📚 KS3 Chemistry Review: Key Concepts and Common Misconceptions | KS3 化学复习：核心概念与常见误区
This KS3 chemistry review brings together the most important ideas from atomic structure, the periodic table, chemical reactions, acids and alkalis, and Earth chemistry. Use it to check your understanding and to practise for end-of-topic or end-of-year assessments.
本 KS3 化学复习总结了原子结构、元素周期表、化学反应、酸碱以及地球化学中最重要的知识。用它来检查你的理解，并为单元测试或年末考试进行练习。
1. The Particle Model and States of Matter | 粒子模型与物质状态
In KS3 science, all matter is made of tiny particles. The particle model explains the properties of solids, liquids and gases by considering how particles are arranged and how they move.
在 KS3 科学中，所有物质都由微小粒子组成。粒子模型通过粒子排列和运动方式解释固体、液体和气体的性质。
Particles in a solid are closely packed in a regular arrangement and vibrate in fixed positions. In a liquid they are close but can slide past one another, and in a gas they are far apart and move rapidly in all directions.
固体中的粒子紧密、规则排列，并在固定位置振动；液体中的粒子相距较近但能相互滑动；气体中的粒子相距很远，并朝各个方向快速运动。
Melting, freezing, boiling, condensation and sublimation are physical changes. The mass stays the same because no particles are created or destroyed during a change of state.
熔化、凝固、沸腾、凝结和升华都是物理变化。由于状态变化过程中粒子没有被创造或消灭，物质的质量保持不变。
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. It happens fastest in gases because gas particles move more freely and have larger spaces between them.
扩散是粒子从高浓度区域向低浓度区域移动的过程。扩散在气体中发生得最快，因为气体粒子运动更自由，粒子之间的空隙也更大。
2. Atoms, Elements and the Periodic Table | 原子、元素与周期表
An element is a substance made of only one type of atom. Each element is represented by a chemical symbol, such as H for hydrogen, O for oxygen, Na for sodium and Fe for iron.
元素是仅由一种原子组成的物质。每种元素都有化学符号，例如 H 表示氢、O 表示氧、Na 表示钠、Fe 表示铁。
Atoms contain protons, neutrons and electrons. Protons are positive, neutrons are neutral, and electrons are negative. The number of protons decides which element an atom is and is called the atomic number.
原子包含质子、中子和电子。质子带正电，中子不带电，电子带负电。质子数目决定原子属于哪种元素，称为原子序数。
The periodic table arranges elements in groups and periods. Elements in the same group have similar chemical properties, such as Group 1 alkali metals and Group 7 halogens.
元素周期表按族和周期排列元素。同一族元素具有相似的化学性质，例如第 1 族碱金属和第 7 族卤素。
Metals are found on the left and centre of the periodic table, while non-metals are found on the right. Most elements are metals, and they share properties such as shininess, conductivity and malleability.
金属位于元素周期表的左侧和中部，非金属位于右侧。大多数元素是金属，它们具有光泽、导电性和延展性等共同性质。
3. Compounds and Mixtures: Key Differences | 化合物与混合物：关键区别
A compound forms when two or more different elements chemically join together. A compound has properties that are different from the elements it contains.
当两种或两种以上不同元素通过化学方式结合时，就形成化合物。化合物的性质与组成它的元素不同。
A mixture contains different substances that are not chemically combined. The components keep their own properties and can be separated by physical methods.
混合物含有未通过化学方式结合的不同物质。各组分保持自己的性质，可以用物理方法分离。
| Feature | Element | Compound | Mixture |
| Particles | One type of atom | Different atoms chemically joined | Different substances not joined |
| Separation | Cannot be broken down chemically | Only by chemical reactions | By physical methods |
For example, water (H₂O) is a compound because hydrogen and oxygen are chemically bonded. Air is a mixture because oxygen, nitrogen, argon and carbon dioxide are simply mixed together.
例如，水 (H₂O) 是化合物，因为氢和氧通过化学键结合。空气是混合物，因为氧气、氮气、氩气和二氧化碳只是混合在一起。
4. Representing Chemical Reactions | 化学反应的表示方法
A chemical reaction is a change in which new substances are formed. Evidence includes colour change, gas bubbles, temperature change and precipitate formation.
化学反应是生成新物质的变化。反应证据包括颜色变化、气泡产生、温度变化和沉淀生成。
Word equations show reactants on the left and products on the right. A clean form of a word equation for combustion is shown below.
文字方程式左边写反应物，右边写生成物。下面是一个简洁的燃烧文字方程式。
methane + oxygen → carbon dioxide + water
甲烷 + 氧气 → 二氧化碳 + 水
During a chemical reaction, atoms are rearranged but not created or destroyed. This is the idea of conservation of mass, so the total mass of reactants equals the total mass of products.
在化学反应中，原子重新排列，但没有被创造或消灭。这就是质量守恒的概念，因此反应物的总质量等于生成物的总质量。
5. Acids and Alkalis | 酸与碱
Acids have a pH below 7, alkalis have a pH above 7, and a neutral solution has a pH of 7. Universal indicator changes colour across the pH scale, from red in strong acid to purple in strong alkali.
酸的 pH 小于 7，碱的 pH 大于 7，中性溶液的 pH 为 7。通用指示剂在 pH 范围内显示不同颜色，从强酸中的红色到强碱中的紫色。
Acids contain hydrogen ions (H⁺). Alkalis are water-soluble bases and contain hydroxide ions (OH⁻). Neutralisation occurs when an acid reacts with an alkali to form a salt and water.
酸含有氢离子 (H⁺)。碱是可溶性的碱，含有氢氧根离子 (OH⁻)。酸与碱反应生成盐和水，这一过程称为中和反应。
hydrochloric acid + sodium hydroxide → sodium chloride + water
盐酸 + 氢氧化钠 → 氯化钠 + 水
Everyday examples include citric acid in lemons, ethanoic acid in vinegar, and sodium hydroxide in drain cleaners. Indicators such as litmus and red cabbage can also show whether a solution is acidic or alkaline.
日常生活中的例子包括柠檬中的柠檬酸、醋中的乙酸，以及下水道清洁剂中的氢氧化钠。石蕊和红甘蓝等指示剂也可以显示溶液是酸性还是碱性。
6. Reactions of Metals with Acids and Oxygen | 金属与酸和氧气的反应
o produce a salt and hydrogen gas. The general pattern is: metal + acid → salt + hydrogen.
许多金属与稀酸反应生成盐和氢气。一般规律是：金属 + 酸 → 盐 + 氢气。
magnesium + hydrochloric acid → magnesium chloride + hydrogen
镁 + 盐酸 → 氯化镁 + 氢气
The test for hydrogen gas is a squeaky pop when a lighted splint is held at the mouth of the test tube. More reactive metals, such as magnesium, react faster than less reactive metals such as copper.
氢气的检验方法是将点燃的木条放在试管口，会听到爆鸣声。反应性较强的金属（如镁）比反应性较弱的金属（如铜）反应更快。
Metals also react with oxygen to form metal oxides. This is known as oxidation and can be shown by the reaction between copper and oxygen.
金属还能与氧气反应生成金属氧化物，这称为氧化反应。铜与氧气的反应可以说明这一过程。
copper + oxygen → copper oxide
铜 + 氧气 → 氧化铜
7. Combustion and Oxidation | 燃烧与氧化
Combustion is a rapid reaction with oxygen that releases heat and light. Complete combustion of a hydrocarbon fuel produces carbon dioxide and water.
燃烧是与氧气发生的快速反应，释放热和光。碳氢燃料完全燃烧生成二氧化碳和水。
propane + oxygen → carbon dioxide + water
丙烷 + 氧气 → 二氧化碳 + 水
Incomplete combustion happens when oxygen is limited and can produce carbon monoxide, which is toxic because it prevents blood from carrying oxygen properly. Soot particles can also form.
当氧气不足时会发生不完全燃烧，可能生成有毒的一氧化碳，因为它会阻止血液正常运输氧气。还可能产生炭黑颗粒。
Oxidation is a broader type of reaction where a substance gains oxygen. Rusting is a slow oxidation of iron that requires both oxygen and water, so keeping iron dry or coated can prevent rust.
氧化是物质获得氧的一类更广泛的反应。铁生锈是铁与氧气和水共同作用发生的缓慢氧化，因此保持铁干燥或涂层可以防止生锈。
8. Earth Chemistry: Rocks and the Atmosphere | 地球化学：岩石与大气
The Earth is made of layers: crust, mantle, outer core and inner core. Rocks can be igneous, sedimentary or metamorphic, depending on how they formed.
地球由地壳、地幔、外地核和内地核等圈层组成。岩石根据形成方式可分为火成岩、沉积岩和变质岩。
Igneous rocks form when magma or lava cools and solidifies. Sedimentary rocks form when layers of sediment are compacted over time, and metamorphic rocks form when existing rocks are changed by heat and pressure.
火成岩由岩浆或熔岩冷却凝固形成；沉积岩由沉积物层长期压实形成；变质岩由原有岩石在高温高压下变质形成。
The early atmosphere came from volcanic gases and has changed over time. Photosynthesis by plants increased the amount of oxygen, allowing animals to evolve and survive.
早期大气来自火山气体，并随时间发生变化。植物光合作用增加了氧气含量，使动物得以进化并生存。
The carbon cycle and the rock cycle show how carbon and rocky materials move through the Earth system. Limestone, chalk and marble are all natural forms of calcium carbonate (CaCO₃).
碳循环和岩石循环展示了碳和岩石物质如何在地球系统中循环。石灰石、白垩和大理石都是碳酸钙 (CaCO₃) 的天然形式。
9. Separating Mixtures | 混合物的分离
Mixtures can be separated using filtration, evaporation, distillation, chromatography and magnetic attraction. The choice depends on the properties of the substances in the mixture.
混合物可以用过滤、蒸发、蒸馏、色谱法和磁铁吸引等方法分离。选择哪种方法取决于混合物中各物质的性质。
Filtration separates an insoluble solid from a liquid. Evaporation recovers a dissolved solid from a solution, while distillation collects the liquid by cooling its vapour.
过滤将不溶性固体与液体分离；蒸发从溶液中回收溶解的固体；蒸馏通过冷却蒸汽来收集液体。
Chromatography separates coloured substances that dissolve in a solvent because different substances move up the paper at different speeds. Magnetic attraction is used when one component is magnetic, such as iron filings in sand.
色谱法能分离溶解在溶剂中的有色物质，因为不同物质在纸上向上移动的速度不同。当混合物中一种组分具有磁性时，例如沙中的铁屑，可使用磁铁吸引进行分离。
For example, to obtain clean salt from rock salt, you grind the mixture, dissolve it in water, filter off insoluble sand, and evaporate the water to leave salt crystals.
例如，要从岩盐中获取纯净的食盐，可以研磨混合物，将其溶于水，过滤掉不溶的沙粒，再蒸发水分，留下盐的晶体。
10. Common Exam Mistakes and Key Tips | 常见考试错误与关键技巧
Students often lose marks by confusing elements, compounds and mixtures. Always check whether a particle diagram shows one type of atom, different atoms joined, or different particles not joined.
学生常因混淆元素、化合物和混合物而失分。一定要检查粒子图表示的是一种原子、不同原子结合在一起，还是不同粒子没有结合。
Another common mistake is forgetting that mass is conserved in a chemical reaction. If a reaction seems to lose mass because gas escapes, include the gas when describing the results.
另一个常见错误是忘记化学反应中质量守恒。如果反应因为气体逸出而看似质量减少，在描述结果时要包括气体。
Practise writing word equations and using the pH scale. Clear scientific keywords such as reactant, product, soluble, insoluble, neutralisation and combustion help earn marks.
练习书写文字方程式和使用 pH 标度。清晰使用反应物、生成物、可溶、不可溶、中和和燃烧等科学关键词有助于得分。
When describing a graph or an experiment, state what was changed, what was measured and what was kept the same. Use comparative language, such as "the higher the temperature, the faster the reaction".
在描述图表或实验时，要说明改变的条件、测量的结果和保持不变的条件。使用比较性语言，例如“温度越高，反应越快”。
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