📚 Year 8 CIE Chemistry: Case Study Practical Drills | CIE 八年级化学：案例分析实战演练
In Year 8 CIE Chemistry, case studies are a powerful way to connect classroom theory with practical, real-world situations. This article presents ten carefully selected case studies that cover key topics such as separation techniques, identifying substances, acids and alkalis, rates of reaction, electrolysis, and more. Read each scenario, think about the underlying chemical principles, and then study the paired explanations to sharpen your analytical skills and exam confidence.
在 CIE 八年级化学中，案例分析是将课堂理论与实际情境紧密结合的有效方法。本文精心挑选了十个案例，涵盖分离技术、物质鉴别、酸与碱、反应速率、电解等核心主题。阅读每个情境，思考背后的化学原理，然后研读对应的双语解析，以提升你的分析能力和应试信心。
1. Case Study 1: The Mystery White Powder | 案例1：神秘的白色粉末
Scenario: A student discovers an unlabelled white powder in the kitchen. It could be table salt (sodium chloride) or sugar (sucrose). The student must identify it without tasting, using only safe laboratory equipment: distilled water, a conductivity tester, a spatula, and a Bunsen burner.
情境：一位学生在厨房发现一包未标记的白色粉末，可能是食盐（氯化钠）或蔗糖。他必须在不品尝的情况下，仅用蒸馏水、导电性检测器、药匙和本生灯进行安全鉴定。
Dissolving in water: Both salt and sugar dissolve readily in cold distilled water, forming clear solutions. Therefore, solubility alone cannot distinguish them.
溶于水：食盐和蔗糖都能在冷蒸馏水中快速溶解，形成透明溶液。因此，单靠溶解性无法区分它们。
Conductivity test: When the salt solution is tested, the bulb lights up brightly because dissolved sodium chloride produces mobile Na⁺ and Cl⁻ ions which carry electric charge. The sugar solution shows no conductivity — the bulb remains off — since sugar dissolves as neutral molecules without ions.
导电性测试：将盐水溶液接入电路，灯泡明亮发光，因为溶解的氯化钠产生可自由移动的 Na⁺ 和 Cl⁻ 离子，能够导电。而糖水溶液不导电——灯泡熄灭——因为蔗糖以中性分子形式溶解，没有离子。
Heating test: Place a small sample on a clean metal spatula and heat in a non-luminous Bunsen flame. Salt does not melt easily and remains a white solid. Sugar first melts, then turns brown and finally black (charring), giving off a characteristic caramel smell and water vapour.
加热测试：用药匙取少量样品放在洁净金属勺上，在本生灯无光焰中加热。食盐不易熔化，保持白色固体。蔗糖先熔化成液体，继而变成棕色，最后碳化变黑，同时释放出特有的焦糖气味和水蒸气。
Conclusion: The powder that conducts electricity in solution and chars on strong heating is sugar; the powder that conducts electricity but does not char is salt. Safety note: always wear eye protection when heating substances.
结论：溶液能导电且强热后碳化的是蔗糖；溶液能导电但加热不碳化的是食盐。安全提示：加热物质时务必佩戴护目镜。
2. Case Study 2: Rusting of Iron Nails | 案例2：铁钉生锈
Scenario: A student sets up three test tubes each containing a clean iron nail. Tube A has dry air (calcium chloride absorbs moisture). Tube B contains boiled distilled water with a layer of oil on top to keep out air. Tube C has ordinary tap water exposed to air. After one week, only the nail in Tube C shows reddish-brown rust.
情境：一位学生准备了三支试管，各放一枚洁净的铁钉。试管A装有干燥空气（用无水氯化钙吸收水蒸气）。试管B中为煮沸过的蒸馏水，上面加一层油以隔绝空气。试管C中是普通自来水，暴露于空气。一周后，只有试管C中的铁钉出现红棕色锈迹。
Observations: Tube A: nail remains shiny; Tube B: nail remains shiny; Tube C: nail is coated with rust and the water level has risen slightly.
观察现象：试管A：铁钉保持光亮；试管B：铁钉保持光亮；试管C：铁钉表面覆盖锈层，水位略微上升。
ir cannot enter because of the oil layer. In Tube C, both water and dissolved oxygen are present, so rust (hydrated iron(III) oxide) forms: iron + oxygen + water → hydrated iron(III) oxide.
化学推理：铁生锈需要同时接触氧气和水。试管A中缺水；试管B中，溶解氧被煮沸去除，且油层阻止空气进入；试管C中，水和溶解氧都具备，因此生成铁锈（水合氧化铁）：铁 + 氧气 + 水 → 水合氧化铁。
Extension: Salt water speeds up rusting because dissolved salts increase the conductivity of water, facilitating the electrochemical process of corrosion. This is why cars in coastal areas rust faster.
拓展：盐水会加速生锈，因为溶解的盐增大了水的导电性，促进电化学腐蚀过程。这就是沿海地区汽车更容易生锈的原因。
3. Case Study 3: Separating a Mixture of Sand and Salt | 案例3：分离沙子与食盐
Scenario: A Year 8 class is given a mixture of sand and common salt. Their task is to obtain pure dry salt and clean sand using standard laboratory apparatus: beakers, filter funnel, filter paper, evaporating dish, tripod, Bunsen burner and distilled water.
情境：八年级学生拿到一份沙子与食盐的混合物。他们的任务是使用烧杯、漏斗、滤纸、蒸发皿、三脚架、本生灯和蒸馏水等标准器材，获得纯净干燥的食盐和清洁的沙子。
Step 1 – Dissolving: Add distilled water to the mixture and stir. Salt dissolves to form a solution, while sand remains insoluble and settles at the bottom.
步骤1 – 溶解：向混合物中加入蒸馏水并搅拌。食盐溶解形成溶液，而沙子不溶，沉于底部。
Step 2 – Filtration: Pour the mixture through a filter paper in a funnel. The residue (wet sand) stays on the filter paper and the filtrate (salt solution) passes through into a beaker.
步骤2 – 过滤：将混合物通过漏斗中的滤纸过滤。残渣（湿沙子）留在滤纸上，滤液（食盐水）透过滤纸流入烧杯。
Step 3 – Evaporation: Transfer the filtrate into an evaporating dish and heat gently on a water bath or with a low flame. Water evaporates, leaving white crystals of salt behind. Do not heat to dryness too quickly to avoid spitting.
步骤3 – 蒸发：将滤液转移至蒸发皿中，用水浴或小火缓慢加热。水分蒸发后，留下白色食盐晶体。注意不要过快地蒸干，以防飞溅。
Step 4 – Drying the sand: Wash the sand on the filter paper with a little distilled water, then dry it in a warm oven or allow it to air-dry.
步骤4 – 干燥沙子：用少量蒸馏水冲洗滤纸上的沙子，然后在烘箱中干燥或自然风干。
Key concept: This method works because one component (salt) is soluble in water while the other (sand) is insoluble. Filtration separates an insoluble solid from a liquid; evaporation then recovers the soluble solid.
核心概念：此方法之所以有效，是因为食盐可溶于水而沙子不溶。过滤将不溶性固体与液体分离；随后用蒸发回收可溶性固体。
4. Case Study 4: Testing Household Liquids with Universal Indicator | 案例4：用通用指示剂测试家用液体
Scenario: A student wants to classify four household liquids — lemon juice, baking soda solution, distilled water and soap solution — as acidic, neutral or alkaline. She adds a few drops of universal indicator to each sample and records the colours: red, blue, green and dark purple.
情境：学生想将四种家用液体——柠檬汁、小苏打溶液、蒸馏水和肥皂水——分类为酸性、中性或碱性。她向每种样品中滴入几滴通用指示剂，记录下颜色：红、蓝、绿和深紫。
Colour chart: Universal indicator shows red/orange in acidic solutions (pH 1-2 lemon juice), green in neutral (pH 7 distilled water), and blue/purple in alkaline solutions (pH 8-10 baking soda, pH 11-14 soap).
颜色对照：通用指示剂在酸性溶液中显红/橙色（pH 1-2 柠檬汁），中性显绿色（pH 7 蒸馏水），碱性中显蓝/紫色（pH 8-10 小苏打，pH 11-14 肥皂）。
Safety: Never mix cleaning products during testing; use separate clean test tubes. Baking soda solution and soap are weak alkalis, often used to neutralise insect stings or clean grease.
安全：测试时不要混合不同的清洁用品；使用独立的洁净试管。小苏打溶液和肥皂水呈弱碱性，常用于中和昆虫叮咬或清洗油污。
Linking to ions: Acids contain excess H⁺ ions, alkalis contain excess OH⁻ ions. Distilled water has equal concentrations of H⁺ and OH⁻, making it neutral.
与离子的联系：酸含有过量的 H⁺ 离子，碱含有过量的 OH⁻ 离子。蒸馏水中 H⁺ 和 OH⁻ 浓度相等，因此呈中性。
Extension: Universal indicator is a mixture of several dyes that change colour gradually over a range of pH, allowing us to estimate pH more precisely than with litmus paper.
拓展：通用指示剂是几种染料的混合物，能在一定pH范围内连续变色，因此比石蕊试纸更精确地估计pH值。
5. Case Study 5: Burning Magnesium | 案例5：燃烧镁
Scenario: A class watches a teacher burn a piece of magnesium ribbon in a crucible. The magnesium burns with a brilliant white light and forms a white ash. The mass of the crucible and its contents is measured before and after the reaction.
情境：全班观察老师将一段镁条在坩埚中燃烧。镁条发出耀眼白光并生成白色灰烬。测量反应前后坩埚及内容物的总质量。
Observation: Magnesium ribbon starts as a shiny grey metal. After burning, it turns into a dull white powder. The final mass is greater than the initial mass of magnesium alone.
观察：镁条起初为光亮的银灰色金属。燃烧后变成无光泽的白色粉末。最终总质量大于原来镁条的质量。
The increase in mass is exactly equal to the mass of oxygen that has chemically joined with the magnesium. This demonstrates the Law of Conservation of Mass – matter is not created or destroyed, only rearranged.
科学解释：镁与空气中的氧气发生化学反应：镁 + 氧气 → 氧化镁 (2Mg + O₂ → 2MgO)。质量增加的部分恰好等于与镁化合的氧气质量。这证明了质量守恒定律——物质既不能创造也不能消灭，只能重新组合。
Safety: The bright UV light can damage eyes; students should observe through safety screens or use tinted goggles. Never look directly at burning magnesium.
安全：强烈的紫外光会损伤眼睛；学生应通过安全屏或佩戴深色护目镜观察。千万不要直视燃烧的镁条。
6. Case Study 6: Making Copper Sulfate Crystals | 案例6：制备硫酸铜晶体
Scenario: A student wants to prepare pure blue copper sulfate crystals (CuSO₄・5H₂O) starting from black copper(II) oxide powder and warm dilute sulfuric acid. She follows a multi-step procedure: reaction, filtration, evaporation and crystallisation.
情境：一位学生打算用黑色氧化铜粉末与温热的稀硫酸为原料，制备纯净的蓝色硫酸铜晶体 (CuSO₄・5H₂O)。她依次进行反应、过滤、蒸发和结晶操作。
Step 1 – s, and a clear blue solution of copper sulfate forms. Equation: CuO + H₂SO₄ → CuSO₄ + H₂O.
步骤1 – 反应：在不断搅拌下，将氧化铜分次加入温热的稀硫酸中。黑色粉末消失，得到透明的蓝色硫酸铜溶液。化学方程式：CuO + H₂SO₄ → CuSO₄ + H₂O。
Step 2 – Filtration: Once no more CuO dissolves, filter the mixture to remove any unreacted black solid. The filtrate is a pure blue solution.
步骤2 – 过滤：当没有更多氧化铜溶解时，过滤混合物以除去未反应的黑色固体。滤液为纯净的蓝色溶液。
Step 3 – Evaporation and crystallisation: Gently heat the filtrate in an evaporating dish to concentrate it. Once crystals start to appear at the edge, stop heating and let it cool slowly. Large, well-shaped blue crystals form overnight.
步骤3 – 蒸发与结晶：将滤液放在蒸发皿中，小火加热浓缩。当边缘开始出现晶膜时，停止加热，让其缓慢冷却。静置一夜，可获得大颗粒、形状规则的蓝色晶体。
Why crystallisation rather than drying to dryness? Rapid evaporation gives a messy powder. Slower cooling yields purer, larger crystals because particles have time to arrange in a regular lattice.
为何不直接蒸干？快速蒸干只能得到杂乱粉末。缓慢冷却能析出更大且更纯净的晶体，因为微粒有充足时间排列成规则晶格。
7. Case Study 7: Distinguishing Oxygen, Hydrogen and Carbon Dioxide | 案例7：区分氧气、氢气和二氧化碳
Scenario: Three gas jars are unlabelled, each containing one of the colourless gases: oxygen (O₂), hydrogen (H₂) and carbon dioxide (CO₂). You have a glowing splint, a burning splint and limewater. Devise a test sequence to identify each gas safely.
情境：三个集气瓶没有标签，分别装有氧气 (O₂)、氢气 (H₂) 和二氧化碳 (CO₂)，均为无色气体。你手头有带火星的木条、燃着的木条和石灰水。设计一个安全的测试顺序来鉴别每种气体。
Test 1 – Glowing splint: A glowing splint lowered into a gas jar relights immediately if the gas is oxygen. The splint bursts into flame because oxygen supports combustion.
测试1 – 带火星木条：将带火星的木条伸入集气瓶，如气体为氧气，木条立即复燃。氧气助燃，因此木条重新燃烧起来。
Test 2 – te), the gas is carbon dioxide. The milky substance is calcium carbonate formed in the reaction: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O.
测试2 – 石灰水：向石灰水中通入气体样品（或用少量石灰水与气体一同摇荡）。若石灰水变浑浊（白色乳状），气体为二氧化碳。浑浊物是生成的碳酸钙：CO₂ + Ca(OH)₂ → CaCO₃ + H₂O。
Test 3 – Burning splint: Hold a burning splint at the mouth of the third gas jar. If the gas is hydrogen, it burns with a 'squeaky pop' sound as the hydrogen ignites and rapidly combines with oxygen in the air. This test must be performed on a small scale, with a safety screen.
测试3 – 燃着的木条：将燃着的木条置于第三个集气瓶口。如果气体是氢气，会听到尖锐的爆鸣声（'squeaky pop'），因为氢气被点燃，与空气中氧气迅速化合。此测试必须小规模进行，并使用安全防护屏。
Result summary: Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Hydrogen gives a squeaky pop with a burning splint.
结果总结：氧气使带火星木条复燃；二氧化碳使石灰水变浑浊；氢气遇燃着木条发出爆鸣声。
8. Case Study 8: Investigating Reaction Rate – Marble Chips and Acid | 案例8：探究反应速率——大理石与酸
Scenario: A group investigates how the concentration of hydrochloric acid affects the rate of reaction with marble chips (calcium carbonate). They use the apparatus shown: a conical flask with acid and marble chips, connected to a gas syringe. They measure the volume of carbon dioxide gas collected every 30 seconds.
情境：一个小组探究盐酸浓度对大理石（碳酸钙）反应速率的影响。他们使用锥形瓶装盐酸和大理石，连接气体注射器，测量每30秒收集到的二氧化碳体积。
Variables: Independent variable: acid concentration (0.5 mol/dm³, 1.0 mol/dm³, 1.5 mol/dm³). Dependent variable: volume of CO₂ produced. Controlled variables: mass of marble chips, volume of acid, temperature, surface area of chips.
变量：自变量：酸的浓度（0.5 mol/dm³, 1.0 mol/dm³, 1.5 mol/dm³）。因变量：生成的 CO₂ 体积。控制变量：大理石质量、酸体积、温度、大理石的表面积。
Observations: With higher acid concentration, bubbles of CO₂ appear faster, and the gas syringe plunger moves out more quickly. The graph of volume vs. time shows a steeper initial slope for the most concentrated acid. All reactions eventually produce the same final volume of CO₂ because the same mass of marble is used in each run.
观察：酸浓度越高，CO₂ 气泡冒出越快，气体注射器活塞外移也越快。体积-时间曲线显示，浓度最高的酸初始斜率最大。所有反应最终都产生相同的 CO₂ 最终体积，因为每次使用的大理石质量相同。
Particle explanation: A higher concentration means more HCl particles per unit volume. They collide more frequently with the surface of marble chips, increasing the frequency of successful collisions, hence a faster rate.
微粒解释：浓度越高，单位体积内的 HCl 微粒越多。它们与大理石表面的碰撞更加频繁，成功碰撞的频率增加，因此反应速率加快。
Equation: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O.
化学方程式：CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O。
9. Case Study 9: Electrolysis of Water | 案例9：水的电解
Scenario: A teacher uses a Hofmann voltameter to electrolyse water containing a little dilute sulfuric acid (to improve conductivity). Two gases collect at the electrodes. Students note the volumes and test the gases.
情境：老师使用霍夫曼电解器，电解含有少量稀硫酸的水（以增强导电性）。两个电极上收集到气体。学生们记录气体体积并进行检验。
Observations: At the cathode (negative electrode), gas is produced; at the anode (positive electrode), gas is also produced. The volume of gas at the cathode is twice the volume at the anode.
观察：在阴极（负极）产生气体；阳极（正极）也产生气体。阴极气体的体积是阳极气体体积的两倍。
Gas tests: Gas from the cathode burns with a 'squeaky pop', confirming it is hydrogen (H₂). Gas from the anode relights a glowing splint, confirming it is oxygen (O₂).
气体检验：阴极收集的气体遇火发出爆鸣声，确认为氢气 (H₂)。阳极收集的气体使带火星木条复燃，确认为氧气 (O₂)。
o of hydrogen to oxygen volumes is 2:1 because water contains twice as many hydrogen atoms as oxygen atoms. The added sulfuric acid does not participate in the electrolysis itself; it merely provides ions that allow the current to flow.
解释：水分解为其组成元素：2H₂O → 2H₂ + O₂。氢气与氧气的体积比为 2:1，因为水分子中氢原子数是氧原子数的两倍。加入的稀硫酸本身不参与电解过程，只是提供导电离子。
Safety: Hydrogen is highly flammable; test small samples only. The apparatus must be sealed properly to avoid gas leakage.
安全：氢气高度易燃，仅取少量样品检验。装置必须良好密封，防止气体泄漏。
10. Case Study 10: Purifying Water | 案例10：净水
Scenario: After a flood, a village's water supply becomes muddy and possibly contaminated. A science student explains how the water can be made safe to drink using simple physical and chemical processes: sedimentation, filtration and chlorination.
情境：洪水过后，村庄的水源变得浑浊，并可能受到污染。一位理科生解释如何使用沉降、过滤和氯化等简单的物理化学方法，让水恢复可饮用。
Step 1 – Sedimentation: The muddy water is left to stand in a large tank. Large, heavy particles settle to the bottom by gravity. Alum (aluminium sulfate) can be added to help fine particles clump together (flocculation) and settle faster.
步骤1 – 沉降：将浑浊水引入大池中静置。依靠重力，大而重的颗粒沉入池底。可加入明矾（硫酸铝）帮助细小颗粒聚集成团（絮凝），加速沉降。
Step 2 – Filtration: The clearer water from the top is passed through layers of sand and gravel. These filter out remaining suspended solids and some microorganisms. At home, a simple filter can be made using gravel, sand and cloth.
步骤2 – 过滤：将上层较清的水通过沙层和砾石层过滤。这些滤层截留剩余的悬浮固体和部分微生物。家庭中可用砾石、沙子和布制作简易过滤器。
Step 3 – Chlorination: A carefully measured amount of chlorine (or sodium hypochlorite) is added to the filtered water. Chlorine kills bacteria, viruses and other harmful pathogens, making the water microbiologically safe. The treated water is then stored in clean, covered tanks.
步骤3 – 氯化消毒：向过滤后的水中加入精确计量的氯（或次氯酸钠）。氯能杀灭细菌、病毒及其他有害病原体，确保水的微生物安全。处理后水储存在洁净、有盖的水箱中。
Limitations: Distillation would produce even purer water, but it requires a lot of energy. The three-stage method is practical for communities. However, it does not remove dissolved chemical pollutants like heavy metal ions. Advanced treatment would be needed for that.
局限：蒸馏能制备更纯净的水，但能耗很高。三级处理法对社区而言切实可行。不过它无法去除溶解的化学污染物，如重金属离子。若存在此类污染物，则需先进处理手段。
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