📚 Cambridge Lower Secondary Science Stage 8 Key Revision Notes | 剑桥初中科学第8阶段核心复习笔记
The Cambridge Lower Secondary Science Learner's Book 8 with Digital Access builds a strong foundation in biology, chemistry, and physics for learners aged 12 to 14. This bilingual revision article highlights the core concepts from the book, organised into ten essential topics. Each section pairs clear English explanations with Chinese translations to support students in mastering scientific vocabulary and principles.
剑桥初中科学学习者用书第8阶段（含数字资源）为12至14岁的学生奠定了生物学、化学和物理学的坚实基础。本双语复习文章提炼了书中的核心概念，整理为十个关键主题。每个部分都将英文解释与中文翻译配对，帮助学生掌握科学词汇和原理。
1. Cells and Organisms | 细胞与生物体
Cells are the basic structural and functional units of all living organisms. Animal cells contain a nucleus, cytoplasm, and a cell membrane, while plant cells also possess a rigid cell wall, chloroplasts for photosynthesis, and a large central vacuole filled with cell sap.
细胞是所有生物体的基本结构和功能单位。动物细胞含有细胞核、细胞质和细胞膜，而植物细胞还具有坚硬的细胞壁、进行光合作用的叶绿体，以及充满细胞液的大中央液泡。
Specialised cells have adaptations that allow them to perform specific functions. For example, red blood cells have no nucleus and contain haemoglobin to carry oxygen, and nerve cells are long to transmit electrical signals quickly.
特化细胞具有适应结构以完成特定功能。例如，红细胞没有细胞核并含有血红蛋白来运输氧气，神经细胞则细长以便快速传递电信号。
Tissues are groups of similar cells working together. Muscular tissue contracts to produce movement. Organs, like the heart, are made of different tissues, and organ systems, such as the circulatory system, consist of several organs interacting to perform vital tasks.
组织是由相似的细胞组成的群体。肌肉组织收缩产生运动。像心脏这样的器官由不同的组织构成，而器官系统（如循环系统）则由多个器官相互作用以完成维持生命的任务。
2. Atoms, Elements, and Compounds | 原子、元素与化合物
All matter is made of atoms. An element is a substance made of only one type of atom. The periodic table lists all known elements, each represented by a unique chemical symbol, such as O for oxygen, Fe for iron, and Na for sodium.
所有物质都由原子组成。元素是仅由一种原子构成的物质。周期表列出了所有已知元素，每种元素都用独特的化学符号表示，如氧为 O，铁为 Fe，钠为 Na。
Compounds are substances formed when atoms of two or more different elements chemically combine in fixed proportions. Water (H₂O) is a compound of hydrogen and oxygen. A mixture contains two or more substances not chemically bonded, so they can be separated by physical methods.
化合物是由两种或多种不同元素的原子按固定比例化学结合形成的物质。水（H₂O）就是氢和氧的化合物。混合物含有两种或多种未化学键合的物质，因此可以通过物理方法分离。
Chemical formulas show the types and ratios of atoms in a compound. The formula of carbon dioxide is CO₂, meaning one carbon atom bonds with two oxygen atoms. The subscript numbers are always written digitally as Unicode characters: ₁ ₂ ₃ ₄.
化学式表示化合物中原子的种类和比例。二氧化碳的化学式是 CO₂，表示一个碳原子与两个氧原子键合。下标数字统一使用 Unicode 字符书写：₁ ₂ ₃ ₄。
3. Chemical Reactions | 化学反应
In a chemical reaction, reactants are transformed into products. Atoms are rearranged, but no atoms are created or destroyed – this is the law of conservation of mass. Signs of a reaction include colour change, temperature change, gas production, or formation of a precipitate.
在化学反应中，反应物转变为生成物。原子重新排列，但原子既不会凭空产生也不会消失——这就是质量守恒定律。反应的迹象包括颜色变化、温度变化、生成气体或形成沉淀。
stance: magnesium + oxygen → magnesium oxide. The arrow points from reactants to products.
文字方程式使用物质名称总结反应。例如：镁 + 氧气 → 氧化镁。箭头从反应物指向生成物。
symbols: 2Mg(s) + O₂(g) → 2MgO(s). The coefficients show the ratio of reacting particles.
简单的符号方程式可以标注状态符号：2Mg(s) + O₂(g) → 2MgO(s)。系数表示反应微粒的比例。
4. Acids and Alkalis | 酸与碱
Acids are substances that release hydrogen ions (H⁺) in water. Common acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). Alkalis are soluble bases that release hydroxide ions (OH⁻) in water, such as sodium hydroxide (NaOH).
酸是在水中释放氢离子（H⁺）的物质。常见的酸包括盐酸（HCl）、硫酸（H₂SO₄）和硝酸（HNO₃）。碱是可溶性碱，在水中释放氢氧根离子（OH⁻），如氢氧化钠（NaOH）。
The pH scale ranges from 0 to 14. Acids have a pH less than 7, neutral solutions have pH 7, and alkalis have pH greater than 7. Universal indicator or litmus paper can be used to test pH.
pH 标度范围从0到14。酸的 pH 小于7，中性溶液的 pH 等于7，碱的 pH 大于7。可使用通用指示剂或石蕊试纸测试 pH。
salt + water. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water.
中和反应是指酸与碱反应生成盐和水。一般文字方程式：酸 + 碱 → 盐 + 水。例如，盐酸 + 氢氧化钠 → 氯化钠 + 水。
5. Forces and Motion | 力与运动
A force is a push or pull that can change an object's speed, direction, or shape. Forces are measured in newtons (N). Contact forces include friction and tension; non-contact forces include gravity and magnetism.
力是能改变物体速度、方向或形状的推或拉。力的单位是牛顿（N）。接触力包括摩擦力和张力；非接触力包括重力和磁力。
When forces on an object are balanced, the object remains stationary or moves at a constant speed. Unbalanced forces cause acceleration or deceleration. The resultant force is the overall force acting on an object after combining all forces.
当作用在物体上的力平衡时，物体会保持静止或匀速运动。不平衡的力将导致加速或减速。合力是组合所有力后作用在物体上的总力。
Speed can be calculated using the formula:
速度可以用公式计算：
average speed = distance ÷ time (v = d / t)
平均速度 = 距离 ÷ 时间 (v = d / t)
A distance–time graph shows how an object moves. A horizontal line indicates the object is stationary; a straight sloping line shows constant speed; a curved line shows acceleration or deceleration.
距离—时间图表示物体的运动状态。水平线表示物体静止；倾斜直线表示匀速运动；曲线表示加速或减速。
6. Energy Transfers | 能量传递
Energy exists in many forms, including kinetic, thermal, light, sound, electrical, and chemical potential energy. Energy is measured in joules (J). The law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed.
能量以多种形式存在，包括动能、热能、光能、声能、电能和化学势能。能量的单位是焦耳（J）。能量守恒定律指出，能量既不能创造也不能消灭，只能转移或转化。
In an energy transfer, energy moves from one store to another. For example, when a ball is dropped, gravitational potential energy is converted into kinetic energy. Sankey diagrams show the useful and wasted energy in a transfer.
在能量传递中，能量从一个储能转移到另一个储能。例如，球掉落时，重力势能转化为动能。桑基图显示传递过程中的有用能量和浪费的能量。
Thermal energy can be transferred by conduction, convection, and radiation. Metals are good conductors, while insulators like plastic reduce heat transfer. Radiation transfers energy as infrared waves and does not require particles.
热能可以通过传导、对流和辐射传递。金属是良导体，而塑料等绝缘体则减缓热传递。辐射以红外波的形式传递能量，不需要介质微粒。
7. Electricity | 电学
An electric current is a flow of electric charge, usually carried by moving electrons in a metal wire. Current is measured in amperes (A) using an ammeter connected in series. Voltage (potential difference) is measured in volts (V) with a voltmeter connected in parallel.
电流是电荷的流动，通常由金属导线中运动的电子携带。电流用安培计串联测量，单位为安培（A）。电压（电位差）用电压表并联测量，单位为伏特（V）。
Circuit symbols represent components such as cells, bulbs, switches, and resistors. In a series circuit, components are connected end to end, and the current is the same everywhere. In a parallel circuit, the current splits across branches, but the voltage is the same across each loop.
电路符号表示电池、灯泡、开关和电阻器等元件。在串联电路中，元件首尾相连，各处电流相等。在并联电路中，电流在各支路分流，但每个回路的电压相同。
Resistance, measured in ohms (Ω), opposes the flow of current. Ohm’s Law states:
电阻的单位是欧姆（Ω），它阻碍电流的流动。欧姆定律表明：
voltage (V) = current (I) × resistance (R)
电压 (V) = 电流 (I) × 电阻 (R)
Safety in circuits is maintained by fuses and circuit breakers that prevent excessive current and overheating.
电路安全由保险丝和断路器维持，防止过电流和过热。
8. Respiration | 呼吸作用
Respiration is a chemical process that releases energy from glucose, occurring in the mitochondria of all living cells. It is not the same as breathing. Aerobic respiration uses oxygen to fully break down glucose:
呼吸作用是所有活细胞线粒体中从葡萄糖释放能量的化学过程，它不同于呼吸动作（通气）。有氧呼吸利用氧气完全分解葡萄糖：
glucose + oxygen → carbon dioxide + water (+ energy)
葡萄糖 + 氧气 → 二氧化碳 + 水 (+ 能量)
Anaerobic respiration occurs without oxygen and releases less energy. In human muscles during vigorous exercise, glucose is converted to lactic acid. In yeast, anaerobic respiration produces ethanol and carbon dioxide – this is fermentation.
无氧呼吸在无氧气情况下进行，释放的能量较少。在人类肌肉剧烈运动时，葡萄糖转化为乳酸。在酵母中，无氧呼吸产生乙醇和二氧化碳——这就是发酵过程。
The energy released by respiration is used for many life processes, including muscle contraction, maintaining body temperature, and active transport across cell membranes.
呼吸作用释放的能量用于许多生命过程，包括肌肉收缩、维持体温和细胞膜上的主动运输。
9. Reproduction in Plants | 植物的生殖
Flowering plants reproduce sexually. The male sex cell is the pollen grain, produced in the anther. The female sex cell is the ovule, contained in the ovary. Pollination is the transfer of pollen from anther to stigma, either by wind, insects, or other animals.
开花植物进行有性繁殖。雄性生殖细胞是花粉粒，产生于花药中。雌性生殖细胞是胚珠，位于子房内。传粉是指花粉从花药传递到柱头，可通过风、昆虫或其他动物进行。
After pollination, a pollen tube grows down the style, carrying the male nucleus to fuse with the ovule during fertilisation. The fertilised ovule develops into a seed, and the ovary becomes the fruit, which aids seed dispersal.
传粉后，花粉管沿花柱向下生长，将雄细胞核送达胚珠，完成受精。受精的胚珠发育为种子，子房发育为果实，有助于种子的传播。
Seeds need water, oxygen, and a suitable temperature for germination. The radicle (embryonic root) emerges first, followed by the plumule (shoot). The seed leaves provide stored food until the seedling can photosynthesise.
种子萌发需要水、氧气和适宜的温度。胚根首先伸出，随后是胚芽。子叶提供储存的养分，直至幼苗能进行光合作用。
10. Rocks, Minerals, and Weathering | 岩石、矿物与风化作用
Rocks are made of minerals. There are three main rock types: igneous, sedimentary, and metamorphic. Igneous rocks, like granite, form when magma or lava cools and solidifies. Sedimentary rocks, like sandstone, are formed from compacted sediment layers. Metamorphic rocks, like marble, result from changes caused by heat and pressure.
岩石由矿物组成。主要有三种岩石类型：火成岩、沉积岩和变质岩。火成岩（如花岗岩）由岩浆冷却凝固形成。沉积岩（如砂岩）由压实的沉积层形成。变质岩（如大理石）由高温和高压引起的变质作用形成。
Weathering is the breakdown of rocks in situ. Physical weathering occurs through temperature changes, ice expansion, or wind abrasion. Chemical weathering involves reactions such as acid rain dissolving limestone. Biological weathering is caused by plant roots or burrowing animals.
风化作用是岩石在原位的分解。物理风化通过温度变化、冰膨胀或风蚀作用发生。化学风化涉及酸雨溶解石灰岩等反应。生物风化由植物根系或穴居动物引起。
Erosion is the removal and transport of weathered material by water, wind, or ice. Together, weathering and erosion shape landscapes, forming features like valleys, caves, and coastlines.
侵蚀是风化物质被水、风或冰搬走和迁移的过程。风化与侵蚀共同塑造了地形，形成了山谷、洞穴和海岸线等地貌。
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