📚 Year 8 WJEC Chemistry: Case Study Practice Drills | 八年级WJEC化学：案例分析实战演练
Welcome to your WJEC Year 8 Chemistry case study workout. In these exercises, you will step into the shoes of a scientific detective, using your knowledge of acids and alkalis, chemical reactions, metals, and gases to crack real-world puzzles. Each case is designed to mirror the style of questions you might meet in a classroom test, practical assessment, or even a fun science challenge. Read the scenario, think about what you already know, and work through the evidence one clue at a time.
欢迎来到WJEC八年级化学案例分析实战训练。在这些练习中，你将化身为科学侦探，运用你所学的酸碱知识、化学反应、金属性质和气体鉴别来解决一个个真实世界中的谜题。每个案例都仿照你可能会在课堂测验、实验评估甚至趣味科学挑战中遇到的题型设计。请阅读情景，联想你已经掌握的知识，一步一步地分析线索。
1. What Is a Chemistry Case Study? | 什么是化学案例分析？
A chemistry case study presents you with a practical situation — it might be a mysterious substance found in a kitchen, a rusting nail, or a problem with hard water. Your job is to apply the core ideas from your WJEC chemistry curriculum to explain what is happening, predict outcomes, or plan an investigation. Unlike a simple recall question, a case study asks you to connect several topics and justify your reasoning.
化学案例分析向你展示一个实际情境——可能是厨房里发现的一种神秘物质、一根生锈的钉子，或者硬水带来的麻烦。你的任务是运用WJEC化学课程中的核心概念来解释发生了什么、预测结果或规划探究方案。与简单的记忆题不同，案例分析要求你联系多个知识点，并为你的推理提供合理依据。
2. Key Steps in Solving Chemistry Cases | 解决化学案例的关键步骤
Every good detective follows a method. When tackling a chemistry case, keep these steps in mind. First, identify the substances involved — look for clues about their properties, such as colour, state, solubility, or what happens when they are heated or mixed. Second, recall relevant chemical facts: what do you know about acids, alkalis, metals, gases, and indicators? Third, plan a simple test or logical sequence of tests that will let you distinguish between the possibilities. Finally, explain your conclusion using scientific vocabulary, linking your observations to chemical reactions or principles.
每位优秀的侦探都会遵循一套方法。在处理化学案例时，请记住以下步骤。第一，识别涉及的物质——寻找有关性质的线索，例如颜色、状态、溶解度，或者它们受热或混合时发生的现象。第二，回忆相关的化学事实：你对酸、碱、金属、气体和指示剂有哪些了解？第三，设计一个简单的测试或一系列合乎逻辑的测试，以便区分各种可能性。最后，用科学词汇解释你的结论，将观察到的现象与化学反应或原理联系起来。
3. Case Study 1: Mystery Kitchen Powders | 案例一：神秘的厨房粉末
You find three unlabelled white powders in the kitchen cupboard. One is baking soda (sodium hydrogencarbonate), one is table salt (sodium chloride), and one is sugar (sucrose). Design a safe experiment, without tasting, to identify each powder using only household vinegar and a teaspoon that can be heated gently over a candle.
你在厨房橱柜里发现了三种没有标签的白色粉末。它们分别是小苏打（碳酸氢钠）、食盐（氯化钠）和白糖（蔗糖）。请设计一个安全的实验，不能品尝，仅利用家中的白醋和一把可以在蜡烛上稍微加热的金属勺子来鉴别各种粉末。
Decoding the mystery: Add a few drops of vinegar to a small sample of each powder on a clean saucer. The baking soda will immediately fizz and produce bubbles of carbon dioxide gas. The salt and sugar will not react with vinegar. To tell salt and sugar apart, place a tiny amount of each on the spoon and heat it gently. Sugar will melt, turn brown, and give off a sweet caramel smell, while salt will not melt (it has a very high melting point) and may just crackle a little. This uses the fact that carbonates react with acids to release CO2, and that sugar is an organic compound that decomposes on heating.
破译谜团：在干净的碟子上，每种粉末取少量样品，分别滴入几滴白醋。小苏打会立即起泡，产生二氧化碳气体。食盐和白糖则不会与醋反应。为了区分食盐和白糖，取少量分别放在勺子上， 用蜡烛微火加热。白糖会熔化，变成棕色，并散发出甜美的焦糖气味；而食盐不会熔化（熔点很高），可能只会轻微爆响。这里用到的原理是：碳酸盐与酸反应会释放二氧化碳，而白糖是一种有机化合物，加热时会分解。
NaHCO3 + CH3COOH → CH3COONa + CO2 + H2O
4. Case Study 2: Acid or Alkali? Testing Household Drinks | 案例二：酸性还是碱性？测试家用饮品
Your friend thinks that all clear drinks are neutral. You want to test three drinks: lemonade, mineral water, and a solution of baking soda in water. You make a red cabbage indicator by boiling chopped red cabbage in water and draining the purple liquid. Predict the colour changes and explain what they tell you about each drink.
你的朋友认为所有透明饮品都是中性的。你想测试三种饮品：柠檬水、矿泉水和一杯小苏打水溶液。你用切碎的红甘蓝在水中煮沸后滤出紫色液体，制作了红甘蓝指示剂。请预测每种饮品的颜色变化，并解释颜色变化说明了什么。
When you add red cabbage indicator, lemonade will turn pink or red because it contains citric acid, making it acidic. Mineral water (still) is usually neutral, so the indicator will stay purple. However, some sparkling mineral waters contain dissolved carbon dioxide, which forms weak carbonic acid and could turn the indicator slightly pinkish. The baking soda solution is an alkali; it will turn the indicator green or blue-green. These observations rely on the fact that natural indicators change colour at different pH levels: red‑cabbage indicator is purple in neutral solutions, red in acids, and green to yellow in alkalis.
当你加入红甘蓝指示剂时，柠檬水会变成粉红或红色，因为它含有柠檬酸，呈酸性。普通矿泉水通常是中性的，所以指示剂会保持紫色。不过，有些含气矿泉水溶有二氧化碳，会形成弱碳酸，可能使指示剂变成淡粉色。小苏打溶液是碱性的，会让指示剂变成绿色或蓝绿色。这些观察依据的是天然指示剂在不同pH值下的颜色变化：红甘蓝指示剂在中性溶液中呈紫色，在酸性溶液中呈红色，在碱性溶液中呈绿色至黄色。
5. Case Study 3: Why Did the Nail Rust? | 案例三：钉子为什么会生锈？
Josh set up four test tubes. Each contained a clean iron nail. Tube A had the nail completely under boiled water with a layer of oil on top. Tube B had the nail in air with a drying agent. Tube C had the nail half in water and half in air. Tube D had the nail in water with some table salt dissolved. After a week, Josh recorded the amount of rust. Which nail rusted the most, and why? What does this tell you about the conditions needed for rusting?
乔什准备了四支试管，每支放了一根干净的铁钉。试管A中，铁钉完全浸没在煮沸过的水中，水上有一层油封住。试管B中，铁钉放在空气里，旁边有干燥剂。试管C中，铁钉一半浸在水中，一半暴露在空气中。试管D中，铁钉浸在溶有少量食盐的水里。一周后，乔什记录了生锈的情况。哪根钉子生锈最严重，为什么？这个实验告诉你锈蚀需要哪些条件？
Nail D rusts the most because it has access to both water and oxygen, and the presence of salt speeds up the rusting process by acting as an electrolyte. Nail C also rusts significantly because it is exposed to both air and water — exactly the conditions needed for iron to oxidise. Nail A hardly rusts: the boiled water has driven out dissolved oxygen, and the oil layer prevents more oxygen from entering. Nail B stays shiny because the drying agent removes moisture, so one essential ingredient for rust — water — is missing. The experiment shows that both oxygen and water are necessary for rusting, and salt accelerates the reaction.
铁钉D生锈最严重，因为它同时接触水和氧气，而盐作为电解质加速了生锈过程。铁钉C也明显生锈，因为它暴露在空气和水中——这正是铁氧化所需的条件。铁钉A几乎没有生锈：煮沸过的水赶走了溶解氧，上面的油层阻止了更多氧气进入。铁钉B保持光亮，因为干燥剂去除了水汽，缺少了生锈必需的水。这个实验表明，氧气和水都是生锈必不可少的，而盐会加快反应。
6. Case Study 4: Choosing the Right Limescale Remover | 案例四：选择合适的水垢去除剂
Your kettle has a thick layer of limescale (mainly calcium carbonate, CaCO3). You have three liquids at home: distilled water, diluted citric acid solution (from lemon juice), and dilute ammonia solution (a common alkali cleaner). Which one will effectively remove the limescale, and why? What observation will tell you the reaction is working?
你的水壶积了厚厚一层水垢（主要成分是碳酸钙，CaCO3）。家里有三种液体：蒸馏水、稀释的柠檬酸溶液（来自柠檬汁）和稀释的氨水（一种常见的碱性清洁剂）。哪一种能有效去除水垢？为什么？什么现象能够告诉你反应正在进行？
The citric acid is the correct choice. Limescale is a carbonate, and carbonates react with acids to produce a salt, water, and carbon dioxide gas. When you pour the citric acid onto the limescale, you will see fizzing or bubbling as CO2 a carbonate; in fact, it may leave a slippery residue. The word equation for the reaction is: calcium carbonate + citric acid → calcium citrate + water + carbon dioxide.
正确选择是柠檬酸。水垢是一种碳酸盐，碳酸盐与酸反应会生成盐、水和二氧化碳气体。当你把柠檬酸倒在水垢上时，会看到起泡或冒泡，这就是二氧化碳释放的现象。蒸馏水没有效果，因为碳酸钙几乎不溶于纯水。氨水是碱，不会与碳酸盐反应，反而可能留下滑腻的残留物。该反应的文字方程式是：碳酸钙 + 柠檬酸 → 柠檬酸钙 + 水 + 二氧化碳。
7. Case Study 5: Identifying Unknown Gases | 案例五：未知气体的鉴别
Three gas jars are filled with colourless gases labelled X, Y, and Z. Here is what you find: Gas X relights a glowing splint. Gas Y makes a 'pop' sound when a lit splint is held to the mouth of the jar. Gas Z turns limewater milky when bubbled through it. Identify the three gases and write the test method for each.
三个集气瓶里分别装有无色气体，标签为X、Y和Z。你观察到：气体X能使带火星的木条复燃。气体Y在瓶口点燃时发出“噗”的一声。气体Z通入石灰水时，石灰水变浑浊。请鉴别这三种气体，并写出每种气体的检验方法。
Gas X is oxygen (O2). The standard test for oxygen is to place a glowing splint into a sample; if it relights, oxygen is present. Gas Y is hydrogen (H2). Hydrogen can be tested with a lit splint held at the open end of an upside-down test tube; it burns rapidly with a squeaky 'pop'. Gas Z is carbon dioxide (CO2). Carbon dioxide is confirmed by bubbling the gas through limewater (calcium hydroxide solution); a cloudy, milky appearance means CO2 is present because fine particles of calcium carbonate form. These are classic gas tests you need to remember for your WJEC practicals.
气体X是氧气（O2）。检验氧气的标准方法是将带火星的木条伸入气体样品中；如果复燃，说明有氧气。气体Y是氢气（H2）。检验氢气时，将点燃的木条移至倒置试管口；氢气会快速燃烧，发出轻微的“噗”声。气体Z是二氧化碳（CO2）。检验二氧化碳可将气体通入石灰水（氢氧化钙溶液）中；石灰水变浑浊或呈乳白色，说明有二氧化碳，因为生成了细小的碳酸钙颗粒。这些都是你在WJEC实验考试中需要牢记的经典气体检验方法。
8. Common Pitfalls and How to Avoid Them | 常见误区及如何避免
- Assuming one observation is enough — always look for a second confirmatory test if possible.
- 认为一次观察就足够了——如果可能，一定要寻找第二个确认性检验。
- Forgetting to control variables — when planning an investigation, only change one factor at a time.
- 忘记控制变量——在规划探究方案时，每次只改变一个因素。
- Mixing up acids and alkalis with litmus or indicators — red litmus turns blue in alkali, not in acid.
- 混淆石蕊或指示剂对酸和碱的反应——红色石蕊在碱中变蓝，而不是在酸中。
- Writing a prediction without scientific reasoning — always back up your guess with chemical principles.
- 写出预测但没有科学依据——始终用化学原理支撑你的猜测。
- Misidentifying gases — hydrogen pops, oxygen relights, carbon dioxide clouds limewater.
- 气体鉴别错误——氢气会爆鸣，氧气能复燃，二氧化碳使石灰水变浑浊。
9. Practice Makes Perfect | 练习成就完美
Now that you have worked through five detective cases, try creating your own. Pick an everyday chemical puzzle — perhaps an unlabelled bottle of liquid, a stain that needs removing, or a food item that behaves unexpectedly — and write a step‑by‑step plan to investigate it. Practising in this way will sharpen your ability to think scientifically, and it will make exam questions feel like familiar stories rather than challenging riddles.
现在你已经完成了五个侦探案例，不妨试着设计你自己的案例。选择一项日常生活的化学谜题——也许是一瓶没有标签的液体、一处需要清除的污渍，或者一种表现意外的食物——然后写一份分步骤的探究计划。这样的练习能擦亮你科学思考的能力，也会让考试题目变得像熟悉的故事，而不是棘手的谜语。
Remember, every case you solve builds your confidence in using the key ideas of the WJEC Chemistry syllabus: acids and carbonates, acids and metals, reactivity, rusting, gases, and indicators. Keep asking 'why' and 'how do I know that?' — that is the heart of chemistry case study practice.
请记住，你解决的每个案例都会增进你运用WJEC化学课程核心概念的自信心：酸与碳酸盐、酸与金属、活泼性、生锈、气体以及指示剂。保持追问“为什么”和“我怎么知道？”——这才是化学案例分析实战的核心。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图