📚 Year 9 WJEC Chemistry: Case Study Practical Exercises | 九年级 WJEC 化学：案例分析实战演练
Case studies are a powerful way to see how chemistry explains the world around us. Instead of just memorising facts, you will step into the role of a scientist, analyse real-world scenarios, and apply your knowledge to solve problems. In this article we present three practical case studies that cover key topics from Year 9 WJEC Chemistry: dissolving and temperature, identifying gases, and neutralisation. Each case study challenges you to think critically, spot errors, and suggest improvements - exactly the skills needed for your exams and beyond.
案例研究是一种强有力的方式，能让我们看到化学如何解释周围的世界。与其死记硬背事实，你将扮演科学家的角色，分析真实场景，并运用你的知识来解决问题。在本文中，我们呈现三个实战案例，涵盖 WJEC 九年级化学的关键主题：溶解与温度、气体鉴别以及中和反应。每个案例都要求你批判性思考、发现错误并提出改进建议——这正是考试及以后所需的技能。
1. Why Case Studies Matter in Chemistry | 为什么案例学习在化学中很重要
In WJEC Year 9 Chemistry, you are not only expected to remember theories - you must be able to use them. Case studies train you to connect textbook ideas with practical situations. They often describe an experiment with flaws or a puzzling observation, and your job is to figure out what happened and why.
在 WJEC 九年级化学中，你不仅要记住理论，还要会使用它们。案例研究训练你将课本知识与实际情况联系起来。它们通常会描述一个有缺陷的实验或一个令人困惑的观察结果，而你的任务就是弄清楚发生了什么以及为什么。
The second reason case studies matter is that they build the four skills that examiners reward most highly: experimental design, data interpretation, error analysis, and evaluation. When you explain why a thermometer was used as a stirrer, or why a rough titre must be ignored, you are demonstrating exactly the reasoning that appears in practical assessments and written papers.
案例学习重要的第二个原因是，它能培养考官最看重的四项技能：实验设计、数据解读、误差分析和评价。当你解释为什么温度计被当作搅拌棒使用是错误的，或者为什么粗略滴定值必须被忽略时，你展示的正是实验评估和笔试中出现的推理能力。
Finally, case studies make chemistry memorable. A number recorded from your own experiment - such as a temperature falling from 21.0 °C to 14.0 °C when ammonium nitrate dissolves - sticks in the memory far better than a definition copied from a textbook.
最后，案例研究让化学变得难忘。来自你自己实验的一个数字——例如硝酸铵溶解时温度从 21.0 °C 降到 14.0 °C——远比从课本抄来的定义更容易留在记忆中。
2. The Four-Step Method for Any Case Study | 任何案例研究的四步法
Before you look at the three cases below, learn a repeatable method. Every WJEC case study question, whether it is about dissolving, gas tests or neutralisation, can be tackled with the same four steps.
在阅读下面三个案例之前，先掌握一套可重复使用的方法。每一道 WJEC 案例研究题，无论关于溶解、气体鉴别还是中和反应，都可以用同样的四步来解决。
- Step 1 - Identify the aim and variables: What is being measured? What is changed, what is measured, what must be kept the same?
第一步——明确目的与变量：要测量什么？改变什么、测量什么、必须保持不变的是什么？ - Step 2 - Inspect the method and apparatus: Is the equipment suitable? Is the technique correct? Is the range of measurements sensible?
第二步——检查方法与器材：器材是否合适？操作是否规范？测量范围是否合理？ - Step 3 - Interpret the data: Look for trends, anomalies, and the direction of change. Does the evidence support the conclusion?
第三步——解读数据：寻找趋势、异常值和变化方向。证据是否支持结论？ - Step 4 - Evaluate and improve: Identify the biggest source of error, then suggest one specific, realistic improvement.
第四步——评价与改进：找出最大的误差来源，然后提出一个具体、可行的改进方案。
A useful sentence pattern for Step 4 is: 'The main source of error is..., because..., so I would improve the method by...'. This structure earns marks because it links cause, effect and solution.
第四步有一个实用的句式："主要误差来源是……，因为……，所以我会通过……来改进方法。" 这种结构把原因、结果和解决方案联系起来，因此容易得分。
3. Case Study 1: Dissolving and Temperature - The Scenario | 案例一：溶解与温度——情境设定
A student investigates whether dissolving a solid in water changes the temperature of the water. She measures 50.0 cm³ of distilled water into a polystyrene cup, records the starting temperature, then adds 5.0 g of ammonium nitrate, stirring the mixture with the thermometer itself. She reads the thermometer every 30 seconds for three minutes.
一名学生研究固体溶于水是否会改变水的温度。她用 50.0 cm³ 蒸馏水倒入聚苯乙烯杯中，记录起始温度，然后加入 5.0 g 硝酸铵，并用温度计本身搅拌混合物。她每 30 秒读一次温度计，持续三分钟。
| Time / s | 时间 / 秒 | Temperature / °C | 温度 / °C |
| 0 | 21.0 |
| 30 | 18.5 |
| 60 | 16.0 |
| 90 | 14.4 |
| 120 | 14.0 |
| 150 | 14.2 |
| 180 | 14.5 |
The key observation is that the temperature falls, reaches a minimum of 14.0 °C at 120 seconds, and then slowly begins to rise again towards room temperature.
关键观察结果是温度下降，在 120 秒时达到最低值 14.0 °C，随后又缓慢回升至接近室温。
4. Case Study 1: Analysis, Errors and Improvements | 案例一：分析、误差与改进
The temperature decrease shows that dissolving ammonium nitrate is endothermic: energy is taken in from the water and the surroundings, so the solution cools down. In endothermic dissolving, the energy needed to break the bonds and separate the ions is greater than the energy released when the ions become surrounded by water molecules (hydration).
温度下降表明硝酸铵的溶解是吸热过程：能量从水和周围环境被吸收，因此溶液变冷。在吸热溶解中，破坏化学键并分离离子所需的能量大于离子被水分子包围（水合）时释放的能量。
The overall temperature change can be calculated as:
总体温度变化可以这样计算：
ΔT = 14.0 °C − 21.0 °C = −7.0 °C
heat capacity of water, c = 4.18 J/g°C, and assuming 1 cm³ of water has a mass of 1.00 g:
随后可用下面的公式估算溶液吸收的能量，水的比热容 c = 4.18 J/g°C，并假设 1 cm³ 水的质量为 1.00 g：
energy absorbed = mass × specific heat capacity × ΔT
energy absorbed = 50.0 g × 4.18 J/g°C × 7.0 °C ≈ 1460 J ≈ 1.46 kJ
Now evaluate the method. Several faults stand out.
现在评价实验方法。有几个明显的缺陷。
- The thermometer was used as a stirrer. This is unsafe and can break the thermometer; it also gives inaccurate readings because the bulb is not resting in the liquid.
温度计被当作搅拌棒使用。这既不安全，又可能打碎温度计；同时因为感温泡未静止于液体中，读数也不准确。 - No lid was placed on the cup, so heat from the air entered the solution and made ΔT smaller than the true value.
杯子上没有加盖，空气中的热量进入溶液，使 ΔT 比真实值偏小。 - Reversal after 120 s means the system was simply returning to room temperature, so the trend was disturbed by the surroundings.
120 秒后的回升说明体系只是在恢复至室温，趋势受到了周围环境的干扰。 - The 5.0 g of solid should be weighed accurately on a balance and the exact mass recorded.
5.0 g 固体应用天平准确称量，并记录精确质量。
A stronger method would use a glass rod as the stirrer, a lid or cotton wool plug, a lagged cup, and repeated trials with a slowly cooling curve plotted so the true temperature change can be found by extrapolation.
更严谨的方法应使用玻璃棒搅拌，加盖子或棉花塞，杯子加保温层，并重复实验，绘制缓慢降温曲线，通过外推法求出真实的温度变化。
5. Case Study 2: Identifying Gases - The Scenario | 案例二：气体鉴别——情境设定
Four unlabelled gas jars are delivered to a Year 9 laboratory, labelled A, B, C and D. Each jar contains one of the gases hydrogen, oxygen, carbon dioxide or ammonia. A student is asked to identify each gas using standard tests, recording the observation and the conclusion for each jar.
四个未标记的集气瓶被送到九年级实验室，标号为 A、B、C、D。每个瓶中装着氢气、氧气、二氧化碳或氨气中的一种。学生需要用标准检验方法鉴别每一种气体，并记录每瓶的观察结果与结论。
Safety points matter here: gases must be tested in a fume cupboard if they are toxic or irritant, splints must be handled carefully, and ammonia and carbon dioxide should not be inhaled directly. The tests themselves are quick, but a single careless step - such as using a wet splint - can produce a false result.
这里安全要点很重要：如果有毒或刺激性气体，必须在通风橱内检验；木条要小心操作；氨气和二氧化碳不应直接吸入。检验本身很快，但一个粗心的步骤——例如使用湿润的木条——就可能产生错误结果。
6. Case Study 2: Results, Reasoning and Errors | 案例二：结果、推理与误差
The student's results are summarised below.
学生的实验结果总结如下。
| Jar | 瓶 | Test | 检验 | Observation | 现象 | Conclusion | 结论 |
| A | Lighted splint at mouth of jar | 点燃的木条靠近瓶口 | Squeaky pop | 发出"噗"的爆鸣声 | Hydrogen | 氢气 |
| B | Glowing splint inserted | 伸入带火星的木条 | Splint relights | 木条复燃 | Oxygen | 氧气 |
| C | Bubble through limewater | 通入石灰水 | Limewater turns milky | 石灰水变浑浊 | Carbon dioxide | 二氧化碳 |
| D | Damp red litmus paper | 湿润的红色石蕊试纸 | Turns blue | 变蓝 | Ammonia | 氨气 |
The limewater test depends on a precipitation reaction. Carbon dioxide reacts with calcium hydroxide solution to form insoluble calcium carbonate, which appears as the white cloudiness:
石灰水检验依赖于沉淀反应。二氧化碳与氢氧化钙溶液反应生成不溶于水的碳酸钙，这就是出现白色浑浊的原因：
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Common errors in this case study include: using a wet splint to test for hydrogen, which can give a weak pop or no pop at all; failing to use damp litmus paper for ammonia, since dry litmus will not respond to the alkaline gas; and holding the limewater tube too close to the mouth so that air is also drawn in. In each case, name the error, state its effect on the result, and give the correction.
这个案例中的常见错误包括：用湿润的木条检验氢气，会导致爆鸣声微弱甚至没有声音；检验氨气时未使用湿润的石蕊试纸，因为干燥的石蕊试纸对碱性气体不会发生变化；以及把石灰水试管拿得离瓶口太近，导致空气也被吸入。针对每一种情况，都要指出错误、说明它对结果的影响，并给出改正方法。
7. Case Study 3: Neutralisation - The Scenario | 案例三：中和反应——情境设定
A student titrates 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide solution with 0.100 mol/dm³ hydrochloric acid, using phenolphthalein indicator. She runs one rough titration and then three accurate titrations, reading the burette to the nearest 0.05 cm³.
一名学生用 0.100 mol/dm³ 盐酸滴定 25.0 cm³ 的 0.100 mol/dm³ 氢氧化钠溶液，使用酚酞作指示剂。她先做一次粗略滴定，再做三次精确滴定，滴定管读数精确到 0.05 cm³。
| Titration | 滴定次数 | Volume of acid / cm³ | 酸的体积 / cm³ |
| Rough | 粗略 | 26.00 |
| 1 | 24.30 |
| 2 | 24.40 |
| 3 | 24.30 |
The reaction is a neutralisation. An acid reacts with an alkali to produce a salt and water:
该反应是中和反应。酸与碱反应生成盐和水：
HCl + NaOH → NaCl + H₂O
H⁺ + OH⁻ → H₂O
Neutralisation is exothermic, so the mixture warms up during the titration. If the student measured the temperature change, she would find a rise of a few degrees, and for a strong acid and strong alkali the enthalpy change per mole of water formed is approximately −57 kJ/mol.
中和反应是放热的，因此滴定过程中混合液会升温。如果学生测量温度变化，会发现上升几度；对于强酸与强碱，每生成 1 摩尔水的焓变约为 −57 kJ/mol。
8. Case Study 3: Calculations, Errors and Improvements | 案例三：计算、误差与改进
The expected titre can be calculated from the mole ratio, since HCl and NaOH react in a 1:1 ratio.
由于 HCl 与 NaOH 以 1:1 反应，可以从摩尔比计算理论滴定体积。
moles of NaOH = concentration × volume = 0.100 mol/dm³ × 0.0250 dm³ = 2.50 × 10⁻³ mol
moles of HCl = 2.50 × 10⁻³ mol
e of HCl = moles ÷ concentration = 2.50 × 10⁻³ mol ÷ 0.100 mol/dm³ = 0.0250 dm³ = 25.0 cm³
The student' s about 0.7 cm³ lower than the expected 25.0 cm³ - a difference far larger than the reading uncertainty, so a systematic error is present.
学生三次精确滴定的平均值为 (24.30 + 24.40 + 24.30) ÷ 3 = 24.33 cm³，比理论值 25.0 cm³ 低约 0.7 cm³。这一偏差远大于读数误差，说明存在系统误差。
Likely causes and ways to improve the practical are listed below.
可能的原因以及改进方法列举如下。
- The rough titre was not included in the mean, which is correct, but the student should always quote the concordant titres only. | 粗略滴定值未计入平均值，这是正确的，但学生应始终只引用相互吻合的滴定值。
- Air bubbles in the burette jet reduce the volume delivered; the jet should be filled before starting. | 滴定管尖嘴中的气泡会减小放出的体积；开始前应先排除气泡。
- Reading the meniscus from above or below eye level causes parallax error; the eye must be level with the meniscus and a white tile placed behind the flask. | 俯视或仰视读取液面会造成视差误差；视线必须与液面齐平，并在锥形瓶后放一块白瓷砖。
- The indicator is added to the flask and stays in the final solution, so the salt obtained by evaporation is not pure; for crystal preparation, the titration is repeated without indicator and the same volume is added. | 指示剂加入锥形瓶后会留在最终溶液中，因此蒸发得到的盐不纯；制备晶体时，应不加指示剂重复滴定，并加入相同体积的酸。
- ith a total mass of about 49.4 g and ΔT = 6.7 °C, this is roughly 1380 J, or about 1.4 kJ. | 若测量温度变化以计算放出的能量，使用：放出的能量 = 质量 × 4.18 J/g°C × ΔT。当总质量约为 49.4 g、ΔT = 6.7 °C 时，约为 1380 J，即约 1.4 kJ。
9. Key Practical Skills WJEC Examiners Look For | WJEC 考官关注的实验技能
All three case studies test the same underlying skills. Examiners reward candidates who can
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