📚 Materials Changes | 材料的变化
From melting ice to rusting iron, materials around us are continually changing. Some changes can be reversed easily, while others create new substances that cannot be turned back. In KS3 chemistry, we classify these changes, explain them using particles, and use word equations to describe reactions.
从冰融化成水到铁生锈，我们周围的材料在不断变化。有些变化容易逆转，而另一些变化会生成无法复原的新物质。在 KS3 化学中，我们对这些变化进行分类，用粒子模型解释它们，并用文字方程式描述化学反应。
1. Physical and Chemical Changes | 物理变化与化学变化
A physical change alters appearance or state but does not make a new substance. Melting, boiling, dissolving and cutting are common physical changes.
物理变化只改变外观或状态，不会生成新物质。熔化、沸腾、溶解和切割是常见的物理变化。
A chemical change, or chemical reaction, produces at least one new substance with different properties. Burning, rusting and neutralisation are examples of chemical changes.
化学变化（化学反应）至少生成一种性质不同的新物质。燃烧、生锈和中和反应都是化学变化的例子。
Tearing paper changes its size but not its chemical nature, so it is a physical change. Burning paper turns it into ash and gases, so it is a chemical change.
撕纸只改变纸的大小，而不改变其化学本质，因此是物理变化。燃烧纸张会将其变成灰烬和气体，因此是化学变化。
2. Reversible and Irreversible Changes | 可逆变化与不可逆变化
Most physical changes are reversible. Water can freeze into ice and melt back into liquid water without changing chemically.
大多数物理变化是可逆的。水可以冻结成冰，再融化成液态水，而不会发生化学变化。
Some physical changes require a method to reverse. Dissolved salt can be recovered by evaporating the water, but the salt has not turned into a new substance.
有些物理变化需要通过一定方法才能逆转。溶解的盐可以通过蒸发水分来回收，但盐并没有变成新物质。
Many chemical changes are irreversible because the starting substances are used up and new products form. For example, once bread is baked, it cannot become dough again.
许多化学变化是不可逆的，因为起始物质被消耗，并生成了新的产物。例如，面包一旦烤好，就无法再变回生面团。
3. State Changes and Particle Theory | 状态变化与粒子理论
Materials can exist as solid, liquid or gas. In solids, particles are closely packed and vibrate in fixed positions; in liquids they are close but can slide past each other; in gases they are far apart and move quickly in all directions.
材料可以以固态、液态或气态存在。固体中粒子紧密排列并在固定位置振动；液体中粒子靠近但可以相互滑动；气体中粒子相距很远，并向各个方向快速运动。
During melting, boiling and condensing, particles gain or lose energy. The material itself does not change chemically – it is still the same substance with a different particle arrangement.
在熔化、沸腾和冷凝过程中，粒子获得或失去能量。材料本身没有发生化学变化——它仍然是同一种物质，只是粒子排列方式不同。
Melting and boiling are reversible physical changes. On a heating curve, temperature stays constant at the melting point and boiling point while the state changes.
熔化和沸腾是可逆的物理变化。在加热曲线上，当状态发生变化时，温度在熔点和沸点保持不变。
4. Chemical Reactions and Signs | 化学反应及其迹象
We look for signs that a chemical reaction has happened. Common signs include a colour change, gas bubbles, a temperature change, formation of a precipitate, or a new odour.
我们通过寻找迹象来判断是否发生了化学反应。常见迹象包括颜色变化、产生气泡、温度变化、生成沉淀或出现新的气味。
A precipitate is an insoluble solid that forms when two solutions react. For example, mixing lead nitrate solution and potassium iodide solution produces a yellow precipitate.
沉淀是两种溶液反应时生成的不溶性固体。例如，将硝酸铅溶液和碘化钾溶液混合会产生黄色沉淀。
A single sign is not always proof of a chemical change. Boiling water also gives bubbles, but that is only a physical change. Several signs together usually indicate a chemical reaction.
单一迹象并不总能证明发生了化学变化。沸水也会产生气泡，但那只是物理变化。多种迹象同时出现通常表明发生了化学反应。
5. Word Equations and Conservation of Mass | 文字方程式与质量守恒
In KS3 chemistry, we write word equations to show reactants on the left and products on the right. For example: methane + oxygen → carbon dioxide + water.
在 KS3 化学中，我们书写文字方程式，左边表示反应物，右边表示生成物。例如：甲烷 + 氧气 → 二氧化碳 + 水。
Mass is conserved in a chemical reaction. Atoms are rearranged, not created or destroyed, so the total mass of products equals the total mass of reactants.
化学反应中质量守恒。原子只是重新排列，没有被创造或消灭，因此生成物的总质量等于反应物的总质量。
Word equations help us describe reactions before we learn symbol equations. The arrow means ‘reacts to form’ and shows the direction of the change.
文字方程式帮助我们在学习符号方程式之前描述反应。箭头表示“反应生成”，并显示变化的方向。
6. Acids and Alkalis | 酸与碱
Acids are substances with a sour taste, such as lemon juice and vinegar. Many
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