📚 Case Study Practice in Chemistry | 化学案例分析实战演练
In Year 7 Edexcel Chemistry, case studies bring textbook concepts to life by placing you in real-world or lab-based scenarios. By analysing observations, planning experiments, and drawing evidence-based conclusions, you strengthen your understanding of particles, mixtures, reactions and separation techniques. This article presents a series of guided case studies with paired English–Chinese explanations to help you think like a chemist and prepare for assessments.
在 Edexcel 七年级化学课程中，案例分析通过将你置身于真实世界或实验室场景，让课本概念活起来。通过分析观察结果、设计实验并基于证据得出结论，你能加深对粒子、混合物、化学反应以及分离技术的理解。本文提供一系列引导式案例，并配以中英对照的解释，帮助你像化学家一样思考，为评估做好准备。
1. What Are Chemistry Case Studies? | 什么是化学案例分析？
A case study presents a scenario – perhaps a mystery liquid to identify, a contaminated mixture to separate, or a reaction that goes wrong – and asks you to apply core principles to solve it. Successful case-study work requires careful observation, linking evidence to scientific ideas (such as the particle model or pH scale), and evaluating your method. The Edexcel approach rewards process just as much as the final answer.
案例分析会给出一个场景——也许是一种需要鉴别的神秘液体、一个需要分离的受污染混合物，或者一个出了问题的化学反应——并要求你运用核心原理来解决它。成功的案例分析工作需要仔细观察、将证据与科学概念（如粒子模型或 pH 标度）联系起来，并对自己的方法进行评估。Edexcel 的评价方式既看重过程也看重最终答案。
2. Case Study 1: Separating Sand and Salt | 案例一：分离沙子和盐
A student accidentally mixes 20 g of fine sand with 20 g of table salt in a beaker. She knows sand is insoluble in water while salt is soluble. She decides to recover both solids using only water, a filter funnel, filter paper, an evaporating basin and a Bunsen burner. Explain each step and state the key scientific terms.
一名学生不小心在烧杯中将 20 克细沙与 20 克食盐混合。她知道沙子不溶于水而盐可溶。她决定仅用 水、过滤漏斗、滤纸、蒸发皿和本生灯来回收这两种固体。请解释每一步骤并说出关键科学术语。
Step 1: Add water to the mixture and stir. The salt dissolves to form a solution (salt + water). Sand does not dissolve and stays as suspended solid particles.
步骤 1：往混合物中加水并搅拌。盐溶解形成溶液（盐+水）。沙子不溶解，仍为悬浮的固体颗粒。
Step 2: Set up filtration – fold the filter paper into the funnel and pour the mixture through. The residue (sand) stays on the filter paper, while the filtrate (salt solution) passes through into the evaporating basin. This is a physical separation based on particle size.
步骤 2：安装过滤装置——将滤纸折好放入漏斗，倒入混合物。滤渣（沙子）留在滤纸上，而滤液（盐溶液）通过并流入蒸发皿。这是基于粒子大小的物理分离。
Step 3: Heat the filtrate gently using a Bunsen burner. Water evaporates, leaving behind salt crystals in the basin. This process is called evaporation or crystallisation. The sand can be dried and weighed on the filter paper.
步骤 3：用本生灯缓慢加热滤液。水蒸发，蒸发皿中留下盐晶体。这个过程叫做蒸发或结晶。沙子可以在滤纸上干燥并称重。
Mistake analysis: If the student had heated the sand-salt mixture directly, both sand and salt would have remained together. Filtration only works when one component dissolves.
错误分析：如果学生直接加热沙盐混合物，沙和盐仍会混在一起。只有其中一种组分溶解时过滤才有效。
3. Case Study 2: Investigating Acid Rain | 案例二：探究酸雨
City planners notice that marble statues (made of calcium carbonate, CaCO₃) are wearing away faster than granite ones. They suspect acid rain. A laboratory model is set up: a marble chip is placed in dilute sulfuric acid, and the mass loss is recorded every 30 seconds. Predict the observations, write a word equation, and explain what happens to the particles.
城市规划者注意到大理石雕像（由碳酸钙 CaCO₃ 构成）比花岗岩雕像磨损得更快。他们怀疑是酸雨所致。于是建立了一个实验室模型：将一块大理石碎片放入稀硫酸中，每隔 30 秒记录质量损失。请预测观察结果，写出文字方程式，并解释粒子层面发生的变化。
Observation: Bubbles of gas are seen on the marble chip. The chip becomes smaller over time, and the solution may turn cloudy if the gas is passed through limewater. Limewater turns milky, confirming carbon dioxide (CO₂).
观察：大理石碎片上出现气泡。随着时间推移，碎片变小；若将气体通入石灰水，溶液可能变浑浊。石灰水变乳白色，证实气体为二氧化碳（CO₂）。
Word equation: calcium carbonate + sulfuric acid → calcium sulfate + water + carbon dioxide. In terms of particles, acid H⁺ ions collide with carbonate CO₃²⁻ groups, breaking the ionic lattice and releasing CO₂ molecules.
文字方程式：碳酸钙 + 硫酸 → 硫酸钙 + 水 + 二氧化碳。从粒子角度看，酸的 H⁺ 离子与碳酸根 CO₃²⁻ 基团碰撞，打破离子晶格并释放出 CO₂ 分子。
Why marble but not granite? Granite is largely silicate minerals which do not react with acids as vigorously. Marble’s carbonate composition makes it vulnerable. This case links chemistry to environmental conservation.
为什么是大理石而非花岗岩？花岗岩主要是硅酸盐矿物，与酸反应不剧烈。大理石 的碳酸盐成分使其易受侵蚀。本案例将化学与环境保护联系起来。
4. Case Study 3: Rusting of Iron – Designing a Fair Test | 案例三：铁的锈蚀——设计公平实验
A bicycle chain rusts badly in winter. The owner wants to know whether salt (from gritted roads) or water alone causes rusting. Design a controlled experiment using three iron nails, boiled water, tap water, salty water and oil. Identify the independent, dependent and control variables.
冬天自行车链条锈蚀严重。车主想知道是盐（来自防滑路面）还是水单独导致生锈。请设计一个对照实验，使用三颗铁钉、煮沸过的水、自来水、盐水和油。指出自变量、因变量和控制变量。
Setup: Nail 1 is placed in boiled water (to remove dissolved oxygen) with a layer of oil to exclude air. Nail 2 is placed in tap water open to air. Nail 3 is placed in salty water open to air. All nails are observed over one week.
装置：钉子 1 放入煮沸过的水（去除溶解氧）并覆盖一层油以隔绝空气。钉子 2 放入敞开的自来水中。钉子 3 放入敞开的盐水中。所有钉子观察一周。
Independent variable: the type of liquid (water vs. salty water) and presence/absence of oxygen. Dependent variable: degree of rusting (mass of rust or visual scale). Control variables: nail size, metal type, temperature, volume of liquid.
自变量：液体种类（水 与 盐水）以及氧气的有无。因变量：锈蚀程度（锈的质量或视觉评分）。控制变量：钉子大小、金属种类、温度、液体体积。
Expected result: Nail 3 rusts most, Nail 2 rusts somewhat, Nail 1 hardly rusts. Both oxygen and water are needed for rusting; salt acts as an electrolyte speeding up the electrochemical process. This helps explain why cars rust faster in coastal or winter-salted areas.
预期结果：钉子 3 锈蚀最严重，钉子 2 有部分锈蚀，钉子 1 几乎不锈蚀。锈蚀需要氧气和水同时存在；盐作为电解质加速了电化学过程。这有助于解释为什么汽车在沿海或冬季撒盐地区锈蚀更快。
5. Case Study 4: Making Soluble Salts – Copper Sulfate Crystals | 案例四：制备可溶性盐——硫酸铜晶体
In the lab, a student wants to prepare pure copper sulfate (CuSO₄) crystals by reacting black copper oxide (CuO) with warm dilute sulfuric acid. The procedure involves adding excess copper oxide, filtering, and then evaporating the filtrate. Explain why excess oxide is used, describe safety precautions, and write a balanced word equation.
在实验室，一名学生希望通过让黑色氧化铜 (CuO) 与温热的稀硫酸反应来制备纯净的硫酸铜 (CuSO₄) 晶体。步骤包括加入过量的氧化铜、过滤，然后蒸发滤液。解释为什么使用过量氧化物，描述安全预防措施，并写出配平的文字方程式。
Reason for excess copper oxide: It ensures all the acid reacts completely, so the final solution is neutral and contains only copper sulfate and water. Unreacted solid is removed by filtration.
使用过量氧化铜的原因：确保所有酸都完全反应，因此最终溶液呈中性，只含有硫酸铜和水。未反应的固体通过过滤除去。
Word equation: copper oxide + sulfuric acid → copper sulfate + water. The balanced symbol equation requires equal numbers of each atom: CuO + H₂SO₄ → CuSO₄ + H₂O.
文字方程式：氧化铜 + 硫酸 → 硫酸铜 + 水。配平的符号方程式要求各原子数目相等：CuO + H₂SO₄ → CuSO₄ + H₂O。
Safety: Wear eye protection; acid is corrosive. Heat gently – not to dryness – to allow slow crystallisation. Blue crystals of CuSO₄·5H₂O form, confirming water of crystallisation.
安全：佩戴护目镜；酸具有腐蚀性。缓慢加热——不要蒸干——以便缓慢结晶。形成蓝色 CuSO₄·5H₂O 晶体，证实含有结晶水。
6. Case Study 5: Using Indicators to Classify Household Substances | 案例五：用指示剂对家庭物质进行分类
A student tests five colourless household liquids – vinegar, tap water, window cleaner, lemon juice and baking soda solution – using red and blue litmus paper, and universal indicator solution. The results are recorded in a table. Red litmus stays red in vinegar and lemon juice, turns blue in window cleaner and baking soda. Blue litmus turns red in vinegar and lemon juice, stays blue in window cleaner and baking soda. Tap water shows no change in both. Universal indicator gives pH values: vinegar ~3, lemon juice ~2, tap water ~7, window cleaner ~11, baking soda ~8.5. Explain the results using the concepts of acid, alkali and neutral.
一名学生用红色和蓝色石蕊试纸以及通用指示剂溶液测试了五种无色家庭液体——醋、自来水、窗户清洁剂、柠檬汁和小苏打溶液。结果记录在表格中。红色石蕊在醋和柠檬汁中保持红色，在窗户清洁剂和小苏打溶液中变为蓝色。蓝色石蕊在醋和柠檬汁中变为红色，在窗户清洁剂和小苏打溶液中保持蓝色。自来水在两种试纸上均无变化。通用指示剂给出的 pH 值：醋~3，柠檬汁~2，自来水~7，窗户清洁剂~11，小苏打~8.5。请运用酸、碱和中性的概念解释这些结果。
Acids (pH < 7) turn blue litmus red; alkalis (pH > 7) turn red litmus blue. Vinegar (ethanoic acid) and lemon juice (citric acid) are natural acids, releasing H⁺ ions in water. Window cleaner contains ammonia (an alkali) and baking soda (sodium hydrogen carbonate) is a weak alkali, both releasing OH⁻ ions. Tap water is neutral (pH 7) because it has equal numbers of H⁺ and OH⁻ ions.
酸（pH 值小于 7）使蓝色石蕊变红；碱（pH 值大于 7）使红色石蕊变蓝。醋（乙酸）和柠檬汁（柠檬酸）是天然酸，在水中释放 H⁺ 离子。窗户清洁剂含有氨（一种碱），小苏打（碳酸氢钠）是弱碱，二者都释放 OH⁻ 离子。自来水呈中性（pH 7），因为其 H⁺ 和 OH⁻ 离子数目相等。
This case practises data interpretation. Notice that lemon juice has a lower pH than vinegar, meaning it is more acidic. The universal indicator showed a range of colours: red for pH 2, orange/yellow for pH 3–6, green for 7, blue/purple for pH 8.5–11.
本案例练习数据解读。注意柠檬汁的 pH 低于醋，表明其酸性更强。通用指示剂显示颜色范围：pH 2 为红色，pH 3–6 为橙/黄色，pH 7 为绿色，pH 8.5–11 为蓝/紫色。
7. Case Study 6: Chromatography – The Case of the Forged Pen | 案例六：色谱法——伪造笔迹案
A suspicious note is found using black ink. Police suspect the ink matches one of three brands – Ink A, B, or C. A chromatography experiment is set up: spots of the crime-scene ink and the three reference inks are placed on a pencil line on filter paper. The paper is dipped in water (solvent) and the dyes separate. After 10 minutes, the crime ink shows three spots (blue, red, yellow) at Rf values 0.2, 0.5, and 0.8. Ink A shows only blue (Rf 0.2); Ink B shows blue (0.2) and red (0.5); Ink C shows blue (0.2), red (0.5), and yellow (0.8). Which ink matches? What does this tell us about mixtures?
发现了一张用黑色墨水书写的可疑便条。警方怀疑该墨水与 A、B、C 三种品牌之一相符。设计了一个色谱实验：将犯罪现场墨水和三种参考墨水的点样置于滤纸上的铅笔线上。将纸浸入水（溶剂）中，染料分离。10 分钟后，犯罪墨水显示三个斑点（蓝、红、黄），Rf 值分别为 0.2、0.5 和 0.8。墨水 A 只显示蓝色斑点（Rf 0.2）；墨水 B 显示蓝色（0.2）和红色（0.5）；墨水 C 显示蓝色（0.2）、红色（0.5）和黄色（0.8）。哪种墨水匹配？这告诉我们关于混合物的什么信息？
Match: Ink C, because it contains exactly the same three dyes with identical Rf values. Ink A and B are missing some components. Black ink is a mixture of different coloured dyes. Chromatography separates them based on their different solubilities. A dye that is more soluble in the solvent travels further, giving a higher Rf.
匹配结果：墨水 C，因为它含有完全相同的三种染料，且 Rf 值一致。墨水 A 和 B 缺少某些组分。黑墨水是不同颜色染料的混合物。色谱法根据它们不同的溶解度将其分离。在溶剂中溶解度越大的染料迁移得越远，Rf 值越高。
Calculating Rf: Rf = distance moved by dye ÷ distance moved by solvent front. Always less than 1. This case shows how chemistry helps forensic science. It also reinforces that a pure substance shows a single spot, while a mixture shows multiple spots.
计算 Rf：Rf = 染料移动距离 ÷ 溶剂前沿移动距离。始终小于 1。本案例展示了化学如何助力法证科学。同时也强化了一个概念：纯物质只有一个斑点，而混合物显示多个斑点。
8. Case Study 7: Energy Changes – Dissolving Mystery Solids | 案例七：能量变化——溶解神秘固体
Two white solids, X and Y, are added to separate beakers of water. A thermometer records the temperature every 30 seconds for 3 minutes after stirring. For solid X, temperature rises from 20 °C to 35 °C; for solid Y, temperature drops from 20 °C to 5 °C. Identify the type of energy change for each process, explain it using particle theory, and suggest what X and Y might be.
将两种白色固体 X 和 Y 分别加入装有水的烧杯中。搅拌后用温度计每 30 秒记录一次温度，持续 3 分钟。固体 X 的温度从 20 °C 升至 35 °C；固体 Y 的温度从 20 °C 降至 5 °C。确定各过程的能量变化类型，用粒子理论解释，并推测 X 和 Y 可能是什么。
For X: Exothermic dissolving. Energy is released when new bonds form between solute particles and water molecules (hydration), exceeding the energy needed to break apart the solid lattice. The solution gets warmer.
对于 X：放热溶解。当溶质粒子与水分子之间形成新键（水合）时释放能量，超过破坏固体晶格所需的能量。溶液变热。
For Y: Endothermic dissolving. The energy required to separate the particles and overcome the lattice is greater than the energy released by hydration. The system absorbs energy from the surroundings, so temperature drops.
对于 Y：吸热溶解。分离粒子并克服晶格所需的能量大于水合释放的能量。系统从周围环境吸收能量，因此温度下降。
Examples: X could be sodium hydroxide (NaOH) or calcium chloride (CaCl₂), which are known for exothermic dissolving. Y could be ammonium nitrate (NH₄NO₃) or potassium chloride, common in endothermic cold packs. This case links particle theory to measurable temperature changes.
举例：X 可能是氢氧化钠 (NaOH) 或氯化钙 (CaCl₂)，它们以放热溶解著称。Y 可能是硝酸铵 (NH₄NO₃) 或氯化钾，常见于吸热冷敷袋中。本案例将粒子理论与可测量的温度变化联系起来。
9. Case Study 8: Investigating Combustion – Candle in a Jar | 案例八：探究燃烧——罐中蜡烛
A burning candle is placed in a trough of water and covered with a glass jar. The flame goes out after a few seconds, and water rises into the jar, occupying about one-fifth of its volume. Analyse this classic experiment to explain the rise of water and the composition of air.
将一支燃烧的蜡烛放在水槽中并用玻璃罐罩住。几秒后火焰熄灭，水上升进入罐内，占罐子容积约五分之一。分析这个经典实验，解释水上升的原因以及空气的组成。
When the candle burns, it uses oxygen (O₂) and produces carbon dioxide (CO₂) and water vapour. However, CO₂ is soluble in water and the water vapour condenses, so the overall number of gas molecules inside the jar decreases. The pressure drops, and the higher external air pressure pushes water up into the jar. The water rises until the pressures equalise. The volume of water (about 20%) corresponds roughly to the proportion of oxygen in air.
蜡烛燃烧时消耗氧气 (O₂) 并产生二氧化碳 (CO₂) 和水蒸气。然而，CO₂ 可溶于水，水蒸气会凝结，因此罐内气体分子总数减少。压力下降，外部较高的气压将水推入罐中。水持续上升直到压力平衡。水的体积（约 20%）大致对应于空气中 氧气的比例。
This is a model, not perfect – some CO₂ remains, and the water level also depends on temperature. Nevertheless, it demonstrates that air is not a single substance but a mixture, and that oxygen is needed for combustion. The candle wax is a hydrocarbon; when it burns completely, water and carbon dioxide form.
这是一个模型，并不完美——部分 CO₂ 仍留存，且水位还受温度影响。尽管如此，它证明了空气不是单一物质而是混合物，且燃烧需要氧气。蜡烛蜡是一种碳氢化合物；完全燃烧时生成水和二氧化碳。
10. Key Skills and Common Mistakes in Case Studies | 案例研究中的关键技能与常见错误
Across all case studies, three skills repeatedly appear: (1) Identifying variables – independent (what you change), dependent (what you measure), and control (what you keep the same). (2) Linking observations to particle theory – e.g., dissolving, diffusion, and state changes all involve particle movement and energy. (3) Evaluating methods – discuss reliability, reproducibility and whether the data fully supports the conclusion. Common mistakes include forgetting control variables, confusing residue and filtrate, misinterpreting litmus colours, and assuming all dissolving is endothermic.
在所有的案例研究中，三项技能反复出现：(1) 确定变量——自变量（你改变的量）、因变量（你测量的量）和控制变量（你保持不变的量）。(2) 将观察结果与粒子理论关联起来——例如，溶解、扩散和状态变化都涉及粒子运动与能量。(3) 评估方法——讨论可靠性、可重复性以及数据是否完全支持结论。常见错误包括忘记控制变量、混淆滤渣与滤液、误判石蕊颜色，以及假定所有溶解都是吸热的。
When confronted with a new scenario, read carefully, underline command words (describe, explain, evaluate, plan), and always link back to a core principle you have studied – such as the conservation of mass, pH scale, or the particle model. Practice with these cases will build your confidence for Edexcel Year 7 assessment tasks.
当遇到新场景时，仔细阅读，划出指令词（描述、解释、评估、计划），并始终与你学过的核心原理联系起来——如质量守恒、pH 标度或粒子模型。通过这些案例的练习，你会增强对 Edexcel 七年级评估任务的信心。
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