📚 WJEC Year 11 Chemistry: Case Study Practice Exercises | WJEC 11年级化学：案例分析实战演练
In your WJEC Year 11 Chemistry course, applying knowledge to real-world scenarios is just as important as learning the theory. Case study exercises help you develop practical thinking, data interpretation, and problem-solving skills. This case study walks you through an investigation of a suspected pollution incident in a river, allowing you to practise qualitative analysis, quantitative calculations, and environmental chemistry. Work through each section, and you will see how chemistry concepts come together to solve a realistic problem.
在WJEC 11年级化学课程中，将知识应用到真实场景与理论学习同等重要。案例分析练习能帮助你培养实践思维、数据解释和问题解决能力。本案例将引导你调查一起疑似河流污染事件，让你练习定性分析、定量计算和环境化学。逐一完成每个部分，你会看到化学概念如何结合起来解决一个现实问题。
1. The Scenario: Pollution in the River Afan | 案例情景：阿凡河污染
A local environmental officer reports that fish in the River Afan have been dying and the water has an unusual yellow-brown colour. A nearby industrial estate includes a metal-plating factory, a chemical warehouse, and a sewage outlet. As a trainee analytical chemist, you are asked to collect water samples from three sites: upstream from the industrial zone (Site A), directly at the discharge pipe (Site B), and 500 metres downstream (Site C). You need to identify any pollutants, measure their concentrations, and assess the possible impact on the ecosystem. This exercise will mirror the investigations found in WJEC practical assessments.
当地一名环境官员报告，阿凡河里的鱼正在死亡，河水呈现异常的黄褐色。附近的工业区包括一个电镀厂、一个化学品仓库和一个污水排放口。作为一名见习分析化学师，你被要求从三个地点采集水样：工业区上游（A点）、排放管正下方（B点）和下游500米处（C点）。你需要识别可能的污染物，测量其浓度，并评估对生态系统的潜在影响。这一练习将模拟WJEC实践考核中常见的调查。
Your first task is to record all visual observations and to test the pH of each sample. You notice that the Site B sample smells acidic and contains fine suspended particles, while Site A is clear and odourless.
你的第一项任务是记录所有视觉观察结果并测试每个样品的pH值。你注意到B点水样有酸味且含有细小悬浮颗粒，而A点水样清澈无味。
2. Initial Observations and pH Testing | 初步观察与pH测试
Using universal indicator paper, you find the following pH values: Site A – 7.0, Site B – 3.5, Site C – 5.2. The acidic pH at Site B suggests a strong acid input. The partial recovery at Site C indicates dilution or neutralisation by natural carbonate minerals in the riverbed. In WJEC chemistry, you are expected to link pH to the concentration of hydrogen ions and understand the logarithmic scale. A decrease of 1 pH unit means a tenfold increase in H⁺ concentration. From pH 7 to 3.5, the H⁺ concentration has risen by a factor of 103.5 ≈ 3200 times.
使用通用指示纸，你测得以下pH值：A点 – 7.0，B点 – 3.5，C点 – 5.2。B点的酸性pH表明有强酸输入。C点pH的部分恢复说明发生了稀释，或是河床中的天然碳酸盐矿物进行了中和。在WJEC化学中，你需要将pH与氢离子浓度联系起来，并理解其对数标度。pH值每下降1个单位，H⁺浓度就增加10倍。从pH 7到3.5，H⁺浓度大约上升了103.5约3200倍。
Besides pH, you would also test for temperature and conductivity. High conductivity at Site B would point to dissolved ions from salts or acids. Your field kit shows conductivity: Site A – 120 μS/cm, Site B – 1450 μS/cm, Site C – 680 μS/cm. This pattern confirms that a large amount of ionic material is being discharged at the pipe.
除了pH，你还可以测试温度和电导率。B点的高电导率表明存在来自盐或酸的溶解离子。你的现场套件显示电导率：A点 – 120 μS/cm，B点 – 1450 μS/cm，C点 – 680 μS/cm。这一趋势证实排放管处正在排出大量离子型物质。
3. Cation Analysis: Flame Tests and Precipitation | 阳离子分析：焰色试验与沉淀反应
To identify metal cations, you carry out flame tests on the dried residues from each sample. Site A gives a faint yellow colour (likely trace sodium). Site B produces a bright green flame, which is characteristic of copper(II) ions, Cu²⁺. Site C shows a weaker green. A green flame test strongly suggests the presence of copper. You should recall from WJEC lessons that copper(II) compounds often emit a green-blue colour in a flame.
为鉴定金属阳离子，你对各样品干燥后的残渣进行焰色试验。A点呈现微弱的黄色（可能是痕量钠）。B点产生明亮的绿色火焰，这是铜(II)离子Cu²⁺的特征。C点显示较浅的绿色。绿色焰色强烈表明铜的存在。你应该记得在WJEC课程中，铜(II)化合物在火焰中常发出蓝绿色。
You then perform sodium hydroxide precipitation tests. Adding a few drops of dilute NaOH to the Site B sample forms a pale blue precipitate, which dissolves in excess ammonia forming a deep blue solution. This confirms Cu²⁺:
然后你进行氢氧化钠沉淀试验。向B点样品中滴加稀NaOH溶液，生成淡蓝色沉淀；该沉淀溶于过量氨水，形成深蓝色溶液。这证实了Cu²⁺：
Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) (pale blue)
Cu(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + 2OH⁻(aq) (deep blue)
Additionally, you notice a small amount of brown precipitate, which might be iron(III) hydroxide. Further testing with potassium thiocyanate (KSCN) solution gives a blood-red colour, confirming Fe³⁺:
此外，你还观察到少量棕色沉淀，可能是氢氧化铁(III)。进一步用硫氰酸钾(KSCN)溶液测试，得到血红色，证实了Fe³⁺：
Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) (brown)
Fe³⁺(aq) + SCN⁻(aq) → [FeSCN]²⁺(aq) (blood-red)
So the main cationic pollutants are copper(II) and iron(III) ions, both likely coming from the metal-plating factory.
因此，主要的阳离子污染物是铜(II)和铁(III)离子，两者都可能来自电镀厂。
4. Anion Analysis: Sulfates and Chlorides | 阴离子分析：硫酸根与氯离子
To test for anions, you add barium chloride solution acidified with dilute hydrochloric acid to the samples. Site B produces a heavy white precipitate, indicating sulfate ions. The precipitate is barium sulfate, which is insoluble in acids:
为测试阴离子，你向各样品中加入用稀盐酸酸化的氯化钡溶液。B点产生大量白色沉淀，表明存在硫酸根离子。该沉淀是硫酸钡，不溶于酸：
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white)
For chloride ions, you add silver nitrate solution followed by dilute nitric acid. A white precipitate forms, which is soluble in dilute ammonia (unlike silver sulfate or silver carbonate). This confirms Cl⁻:
对于氯离子，你加入硝酸银溶液，随后加入稀硝酸。生成白色沉淀，该沉淀可溶于稀氨水（与硫酸银或碳酸银不同）。这证实了Cl⁻：
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white)
The presence of sulfate and chloride again points to industrial discharge, possibly from acids and metal salts used in electroplating.
硫酸根和氯离子的存在再次指向工业排放，可能来自电镀中使用的酸和金属盐。
5. Acidity Determination: Acid-Base Titration | 酸度测定：酸碱滴定
You decide to titrate the filtered Site B water against standard 0.100 mol/dm³ sodium hydroxide solution using phenolphthalein as indicator. After pipetting 25.0 cm³ of the water sample into a conical flask, the average titre of NaOH needed to reach the endpoint is 18.50 cm³. The acidic content is assumed to be sulfuric acid, H₂SO₄, which is diprotic. Use this titration data to calculate the concentration of the acid.
你决定用0.100 mol/dm³的氢氧化钠标准溶液滴定过滤后的B点水样，酚酞作指示剂。用移液管量取25.0 cm³水样至锥形瓶中，到达终点所需NaOH的平均滴定体积为18.50 cm³。假设酸性成分是硫酸 H₂SO₄，一种二元酸。请利用滴定数据计算该酸的浓度。
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
s of H₂SO₄ in 25.0 cm³ = (1.85 × 10⁻³) / 2 = 9.25 × 10⁻⁴ mol. Concentration of H₂SO₄ in the sample = (9.25 × 10⁻⁴ mol) / (25.0 / 1000 dm³) = 0.0370 mol/dm³. Expressed in g/dm³, using Mᵣ of H₂SO₄ = 98.1, the concentration = 0.0370 × 98.1 ≈ 3.63 g/dm³. This is highly acidic for natural water.
所用NaOH的物质的量 = c × V = 0.100 mol/dm³ × (18.50 / 1000) dm³ = 1.85 × 10⁻³ mol。根据化学计量关系，1 mol H₂SO₄ 与2 mol NaOH反应，因此25.0 cm³样品中H₂SO₄的物质的量 = (1.85 × 10⁻³) / 2 = 9.25 × 10⁻⁴ mol。样品中H₂SO₄的浓度 = (9.25 × 10⁻⁴ mol) / (25.0 / 1000 dm³) = 0.0370 mol/dm³。换算成g/dm³，H₂SO₄的Mᵣ = 98.1，浓度 = 0.0370 × 98.1 ≈ 3.63 g/dm³。这对天然水体来说酸性极强。
6. Calculating Pollutant Concentration from Mass Data | 根据质量数据计算污染物浓度
e, CuSO₄, and iron(III) sulfate, Fe₂(SO₄)₃. If preliminary tests suggest the residue is 70% the water in mol/dm³. Mᵣ of CuSO₄ = 159.6, and each mole of CuSO₄ contains 1 mole of Cu²⁺.
你同样蒸发100.0 cm³的B点水样至干，残余物质量为0.456 g。该残余物主要为硫酸铜 CuSO₄ 和硫酸铁(III) Fe₂(SO₄)₃。如果初步测试表明残余物中70%为CuSO₄，试计算水中Cu²⁺离子浓度（单位mol/dm³）。CuSO₄的Mᵣ = 159.6，且每摩尔CuSO₄含1 mol Cu²⁺。
cm³, so concentration of CuSO₄ = 0.00200 mol / 0.100 dm³ = 0.0200 mol/dm³. Thus [Cu²⁺] = 0.0200 mol/dm³. In terms of mass concentration, that is 0.0200 × 63.5 = 1.27 g/dm³, far above safe ecological limits.
残余物中CuSO₄的质量 = 0.456 g × 0.70 = 0.3192 g。CuSO₄的物质的量 = 0.3192 / 159.6 = 0.00200 mol（取三位有效数字）。该含量来自100.0 cm³，因此CuSO₄浓度 = 0.00200 mol / 0.100 dm³ = 0.0200 mol/dm³。于是[Cu²⁺] = 0.0200 mol/dm³。换算成质量浓度即0.0200 × 63.5 = 1.27 g/dm³，远高于安全生态限值。
7. Colorimetric Determination of Iron(III) Ions | 铁(III)离子的比色法测定
e to form the red [FeSCN]²⁺ complex. A series of standard solutions of known concentrations is prepared, and their absorbance measured at 480 nm. The calibration graph yields a linear relationship: absorbance = 0.125 × concentration (ppm). The Site B sample, after appropriate dilution, gives an absorbance of 0.450. Calculate the concentration of Fe³⁺ in the original sample.
为更精确地测得铁(III)浓度，你进行比色分析。Fe³⁺离子与硫氰酸钾反应生成红色的[FeSCN]²⁺配合物。配制一系列已知浓度的标准溶液，并在480 nm处测量吸光度。校准曲线给出线性关系：吸光度 = 0.125 × 浓度（ppm）。适当稀释后的B点样品吸光度为0.450。计算原始水样中Fe³⁺的浓度。
, then the original concentration = 3.60 ppm × 10 = 36.0 ppm (mg/dm³). Assuming the density of water is ~ × 10⁻⁴ mol/dm³. Although lower than copper, this is still toxic to many aquatic organisms.
用该公式，稀释后样品浓度 = 0.450 / 0.125 = 3.60 ppm。如果样品稀释了10倍（如取10 cm³稀释至100 cm³），则原始浓度 = 3.60 ppm × 10 = 36.0 ppm (mg/dm³)。假设水的密度约为1 g/cm³，这相当于36.0 mg/dm³，或0.0360 g/dm³。以mol/dm³计，Fe的Mᵣ = 55.8，浓度 = 0.0360 / 55.8 = 6.45 × 10⁻⁴ mol/dm³。虽然低于铜，但仍对许多水生生物有毒。
8. Reaction Rate Considerations: Acid-Catalysed Hydrolysis | 反应速率考量：酸催化水解
Your investigation also considers whether the acidic conditions could accelerate the corrosion of iron structures or the breakdown of organic matter. A simple rate experiment can be designed by placing a known mass of magnesium ribbon into water samples at different pH and measuring the volume of hydrogen gas produced over time. The rate is faster at lower pH because of the higher H⁺ concentration. This directly links to the WJEC topic on activation energy and catalyst effects.
你的调查同时考虑酸性条件是否会加速铁结构的腐蚀或有机物的分解。可以设计一个简单速率实验：将已知质量的镁带放入不同pH的水样中，测量随时间产生的氢气体积。由于H⁺浓度较高，pH低时反应速率更快。这直接关联到WJEC关于活化能和催化剂效应的课题。
Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)
You could also investigate the effect of temperature on the reaction rate by warming the acidic water. A rise of 10°C would roughly double the rate, consistent with the rule of thumb often discussed in class. The case study allows you to predict that in summer, the pollutant's corrosive action would increase, posing a higher risk.
你还可以通过加热酸性水样来探究温度对反应速率的影响。温度每升高10°C，速率大约翻倍，这与课上常讨论的经验规则一致。通过本案例可以预测，夏季时污染物的腐蚀作用会增强，风险更高。
9. Electrochemical Corrosion Potential | 电化学腐蚀可能性
With dissolved copper ions present, there is a possibility of galvanic corrosion if the water flows over iron pipes or boat hulls. Copper has a higher standard electrode potential than iron, so it can deposit as copper metal while iron oxidises. This displacement reaction can be written as:
由于存在溶解的铜离子，若水流经铁质管道或船体，有可能发生电化学腐蚀。铜的标准电极电位比铁高，因此铜会沉积为金属铜，而铁则被氧化。这一置换反应可表示为：
Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)
You can connect this to the reactivity series and the concept of sacrificial protection. In WJEC chemistry, students are expected to explain why zinc blocks are used on ship hulls, since zinc is more reactive and corrodes instead of iron. However, in this contaminated river, the presence of Cu²⁺ will actually speed up the corrosion of any iron-based structures, worsening the environmental damage.
你可以将此与金属活动性顺序及牺牲保护的概念联系起来。在WJEC化学中，学生需要解释为什么船体上使用锌块，因为锌更活泼，会代替铁被腐蚀。然而，在这条受污染的河流中，Cu²⁺的存在实际上会加速任何铁质结构的腐蚀，加剧环境破坏。
10. Environmental Impact and Mitigation | 环境影响与缓解措施
The combined data show that the industrial estate is releasing sulfuric acid, copper(II) sulfate, iron(III) sulfate, and chloride salts into the river. This discharge lowers the pH, increases heavy metal toxicity, and can cause fish gill damage and invertebrate death. In your WJEC exam, you might be asked to suggest clean-up methods. One approach is to add lime (calcium oxide or calcium hydroxide) to neutralise the acid and precipitate the heavy metals as hydroxides:
综合数据表明，工业区正在向河流排放硫酸、硫酸铜(II)、硫酸铁(III)和氯化物盐。这些排放物降低pH，增加重金属毒性，可能导致鱼的鳃损伤及无脊椎动物死亡。在WJEC考试中，你可能会被要求提出清理方法。一种方法是通过添加石灰（氧化钙或氢氧化钙）来中和酸性，并使重金属以氢氧化物形式沉淀：
Ca(OH)₂(s) + H₂SO₄(aq) → CaSO₄(s) + 2H₂O(l)
Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
Another strategy is ion-exchange columns that can remove cations and anions. You also recommend regular monitoring of pH and metal ion levels, and enforcement of limits on industrial effluent. This case study illustrates how WJEC chemistry practical skills — titration, qualitative tests, colorimetry, and rate experiments — are essential for solving real environmental problems.
另一种策略是使用离子交换柱以去除阳离子和阴离子。你还建议定期监测pH和金属离子水平，并对工业废水实施限值。本案例展示了WJEC化学的实践技能——滴定、定性测试、比色法和速率实验——对于解决真实环境问题至关重要。
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