📚 Year 8 Edexcel Chemistry: Case Study Practice | Edexcel 八年级化学案例分析实战演练
Case study questions in chemistry require you to apply your knowledge to real-world scenarios. This article provides a series of Edexcel Year 8 style case studies, each designed to strengthen your understanding of key topics such as elements, compounds, mixtures, chemical reactions, acids and alkalis, and Earth science. Work through each case carefully, using the evidence provided and your scientific reasoning.
化学案例分析题要求你将所学知识应用到实际情境中。本文提供了一系列 Edexcel 八年级风格的案例研究，每个案例旨在加深你对元素、化合物、混合物、化学反应、酸碱以及地球科学等核心主题的理解。请仔细阅读每个案例，利用提供的证据和你的科学推理来完成分析。
1. Case 1: White Powder Mystery – Mixture or Compound? | 案例一：白色粉末之谜——混合物还是化合物？
Two white powders, A and B, were found in the lab. When water was added to Powder A, some of it dissolved, leaving a grainy white residue at the bottom. Upon heating strongly, Powder A left behind a black charred mass with a distinct smell of burnt sugar.
实验室里发现了两种白色粉末 A 和 B。向粉末 A 加水后，部分粉末溶解，底部留下颗粒状的白色残渣。对粉末 A 进行强烈加热，最终留下黑色焦状物，并伴有明显的焦糖气味。
Powder B did not dissolve in water. When heated, it produced bubbles of a gas that turned limewater cloudy, and a white solid remained in the test tube.
粉末 B 不溶于水。加热时产生气泡，该气体能使石灰水变浑浊，而试管中留下白色固体。
Question: Which powder is a mixture and which is a compound? Justify your conclusion using the observations.
问题：哪种粉末是混合物，哪种是化合物？利用观察结果证明你的结论。
Analysis: Powder A is a mixture. The fact that part of it dissolved (sugar) while another part did not (salt) shows it contains at least two substances with different properties. The burnt sugar smell on heating confirms one component decomposes on its own. Mixtures can be separated by physical means, such as dissolving and filtering.
分析：粉末 A 是混合物。它部分溶解（糖）而部分不溶（盐），表明它至少含有两种性质不同的物质。加热时焦糖气味证实其中一种组分单独分解。混合物可以通过物理方法（如溶解和过滤）进行分离。
Powder B is a compound. It does not dissolve in water as a whole and undergoes a chemical change on heating, producing carbon dioxide (shown by limewater turning cloudy) and calcium oxide. This demonstrates that the substance breaks down into simpler substances, a characteristic of compounds. Compounds cannot be separated by physical techniques.
粉末 B 是化合物。它整体不溶于水，加热时发生化学变化，生成二氧化碳（使石灰水变浑浊）和氧化钙。这表明该物质分解成了更简单的物质，是化合物的典型特征。化合物不能用物理方法分离。
2. Case 2: Extracting Pure Salt from Rock Salt | 案例二：从岩盐中提取纯盐
Rock salt is a naturally occurring mixture of sodium chloride (common salt) and sand, along with other insoluble impurities. A student needs to obtain pure, dry salt crystals from a sample of rock salt. The available equipment includes a mortar and pestle, beakers, water, filter paper, funnel, evaporating dish, Bunsen burner, and tripod.
岩盐是天然存在的混合物，含有氯化钠（食盐）和沙子，以及其他不溶性杂质。一名学生需要从岩盐样品中获得纯净干燥的食盐晶体。可用器材包括研钵和杵、烧杯、水、滤纸、漏斗、蒸发皿、本生灯和三脚架。
Task: Describe a step-by-step method the student should use, explaining why each step is needed.
任务：描述学生应该采用的分步方法，并解释每一步的必要性。
Step 1: Crush the rock salt into smaller pieces using a mortar and pestle. This increases the surface area, making the salt dissolve faster in water.
步骤 1：用研钵和杵将岩盐碾碎成较小的颗粒。这增加了表面积，使盐在水中溶解得更快。
Step 2: Add the crushed rock salt to a beaker of warm water and stir thoroughly. Salt is soluble in water, but sand is insoluble, so the salt dissolves while sand remains as solid grains.
步骤 2：将碾碎的岩盐加入到盛有温水的烧杯中，充分搅拌。盐可溶于水，而沙子不溶，因此盐溶解，沙子仍以固体颗粒形式存在。
Step 3: Filter the mixture through filter paper placed in a funnel. The sand is trapped on the filter paper as residue, while the salt solution (filtrate) passes through, thus separating the insoluble impurities.
步骤 3：用放在漏斗中的滤纸过滤混合物。沙子作为滤渣留在滤纸上，而盐水（滤液）通过滤纸，从而分离出不溶性杂质。
Step 4: Pour the filtrate into an evaporating dish and gently heat it using a Bunsen burner. The water evaporates, leaving behind white salt crystals. Slow heating near the end prevents spitting and ensures pure, dry crystals are collected.
步骤 4：将滤液倒入蒸发皿中，用本生灯缓缓加热。水蒸发后，留下白色盐晶体。接近结束时缓慢加热可防止迸溅，确保收集到纯净干燥的晶体。
3. Case 3: Candle Burning and the Law of Conservation of Mass | 案例三：蜡烛燃烧与质量守恒定律
A student places a candle on an electronic balance and records a mass of 65.0 g. After burning the candle for 10 minutes, the mass shown decreases to 62.8 g. Another student argues that this proves mass can be destroyed during a chemical reaction. A third student suggests placing the candle inside a sealed jar containing oxygen and measuring the total mass before and after.
一名学生将蜡烛放在电子天平上，记录质量为 65.0 g。蜡烛燃烧 10 分钟后，天平显示的质量减少至 62.8 g。另一名学生认为这证明化学反应中质量可以被消灭。第三名学生建议将蜡烛放在一个含有氧气的密封罐中，然后测量燃烧前后的总质量。
Question: Evaluate the claim that mass is destroyed using scientific principles. What would be observed in the sealed jar experiment?
问题：运用科学原理评估“质量被消灭”的说法。在密封罐实验中会观察到什么？
Answer: Mass is not destroyed; the law of conservation of mass states that in a chemical reaction, the total mass of reactants equals the total mass of products. In the open-air experiment, the candle wax (a hydrocarbon) reacts with oxygen to form carbon dioxide and water vapour, which escape into the air. That is why the remaining solid appears to lose mass.
答案：质量并没有被消灭；质量守恒定律指出，在化学反应中，反应物的总质量等于生成物的总质量。在开放空气的实验中，蜡烛中的蜡（碳氢化合物）与氧气反应，生成二氧化碳和水蒸气，这些气体散逸到空气中，这就是剩余固体似乎质量减少的原因。
The word equation for the reaction is: hydrocarbon + oxygen → carbon dioxide + water
反应的文字方程式为：碳氢化合物 + 氧气 → 二氧化碳 + 水
In the sealed jar, the total mass of the jar, candle and contents before and after burning remains the same, because the gases produced are trapped inside the system. This directly demonstrates the conservation of mass.
在密封罐中，燃烧前后罐体、蜡烛及内容物的总质量保持不变，因为产生的气体被封闭在系统内。这直接证明了质量守恒。
4. Case 4: Acid Rain and Marble Statues | 案例四：酸雨与大理石雕像
Marble is a rock composed mainly of calcium carbonate (CaCO₃). Historic marble statues exposed to rain in polluted cities often show signs of wear – their surfaces become rough and details are lost. Scientists explain that acidic rainwater reacts with the calcium carbonate in a chemical weathering process.
大理石是一种主要由碳酸钙（CaCO₃）组成的岩石。在污染城市中，暴露在雨中的历史大理石雕像常出现风化迹象——表面变得粗糙，细节丢失。科学家解释说，酸性雨水与碳酸钙发生化学反应，导致化学风化。
Acid rain typically contains sulfuric acid (H₂SO₄) and nitric acid (HNO₃). When sulfuric acid falls on marble, the following reaction takes place:
酸雨中通常含有硫酸（H₂SO₄）和硝酸（HNO₃）。当硫酸落在大理石上时，会发生如下反应：
calcium carbonate + sulfuric acid → calcium sulfate + carbon dioxide + water
碳酸钙 + 硫酸 → 硫酸钙 + 二氧化碳 + 水
Task: Explain why marble is particularly vulnerable to acid rain and suggest a variable that could be changed in an investigation to see how the rate of weathering is affected.
任务：解释为什么大理石特别容易受到酸雨的侵蚀，并指出在一项研究风化速率受何因素影响的实验中，可以改变哪一个变量？
Marble is vulnerable because its main component, calcium carbonate, reacts chemically with acids. This converts solid marble into soluble calcium sulfate or calcium nitrate (depending on the acid), which is washed away by rain, along with CO₂ gas escaping. Over time, the statue loses mass and its carved features are destroyed.
大理石之所以脆弱，是因为其主要成分碳酸钙会与酸发生化学反应。这将固态大理石转化为可溶的硫酸钙或硝酸钙（取决于酸的种类），这些盐被雨水冲走，同时 CO₂ 气体逸散。久而久之，雕像质量损失，雕刻特征被破坏。
In an investigation, one controlled variable to change could be the concentration of the acid. By using different concentrations of sulfuric acid on identical marble chips and measuring mass loss over time, you could determine how acidity affects the rate of weathering. Other variables such as temperature and surface area must be kept constant.
在实验中，可以改变的一个变量是酸的浓度。通过对相同的大理石碎片使用不同浓度的硫酸，并测量随时间变化的质量损失，可以确定酸度如何影响风化速率。其他变量如温度和表面积必须保持一致。
5. Case 5: Obtaining Copper from Copper Oxide | 案例五：从氧化铜中提取铜
Copper is an important metal used in electrical wiring. It can be extracted from copper oxide (CuO) by heating with carbon. The reactivity series shows that carbon is more reactive than copper, so carbon can displace copper from its compound. The experiment is performed by heating a mixture of copper oxide powder and charcoal strongly.
铜是用于电线的重要金属。它可以通过与碳一起加热从氧化铜（CuO）中提取。金属活动性顺序表明碳比铜更活泼，因此碳可以从其化合物中置换出铜。该实验通过强烈加热氧化铜粉末和木炭的混合物来进行。
During the reaction, a reddish-brown powder of pure copper forms, and a colourless gas is evolved that extinguishes a burning splint and turns limewater milky.
反应过程中，生成红棕色的纯铜粉末，同时产生一种无色气体，该气体能使燃烧的木条熄灭，并使石灰水变浑浊。
Question: Write the word equation for the reaction and explain why this is a displacement reaction.
问题：写出该反应的文字方程式，并解释为什么这是一个置换反应。
copper oxide + carbon → copper + carbon dioxide
氧化铜 + 碳 → 铜 + 二氧化碳
This is a displacement reaction because a more reactive element (carbon) takes the place of a less reactive element (copper) in its compound. Carbon removes the oxygen from copper oxide, reducing it to copper metal, while carbon itself is oxidised to carbon dioxide. This method is commonly used to extract metals of low to medium reactivity from their ores.
这是一个置换反应，因为更活泼的元素（碳）取代了化合物中较不活泼的元素（铜）。碳从氧化铜中夺走氧，将氧化铜还原为铜金属，而碳本身被氧化成二氧化碳。这种方法常用于从矿石中提取中低活泼性的金属。
6. Case 6: Classifying Unknown Solutions Using pH | 案例六：利用 pH 值分类未知溶液
Three colourless solutions, labelled X, Y and Z, are tested with universal indicator solution. Solution X turns the indicator red and gives a pH of 2. Solution Y turns the indicator green and reads pH 7. Solution Z turns the indicator purple with a pH of 11.
三种无色溶液，分别标记为 X、Y 和 Z，用通用指示剂进行测试。溶液 X 使指示剂变红，pH 读数为 2。溶液 Y 使指示剂变绿，pH 为 7。溶液 Z 使指示剂变紫，pH 为 11。
Task: Identify each solution as acidic, neutral or alkaline. Then suggest a real-life example for each type, and describe what happens when X and Z are mixed together.
任务：判断每种溶液属于酸性、中性还是碱性。然后为每种类型给出一个生活中的例子，并描述当 X 和 Z 混合时会发生什么。
Solution X is strongly acidic (pH 2). An example is lemon juice or stomach acid. Solution Y is neutral (pH 7), like pure water. Solution Z is strongly alkaline (pH 11), similar to household ammonia solution.
溶液 X 呈强酸性（pH 2）。例如柠檬汁或胃酸。溶液 Y 呈中性（pH 7），类似纯水。溶液 Z 呈强碱性（pH 11），类似家用氨水。
When an acid (X) and an alkali (Z) are mixed, a neutralisation reaction occurs. The general word equation is: acid + alkali → salt + water. In this case, the pH of the resulting solution moves toward 7, and heat may be given out. The exact salt produced depends on the specific acid and alkali, but the reaction always involves H⁺ ions from the acid combining with OH⁻ ions from the alkali to form water.
当酸 (X) 与碱 (Z) 混合时，发生中和反应。一般的文字方程式为：酸 + 碱 → 盐 + 水。在此过程中，混合溶液的 pH 值趋向 7，并可能释放热量。生成的盐取决于具体的酸和碱，但反应总是涉及酸中的 H⁺ 离子与碱中的 OH⁻ 离子结合生成水。
7. Case 7: The Rock Detective | 案例七：岩石侦探
A geologist finds
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