📚 Case Study Practical Exercises for Year 7 CIE Chemistry | 七年级CIE化学案例分析实战演练
Welcome to this collection of case study practical exercises designed for Year 7 CIE Chemistry. Each case presents a real-world scenario that challenges you to apply core chemical concepts, from states of matter and separation techniques to acids, alkalis, and the rock cycle. These exercises will strengthen your understanding and prepare you for both coursework and examinations. Read each scenario carefully, think like a scientist, and enjoy connecting classroom theory to everyday life.
欢迎来到为七年级CIE化学设计的案例分析实战演练合集。每个案例都展现了一个现实世界中的情景，挑战你运用核心化学概念，从物质状态和分离技术到酸碱以及岩石循环。这些练习将加深你的理解，并为你的课程作业和考试做好准备。请仔细阅读每个情景，像科学家一样思考，并享受将课堂理论与日常生活联系起来的过程。
1. A Mysterious Puddle: States of Matter and the Particle Model | 神秘的水洼：物质状态与粒子模型
Scenario: After a heavy summer rain, a large puddle forms on the pavement. By midday, the puddle is completely gone, even though no one swept the water away. Your friend insists the water simply disappeared into nothing.
情景：一场夏季大雨过后，人行道上形成了一个大水洼。到了中午，水洼完全消失了，尽管没有人把水扫走。你的朋友坚持认为水就这样消失得无影无踪了。
Your task: Use the particle model to explain what really happened. Why does the puddle disappear faster on a warm, windy day? How is this different from boiling?
你的任务：用粒子模型解释真正发生了什么。为什么在温暖有风的日子里水洼消失得更快？这与沸腾有何不同？
Thinking and answer: Water is made of tiny particles that are constantly moving. In a puddle, particles at the surface have different amounts of energy. Some fast-moving particles overcome attractive forces and escape into the air as a gas – this process is called evaporation. On a warm day, particles gain more thermal energy, so they move faster and escape more easily. Wind carries away the escaped particles, preventing them from returning to the liquid, which speeds up evaporation. Evaporation occurs only at the surface and at any temperature, while boiling happens throughout the liquid at a specific temperature (the boiling point). No water ‘disappeared’ – it simply changed state from liquid to gas, spreading into the atmosphere as invisible water vapour.
思考与解答：水由不断运动的微小粒子组成。在水洼中，表面的粒子具有不同的能量。一些运动较快的粒子克服了吸引力，以气体的形式逃逸到空中——这个过程叫做蒸发。在温暖的日子里，粒子获得更多热能，因此运动更快，更容易逃逸。风将逃逸的粒子带走，阻止它们回到液体中，从而加速蒸发。蒸发仅发生在液体表面，且在任何温度下都可发生，而沸腾则在特定温度（沸点）下整个液体内部同时发生。水并没有“消失”——它只是从液态变为气态，以看不见的水蒸气形式扩散到大气中。
2. The Messy Mixture: Separation Techniques in Action | 混乱的混合物：分离技术在行动
Scenario: In a Year 7 lab, a clumsy student accidentally mixes together sand, salt, and iron filings into a single beaker. The challenge is to recover all three components as pure as possible, using only equipment available in the school laboratory.
情景：在七年级实验室里，一个粗心的学生不小心将沙子、盐和铁屑混在一个烧杯里。挑战是只使用学校实验室可用的设备，尽可能纯地回收这三种成分。
Your task: Design a step-by-step plan to separate the mixture. Name each separation method and explain why it works for that particular substance.
你的任务：设计一个逐步分离该混合物的计划。命名每种分离方法，并解释它为什么适用于那种特定物质。
Thinking and answer: First, wrap a magnet in a plastic bag and move it through the mixture. Iron is magnetic, so iron filings will be attracted to the magnet, leaving the sand and salt behind. This is magnetic separation. Next, add the remaining sand–salt mixture to water and stir. Salt is soluble in water, but sand is insoluble. Then filter the mixture using filter paper and a funnel. The sand stays on the filter paper as a residue, while the salt solution (filtrate) passes through. This is filtration. Finally, pour the salt solution into an evaporating basin and gently heat it. Water will evaporate, leaving solid salt crystals behind. This is evaporation or crystallisation. In this way, all three components are recovered physically, without any chemical reactions occurring.
思考与解答：首先，用塑料袋包裹磁铁并在混合物中移动。铁具有磁性，因此铁屑会被磁铁吸引，留下沙子和盐。这是磁性分离。接着，将剩余的沙盐混合物加入水中搅拌。盐可溶于水，而沙子不溶。然后用滤纸和漏斗过滤混合物。沙子作为残渣留在滤纸上，而盐溶液（滤液）透过滤纸。这是过滤。最后，将盐溶液倒入蒸发皿中，温和加热。水分蒸发，留下固态盐晶体。这是蒸发或结晶。通过这种方式，三种组分均通过物理方法回收，没有发生任何化学反应。
3. Rescuing the Garden: Acids, Bases and Indicators | 拯救花园：酸、碱与指示剂
Scenario: Farmer Emily notices that most of her vegetables are growing poorly, and the soil has turned a strange reddish colour. A simple test with universal indicator paper shows a pH value of 4.5. She wants to know how to fix the soil to make it more suitable for plants, which usually prefer a pH between 6 and 7.
情景：农民艾米丽注意到她的大部分蔬菜长势不佳，而且土壤变成了奇怪的微红色。用通用指示剂试纸进行简单测试，显示pH值为4.5。她想知道如何改良土壤，使其更适合植物生长，植物通常喜欢pH值在6到7之间。
Your task: Explain what the pH value tells us about the soil. Suggest a safe substance she could use to treat the soil, and write a word equation for the chemical reaction that will occur. Explain how an indicator helps monitor the change.
你的任务：解释pH值告诉我们土壤的什么信息。建议一种她可以用来处理土壤的安全物质，并写出将要发生的化学反应的文字方程式。解释指示剂如何帮助监测变化。
Thinking and answer: A pH of 4.5 means the soil is acidic. Acids have a pH below 7, with stronger acids having lower values. To bring the pH closer to neutral, Emily should add a base. A common and safe base used in gardening is powdered limestone (calcium carbonate) or slaked lime (calcium hydroxide). When a base reacts with an acid, a neutralisation reaction occurs. For instance: acid (in soil) + calcium carbonate → calcium salt + water + carbon dioxide. The pH can be monitored repeatedly with universal indicator paper; as the soil becomes less acidic, the colour changes from red/orange to green. Once the pH reaches about 6.5, the soil is ready for planting. This is a classic application of acids, alkalis, and indicators in agriculture.
思考与解答：pH值为4.5意味着土壤是酸性的。酸的pH值低于7，酸性越强数值越低。为了使pH值更接近中性，艾米丽应该添加碱。园艺中常用的一种安全碱是粉末状石灰石（碳酸钙）或熟石灰（氢氧化钙）。当碱与酸反应时，发生中和反应。例如：酸（土壤中的）+ 碳酸钙 → 钙盐 + 水 + 二氧化碳。可以用通用指示剂试纸反复监测pH值；随着土壤酸性减弱，颜色从红色/橙色变为绿色。一旦pH值达到6.5左右，土壤就可以种植了。这是酸、碱和指示剂在农业中的一个经典应用。
4. The Perfect Cake: Physical and Chemical Changes | 完美的蛋糕：物理变化与化学变化
Scenario: Leo is baking a sponge cake. He carefully weighs flour, sugar, butter, and eggs. He notices that butter melts when heated, the mixture rises in the oven, and the final cake has a completely different taste and texture from the raw ingredients. He wonders whether these steps are physical or chemical changes.
情景：利奥正在烤一个海绵蛋糕。他小心地称量了面粉、糖、黄油和鸡蛋。他注意到黄油加热时融化，混合物在烤箱中膨胀起来，最终的蛋糕在味道和质地上与原材料完全不同。他想知道这些步骤是物理变化还是化学变化。
Your task: Identify which steps in the cake-making process are physical changes and which are chemical changes. Provide clear evidence to support your classification for each change.
你的任务：找出蛋糕制作过程中哪些步骤是物理变化，哪些是化学变化。为每种变化的分类提供明确的证据。
Thinking and answer: Melting butter is a physical change because it only involves a change of state from solid to liquid. No new substance is formed; the melted butter is still butter and can be solidified again by cooling. Mixing the ingredients together is also mainly a physical change, as the substances are simply dispersed among each other without altering their chemical identities. However, baking the cake is a chemical change. The heat causes reactions such as the decomposition of baking powder to produce carbon dioxide gas, which makes the cake rise, and the browning of sugars (caramelisation). New substances are formed, and the process is irreversible – you cannot get back the raw flour, butter, or eggs from the baked cake. The change in colour, production of gas bubbles, and an irreversible change in taste are all strong indicators of a chemical reaction.
思考与解答：融化黄油是一个物理变化，因为它仅涉及从固态到液态的状态变化。没有新物质生成；融化的黄油仍然是黄油，可以通过冷却再次固化。将各种配料混合在一起也主要是物理变化，因为各种物质只是相互分散，而没有改变它们的化学特性。然而，烘烤蛋糕是一个化学变化。热量引发了诸如发酵粉分解产生二氧化碳气体（使蛋糕膨胀）以及糖类褐变（焦糖化）等反应。有新物质生成，并且该过程不可逆——你无法从烤好的蛋糕中重新获得生的面粉、黄油或鸡蛋。颜色变化、气泡的产生以及不可逆的味道变化都是化学反应的强烈指示。
5. The White Crystal Mystery: Elements, Compounds and Mixtures | 白色晶体之谜：元素、化合物与混合物
Scenario: A student is given a small jar containing shiny white crystals, labelled only as ‘Sample X’. The sample has a fixed melting point of 801 °C, dissolves in water to form a conducting solution, and when heated strongly does not decompose into simpler substances but melts as a whole. It is also not attracted to a magnet and cannot be separated into different parts by hand-picking.
情景：一位学生拿到一个小瓶子，里面装着有光泽的白色晶体，标签上仅写着“样品X”。该样品有固定的熔点801°C，溶于水形成可导电的溶液，强热时不会分解为更简单的物质，而是整体熔化。它也不能被磁铁吸引，无法通过人工挑选分离成不同的部分。
Your task: Based on these observations, decide whether Sample X is best classified as an element, a compound, or a mixture. Justify your reasoning using the definitions of each category.
你的任务：根据这些观察结果，判断样品X最适合被归类为元素、化合物还是混合物。用每个类别的定义来证明你的推理。
Thinking and answer: A mixture consists of two or more substances not chemically combined, so it would not have a single fixed melting point – it would melt over a range of temperatures. Since Sample X melts sharply at 801 °C, it is unlikely to be a mixture. Also, it cannot be separated by physical means like hand-picking or magnetism. An element is a pure substance made of only one type of atom. A compound is a pure substance made of two or more types of atoms chemically bonded together. Sample X dissolves in water to give a conducting solution, which suggests the presence of ions; many elements (e.g., metals) conduct in solid state or when molten, but elements rarely dissolve in water to produce a conducting solution without reacting. A typical compound like sodium chloride (table salt) matches this behaviour: fixed melting point, dissolves to give a conducting solution. Therefore, Sample X is best classified as a compound. It is pure and has properties distinct from its constituent elements.
思考与解答：混合物由两种或多种未化学结合的物质组成，因此它不会有单一的固定熔点——它会在一定的温度范围内熔化。由于样品X在801°C时急剧熔化，它不太可能是混合物。此外，它无法通过诸如手选或磁力等物理方法分离。元素是仅由一种原子组成的纯净物。化合物是由两种或多种原子通过化学键结合而成的纯净物。样品X溶于水形成导电溶液，这表明存在离子；许多元素（例如金属）在固态或熔融状态下导电，但元素很少能在不反应的情况下溶于水产生导电溶液。一种典型的化合物，如氯化钠（食盐），符合这种行为：固定熔点，溶解后形成导电溶液。因此，样品X最好归类为化合物。它是纯净的，并且具有与其组成元素不同的性质。
6. Identifying the Unlabelled Substances: Metals and Non-metals | 鉴别未标记物质：金属与非金属
Scenario: In the stockroom, two containers have lost their labels. One contains a shiny, grey solid that can be hammered into sheets, conducts electricity, and reacts with dilute acid to produce bubbles of gas. The other contains a dull, brittle solid that does not conduct electricity and does not react with acid under the same conditions.
情景：在储藏室里，有两个容器丢失了标签。一个装有有光泽的灰色固体，可被锤打成薄片，导电，并且与稀酸反应产生气泡。另一个装有暗沉的脆性固体，不导电，在相同条件下不与酸反应。
Your task: Using your knowledge of the properties of metals and non-metals, classify each substance. Identify key properties that led to your conclusion, and predict what gas might be produced during the acid test if the first substance is a metal.
你的任务：运用你对金属和非金属性质的知识，对每种物质进行分类。指出导致你得出结论的关键性质，并预测如果第一种物质是金属，在酸测试中可能产生什么气体。
Thinking and answer: The first substance shows typical metallic properties: shiny (lustrous), malleable (can be hammered into sheets), good electrical conductivity, and reaction with acid to release gas. These are characteristic of metals. Most metals react with dilute acids to produce hydrogen gas, so the bubbles are likely hydrogen (tested with a burning splint producing a squeaky pop). The second substance is dull, brittle (breaks easily rather than bending), an insulator for electricity, and unreactive with acid. These are typical non-metal properties. It could be a solid non-metal like carbon or sulfur. Metals tend to be on the left side of the periodic table and have free electrons that allow conductivity, while non-metals lack these free electrons. This case study reinforces the clear contrast between the two classes of elements.
思考与解答：第一种物质显示出典型的金属性质：有光泽（光亮），有延展性（可锤成薄片），导电性良好，并与酸反应放出气体。这些都是金属的特征。大多数金属与稀酸反应生成氢气，因此这些气泡很可能是氢气（使用点燃的木条进行测试会产生“噗”的一声）。第二种物质暗淡无光，脆性（易碎而非弯曲），电绝缘，且不与酸反应。这些是典型的非金属性质。它可能是像碳或硫这样的固态非金属。金属通常位于元素周期表的左侧，并具有允许导电的自由电子，而非金属缺乏这些自由电子。这个案例研究加强了这两类元素之间的鲜明对比。
7. The Rock Detective: Journey Through the Rock Cycle | 岩石侦探：穿越岩石循环之旅
Scenario: A geologist discovers a rock layer containing fossilised sea creatures high up in a mountain range. The rock itself is soft and crumbly, with clear layers. Nearby, she finds a much harder, banded rock with interlocking crystals that does not contain fossils. Deep beneath the area, there is evidence of magma activity.
情景：一位地质学家在山脉高处发现了一个含有海洋生物化石的岩层。岩石本身松软易碎，层理清晰。在附近，她找到了一种坚硬得多、具有交错晶体且不含化石的条纹状岩石。在该区域深处，有岩浆活动的证据。
Your task: Name the two main types of rock described, and explain how each likely formed. Use the rock cycle to describe a possible path from the fossil-containing rock to the banded rock. What role does magma play in this transformation?
你的任务：为所描述的两种主要岩石类型命名，并解释每种岩石可能如何形成。利用岩石循环描述从含有化石的岩石到条纹状岩石的可能路径。岩浆在这一转变中扮演什么角色？
Thinking and answer: The soft, layered rock containing fossils is sedimentary rock, such as limestone or shale. It formed millions of years ago from the accumulation and compression of sediments, including shells and skeletons of marine organisms, at the bottom of an ancient sea. Over time, tectonic movements uplifted these layers to form mountains. The hard, banded rock without fossils is metamorphic rock, such as gneiss or schist. It formed when the original sedimentary rock was subjected to intense heat and pressure due to burial and nearby magma. The minerals recrystallised into larger, interlocking crystals, and layers of minerals created the banded texture. Magma provides the thermal energy and surrounding pressure. If the rock had melted completely and then cooled, it would have become an igneous rock, but the presence of banding indicates it remained solid during the change. This journey shows the dynamic nature of Earth’s crust through the rock cycle.
思考与解答：含有化石的松软层状岩石是沉积岩，例如石灰岩或页岩。它形成于数百万年前，由沉积物（包括海洋生物的贝壳和骨骼）在古海底积累并压缩而成。随着时间的推移，构造运动将这些岩层抬升形成山脉。不含化石的坚硬条纹状岩石是变质岩，例如片麻岩或片岩。它是由原来的沉积岩在埋藏和附近岩浆的作用下经受高温高压而形成。矿物重结晶成更大、交错的晶体，矿物的分层形成了条纹状纹理。岩浆提供了热能和周围压力。如果岩石完全熔化然后冷却，它就会变成火成岩，但条纹的存在表明它在变化过程中保持固态。这段旅程显示了地壳通过岩石循环的动态特性。
8. Surviving in the Wild: Water Purification Challenge | 野外生存：水净化挑战
Scenario: A group of hikers is lost in the wilderness and has run out of clean drinking water. They find a muddy river and are aware that the water contains suspended dirt, bacteria, and possibly dissolved salts. They have basic camping equipment: pots, cloth, a gas stove, and a clear plastic sheet.
情景：一群徒步旅行者在荒野中迷路，且已用尽干净的饮用水。他们发现了一条浑浊的河流，意识到水中含有悬浮的泥土、细菌，可能还有溶解的盐类。他们只有基本的露营设备：锅、布料、燃气炉和一张透明塑料布。
Your task: Design a sequence of purification steps to make the river water safe to drink. For each step, explain which impurities it removes. What step is essential to kill harmful microorganisms?
你的任务：设计一系列净化步骤，使河水可以安全饮用。对每一步，解释它去除了哪些杂质。哪一步对杀灭有害微生物至关重要？
Thinking and answer: Step 1: Filtration through cloth. Pour the muddy water through a folded piece of clean cloth to remove large suspended particles like silt and sand. This improves clarity but does not remove dissolved substances or germs. Step 2: Settling and decantation. Allow the filtrate to stand so remaining fine particles settle at the bottom, then carefully pour off the clear water. Step 3: Boiling. Bring the clear water to a rolling boil for at least one minute (longer at high altitude). Boiling kills bacteria, viruses, and other pathogens, making water microbiologically safe. It does not remove dissolved salts or chemicals. Step 4 (optional): If they suspected dissolved salts, they could create a simple solar still using the plastic sheet to evaporate and condense pure water, leaving salts behind. Thus, the crucial step to destroy microorganisms is boiling. Chemical tablets or UV pens are modern alternatives, but with basic gear, boiling is the most reliable method.
思考与解答：第一步：用布过滤。将泥水倒过一块折叠的干净布料，去除大的悬浮颗粒，如淤泥和沙子。这改善了澄清度，但不能去除溶解性物质或病菌。第二步：沉降与倾析。让滤液静置，使剩余的细小颗粒沉至底部，然后小心地倒出清澈的水。第三步：煮沸。将清澈的水加热至沸腾翻滚至少一分钟（在高海拔地区时间更长）。煮沸能杀死细菌、病毒和其他病原体，使水在微生物学上变得安全。它不能去除溶解的盐类或化学物质。第四步（可选）：如果他们怀疑有溶解的盐类，可以利用塑料布制作一个简单的太阳能蒸馏器，通过蒸发和凝结得到纯水，将盐类留下。因此，灭活微生物的关键步骤是煮沸。化学药片或紫外线笔是现代替代品，但在只有基本装备的情况下，煮沸是最可靠的方法。
9. The Candle Experiment: Uncovering the Gases in Air | 蜡烛实验：揭示空气中的气体
Scenario: In a classic investigation, a student places a lit candle on a tray of water and covers it with a glass jar. After a short time, the candle flame goes out, and water rises into the jar, occupying about one-fifth of the jar’s volume. The student wonders why this happens and what it tells us about the composition of air.
情景：在一个经典探究中，一名学生将点燃的蜡烛放在盛有水的托盘上，并用一个玻璃罐罩住。片刻之后，蜡烛火焰熄灭，罐内水位上升，占据罐子大约五分之一的容积。学生想知道为什么会发生这种现象，以及它告诉了我们关于空气组成的什么信息。
Your task: Explain in terms of chemical reactions why the candle goes out and why water rises. Relate this to the main gases in air and their proportions. What simple test could you perform on the remaining gas inside the jar?
你的任务：用化学反应解释蜡烛为何熄灭以及水为何上升。将此与空气中的主要气体及其比例联系起来。你能对罐内剩余气体进行什么简单测试？
Thinking and answer: The candle wax (a hydrocarbon) burns by reacting with oxygen in the air, producing carbon dioxide and water vapour. The reaction is: wax + oxygen → carbon dioxide + water. When the oxygen inside the jar is used up, combustion stops and the flame extinguishes. Initially, the air inside heats up and expands, but as the candle goes out, the air cools and contracts. Additionally, the carbon dioxide produced dissolves slightly in water, and water vapour condenses. These effects reduce the pressure inside the jar, so the higher external atmospheric pressure pushes water up into the jar. The water rise of about one-fifth indicates that oxygen made up roughly 20% of the air, which matches the true proportion (21% oxygen, 78% nitrogen). The remaining gas is mainly nitrogen. To test it, one could introduce a glowing splint – it would be immediately extinguished, confirming the absence of oxygen. This classic experiment illustrates the concept of active and inactive parts of air.
思考与解答：蜡烛蜡（一种碳氢化合物）通过空气中的氧气燃烧，生成二氧化碳和水蒸气。反应为：蜡 + 氧气 → 二氧化碳 + 水。当罐内的氧气耗尽时，燃烧停止，火焰熄灭。起初，罐内空气受热膨胀，但随着蜡烛熄灭，空气冷却收缩。此外，产生的二氧化碳部分溶解于水，水蒸气凝结。这些效应降低了罐内气压，因此较高的外部大气压将水推入罐中。水位上升约五分之一表明氧气约占空气的20%，这符合真实比例（21%氧气，78%氮气）。剩余气体主要是氮气。要测试它，可以放入一根带火星的木条——它会立即熄灭，证实没有氧气的存在。这个经典实验展示了空气中活性部分与非活性部分的概念。
10. Acid Rain and Ancient Monuments: Environmental Chemistry at Work | 酸雨与古迹：环境化学在行动
Scenario: A historic statue made of marble (calcium carbonate) has been standing in a city for over 200 years. In recent decades, the statue’s features have become blurred, and the surface shows signs of wearing away. Scientists record that the local rainwater has a pH of around 4, significantly lower than the expected 5.6 for clean rain.
情景：一座由大理石（碳酸钙）制成的历史雕像矗立在某城市已有200多年。近几十年来，雕像的面部特征变得模糊，表面出现被侵蚀的迹象。科学家记录显示当地雨水的pH值约为4，明显低于洁净雨水的预期值5.6。
Your task: Explain how acid rain forms from air pollutants. Write a word equation for the reaction between marble and acid rain. Suggest what could be done to slow down the damage, and link your answer to the chemistry of neutralisation.
你的任务：解释酸雨如何由空气污染物形成。写出大理石与酸雨反应的文字方程式。提出减缓损害的建议，并将你的答案与中和反应的化学原理联系起来。
Thinking and answer: Acid rain is primarily caused by sulfur dioxide and nitrogen oxides released from burning fossil fuels in power stations and vehicles. These gases dissolve in water droplets in clouds, forming sulfuric acid and nitric acid, which fall as rain with a pH lower than 5.6. The marble statue is made of calcium carbonate, which reacts with the acids: calcium carbonate + sulfuric acid → calcium sulfate + water + carbon dioxide. The calcium sulfate is slightly soluble and can be washed away, gradually dissolving the statue. To slow down the damage, we can reduce emissions by using cleaner energy sources, fitting scrubbers in chimneys, and driving electric vehicles. In parallel, localised neutralisation could involve applying a mildly alkaline protective coating (like limewash) to the statue, providing a sacrificial layer that reacts with acid before reaching the marble. This is a direct application of neutralisation chemistry to conserve cultural heritage.
思考与解答：酸雨主要由燃烧化石燃料的发电站和车辆释放的二氧化硫和氮氧化物引起。这些气体溶解在云层的水滴中，形成硫酸和硝酸，并以pH值低于5.6的雨水形式降落。大理石雕像由碳酸钙制成，与酸反应：碳酸钙 + 硫酸 → 硫酸钙 + 水 + 二氧化碳。硫酸钙微溶，可被冲刷掉，逐渐溶解雕像。为减缓损害，我们可以通过使用更清洁的能源、在烟囱中安装洗涤器以及推广电动汽车来减少排放。同时，局部中和处理可以包括在雕像上涂抹温和的碱性保护涂层（如石灰水），提供一个牺牲层，在酸接触到石头之前与之反应。这是利用中和化学保护文化遗产的一个直接应用。
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