📚 Material Changes | 物质变化
From melting ice to burning wood, materials around us are constantly changing. Some changes only alter the appearance or state of a substance, while others create completely new substances with different properties. This article explains the key types of material changes you need to know for KS3 chemistry.
从冰块融化到木材燃烧，我们周围的物质在不断变化。有些变化只改变物质的外观或状态，而另一些变化则生成性质完全不同的新物质。本文讲解 KS3 化学中你必须掌握的主要物质变化类型。
1. What Is a Material Change? | 什么是物质变化？
A material change is any process in which a substance changes its form, state, or chemical identity. Scientists group material changes into two broad types: physical changes and chemical changes.
物质变化是指物质的形式、状态或化学身份发生变化的过程。科学家将物质变化分为两大类：物理变化和化学变化。
The key question is always: has a new substance been made? If yes, it is a chemical change. If no, it is a physical change.
关键问题始终是：是否生成了新物质？如果生成了新物质，就是化学变化；如果没有，则是物理变化。
2. Physical Changes: No New Substance | 物理变化：不生成新物质
A physical change alters the state, shape, or appearance of a material but does not create a new chemical substance. The particles remain the same; only their arrangement or energy changes.
物理变化改变物质的状态、形状或外观，但不生成新的化学物质。粒子本身不变，只是排列方式或能量发生变化。
Common examples include melting, freezing, boiling, condensing, dissolving sugar in water, and cutting paper. These changes are often easy to reverse.
常见例子包括熔化、凝固、沸腾、凝结、糖溶于水以及剪纸。这些变化通常容易逆转。
For example, ice melting into water is a physical change because the chemical formula H₂O stays the same in both solid and liquid states.
例如，冰融化成水是物理变化，因为固态和液态的化学式 H₂O 都保持不变。
3. Chemical Changes: New Substances Form | 化学变化：生成新物质
A chemical change, also called a chemical reaction, produces at least one new substance. The atoms are rearranged, chemical bonds break and form, and the new substance usually has different properties from the starting materials.
化学变化也叫化学反应，至少生成一种新物质。原子重新排列，化学键断裂并形成，新物质通常具有与起始物质不同的性质。
Chemical changes are usually difficult to reverse by simple physical methods. Burning, rusting, cooking an egg, and adding acid to bicarbonate are all chemical changes.
化学变化通常难以通过简单的物理方法逆转。燃烧、生锈、煎鸡蛋以及将酸加入碳酸氢盐都是化学变化。
In a chemical reaction, the starting substances are called reactants, and the substances formed are called products.
在化学反应中，起始物质称为反应物，生成的物质称为生成物。
| Feature / 特征 | Physical change / 物理变化 | Chemical change / 化学变化 |
|---|---|---|
| New substance? / 新物质？ | No / 否 | Yes / 是 |
| Particles / 粒子 | Same particles / 相同粒子 | Atoms rearranged / 原子重新排列 |
| Reversibility / 可逆性 | Usually easy / 通常容易 | Usually difficult / 通常困难 |
| Examples / 例子 | Melting, dissolving / 熔化、溶解 | Burning, rusting / 燃烧、生锈 |
4. Reversible and Irreversible Changes | 可逆变化与不可逆变化
Many physical changes are reversible: melted wax can freeze again, and dissolved salt can be recovered by evaporating the water. Reversibility is often a clue that no new substance has been made.
许多物理变化是可逆的：熔化的蜡可以再次凝固，溶解的盐可以通过蒸发水分而回收。可逆性往往提示没有生成新物质。
Most chemical changes are irreversible in practice. Once wood is burned to ash and gases, you cannot easily turn the ash back into wood.
大多数化学变化实际上是不可逆的。木材一旦燃烧成灰烬和气体，你很难将灰烬变回木材。
However, some chemical reactions are reversible under special conditions, such as the decomposition and re-formation of some hydrated salts. Do not rely on reversibility alone; always check whether a new substance forms.
然而，有些化学反应在特殊条件下是可逆的，例如某些水合盐的分解与重新形成。不要只依赖可逆性判断；始终检查是否生成了新物质。
5. Evidence of a Chemical Reaction | 化学反应的证据
You can look for several signs to decide whether a chemical change has happened. The main signs are: a colour change, a gas being produced (bubbles or fizzing), a temperature change (exothermic or endothermic), a precipitate forming, and light or sound being released.
你可以通过以下几个迹象判断是否发生了化学变化。主要迹象包括：颜色变化、产生气体（气泡或嘶嘶声）、温度变化（放热或吸热）、生成沉淀，以及释放光或声音。
For example, when a magnesium ribbon burns, it gives out brilliant white light and heat, and a white ash of magnesium oxide is formed.
例如，镁条燃烧时会发出明亮的白光和热，并生成白色的氧化镁粉末。
Not every fizz means a chemical reaction: opening a fizzy drink releases dissolved carbon dioxide, which is a physical change because no new substance is made.
并非所有冒泡都表示化学反应：打开汽水时释放出溶解的二氧化碳，这是物理变化，因为没有生成新物质。
6. Combustion: Burning Fuels | 燃烧：燃料的燃烧
Combustion is a chemical reaction in which a fuel reacts rapidly with oxygen, releasing energy as heat and light. Fuels include wood, natural gas, petrol, and hydrogen.
燃烧是一种化学反应，燃料与氧气迅速反应，以热和光的形式释放能量。燃料包括木材、天然气、汽油和氢气。
The general word equation for complete combustion of a hydrocarbon fuel is:
碳氢燃料完全燃烧的一般文字方程式为：
Fuel + Oxygen → Carbon dioxide + Water
燃料 + 氧气 → 二氧化碳 + 水
For example, methane burns with a blue flame:
例如，甲烷燃烧时产生蓝色火焰：
CH₄ + 2O₂ → CO₂ + 2H₂O
甲烷 + 氧气 → 二氧化碳 + 水
7. Thermal Decomposition | 热分解
Thermal decomposition is a chemical change in which one substance breaks down into two or more simpler substances when heated. It is an endothermic process because it takes in heat energy.
热分解是一种化学变化，物质在加热时分解成两种或更多更简单的物质。它是吸热过程，因为需要吸收热量。
A common example is heating copper carbonate, a green powder, which turns black as copper oxide forms and carbon dioxide gas is released.
一个常见例子是加热碳酸铜，这种绿色粉末会变黑，因为生成了氧化铜并释放出二氧化碳气体。
Copper carbonate → Copper oxide + Carbon dioxide
碳酸铜 → 氧化铜 + 二氧化碳
CuCO₃ → CuO + CO₂
碳酸铜 → 氧化铜 + 二氧化碳
8. Oxidation and Rusting | 氧化与生锈
Oxidation is a chemical change in which a substance reacts with oxygen. Rusting is the slow oxidation of iron in the presence of oxygen and water, forming hydrated iron(III) oxide.
氧化是物质与氧气发生反应的一种化学变化。生锈是铁在氧气和水存在下发生的缓慢氧化，生成水合氧化铁(III)。
Rusting is a serious problem because rust is weak and flaky, so it damages bridges, cars, and tools. Unlike burning, rusting releases energy very slowly, so it is not obvious.
生锈是一个严重问题，因为铁锈脆弱且易剥落，会损坏桥梁、汽车和工具。与燃烧不同，生锈释放能量非常缓慢，因此不易察觉。
Rusting can be prevented by keeping iron dry, painting, oiling, galvanising with zinc, or using stainless steel.
可以通过保持铁干燥、涂漆、涂油、用锌镀锌或使用不锈钢来防止生锈。
9. Acids, Alkalis and Neutralisation | 酸、碱与中和反应
Acids and alkalis are common chemicals that can cause material changes. Acids have a pH below 7, alkalis have a pH above 7, and a neutral solution has a pH of 7. Indicators such as litmus and universal indicator show the pH.
酸和碱是能引起物质变化的常见化学品。酸的 pH 小于 7，碱的 pH 大于 7，中性溶液的 pH 等于 7。石蕊和通用指示剂等指示剂可以显示 pH。
When an acid reacts with an alkali, the reaction is called neutralisation. The products are a salt and water. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.
酸与碱反应称为中和反应，生成盐和水。例如，盐酸与氢氧化钠反应生成氯化钠和水。
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
盐酸 + 氢氧化钠 → 氯化钠 + 水
Neutralisation is useful in treating acidic soil with lime, treating indigestion with antacid tablets, and controlling pH in swimming pools.
中和反应在农业改良酸性土壤、用抗酸药治疗胃灼热以及控制游泳池 pH 等方面非常有用。
10. Conservation of Mass in Reactions |
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