease.
反应速率是指反应物被消耗或生成物产生的快慢。较快的反应会表现出更迅速的气泡、颜色变化或质量减少。
Four main factors affect the rate: temperature, concentration, surface area and the presence of a catalyst. Increasing temperature or concentration usually speeds up a reaction by increasing successful collisions between particles.
影响速率的主要因素有四个：温度、浓度、表面积和催化剂的存在。升高温度或增大浓度通常通过增加粒子之间的有效碰撞来加快反应。
Breaking a solid into smaller pieces increases its surface area, allowing more particles to react at the same time. For example, powdered marble chips react faster with acid than large chips.
把固体分成更小的块会增大其表面积，使更多粒子同时反应。例如，粉末状大理石碎片与酸反应比大块碎片更快。
Catalysts speed up reactions without being used up themselves. Enzymes in your body are natural catalysts that help break down food quickly at body temperature.
催化剂能加快反应，但自身不被消耗。体内的酶是天然催化剂，能在体温下帮助快速分解食物。
10. Testing Common Gases | 常见气体检验
We can identify gases produced in reactions using simple tests. Hydrogen gives a ‘squeaky pop’ when a lit splint is placed at the mouth of the test tube.
我们可以用简单的方法鉴别反应中产生的气体。将点燃的木条放在试管口，氢气会发出“噗”的爆鸣声。
Oxygen relights a glowing splint. If you insert a splint that has just been blown out but still glows, it will burst back into flame in oxygen.
氧气能使带火星的木条复燃。如果将刚吹灭但仍带火星的木条伸入氧气中，它会重新燃烧起来。
Carbon dioxide turns limewater milky or cloudy, because calcium carbonate is formed. This is the standard test for carbon dioxide gas.
二氧化碳能使石灰水变浑浊，因为生成了碳酸钙。这是检验二氧化碳气体的标准方法。
Chlorine, if tested, turns damp blue litmus paper red and then white, because it is acidic and acts as a bleach. Always test gases in small amounts and with proper ventilation.
如果检验氯气，它会使湿润的蓝色石蕊试纸先变红后变白，因为它呈酸性并具有漂白作用。检验气体时应使用少量并保持良好通风。
11. Everyday Chemical Reactions | 日常化学反应
Chemical reactions are not just in the lab. Respiration in your cells releases energy from glucose; photosynthesis in plants uses light energy to make glucose and oxygen.
化学反应不只发生在实验室里。细胞中的呼吸作用从葡萄糖中释放能量；植物的光合作用利用光能制造葡萄糖和氧气。
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