📚 The River Pollution Challenge: A Year 7 Chemistry Case Study | 河流污染挑战：七年级化学案例研究
Case studies bring chemistry to life by placing you in the role of a scientific detective. In this scenario, you will investigate a mysterious river pollution event using core Year 7 CAIE chemistry concepts such as states of matter, mixtures, separation techniques and indicators. You will collect evidence, perform experiments in your mind, and develop a clean-up plan just like a real environmental chemist.
案例研究通过让你扮演科学侦探，将化学知识变得鲜活。在本情境中，你将运用七年级 CAIE 化学的核心概念，如物质状态、混合物、分离技术和指示剂，来调查一起神秘的河流污染事件。你将收集证据，在脑海中完成实验，并制定一个净化方案，就像一位真正的环境化学家那样。
1. The Distress Call: Observing the Problem | 求救信号：观察问题
A local village has reported dead fish floating on the river surface and a strange tint to the water. Your first responsibility as a chemical investigator is to observe the scene carefully without touching anything. Observation is a fundamental skill in chemistry: you use your senses to gather initial data before any experiment begins. You stand by the riverbank and record what you see, smell and hear. The water appears murky with a distinct greenish-blue colour that is not natural for a freshwater stream. A faint, sharp metallic odour reaches your nose, suggesting a dissolved chemical. You also notice that plants near the effluent pipe look withered. These visual and olfactory clues are your starting points to form a hypothesis about what might be polluting the river.
当地村庄报告河面上漂浮着死鱼，水体出现了奇怪的色调。作为化学调查员，你的首要责任是在不触碰任何东西的情况下仔细观察现场。观察是化学的一项基本技能：在实验开始前，你先用感官收集初步数据。你站在河岸边，记录所见、所闻和所听。河水看上去浑浊，并带有明显的蓝绿色调，这对于淡水溪流来说并不自然。一股微弱而刺鼻的金属气味飘进鼻腔，暗示有溶解的化学物质。你还注意到排污口附近的植物看起来枯萎了。这些视觉和嗅觉线索是你形成关于污染物可能是什么的假设的起点。
2. Gathering Evidence: Sampling Techniques | 收集证据：取样技术
Proper sampling is essential to get reliable data. You put on protective gloves and take three clean, labelled glass bottles. Bottle A is filled directly from the surface near the dead fish. Bottle B is collected mid-stream at a depth of about 20 cm. Bottle C is taken 50 metres upstream from the suspected pipe to serve as a control sample. Each bottle is sealed immediately to prevent contamination. You record the date, time and location on each label. In the laboratory, you shake the bottles and observe any sediment. This systematic approach ensures your investigation follows the fair-test principle often used in chemistry. You note that Bottle A has the strongest blue-green tint, while Bottle C is almost colourless but slightly cloudy.
正确的取样对于获得可靠数据至关重要。你戴上防护手套，拿出三只洁净且贴好标签的玻璃瓶。A 瓶直接取自死鱼附近的水面。B 瓶在河中段约 20 厘米深处采集。C 瓶则在怀疑的管道上游 50 米处采集，作为对照样本。每只瓶子立即密封以避免污染。你在每个标签上记录日期、时间和地点。在实验室里，你摇动瓶子并观察是否有沉淀。这种系统的方法确保你的调查遵循化学中常用的公平测试原则。你注意到 A 瓶的蓝绿色调最深，而 C 瓶几乎无色但略微浑浊。
3. Visual Clues: Colour, Clarity and Odour | 视觉线索：颜色、清澈度和气味
Before using any apparatus, we examine the physical properties of the collected samples. Physical properties include colour, clarity and odour — characteristics that do not change the chemical identity of a substance. Sample A is translucent with a bright blue-green colour that reminds you of copper(II) sulfate solution you once saw in the lab. Sample B has the same colour but is less intense, while Sample C shows only a faint haze. When you waft the vapour from each bottle towards your nose (never directly sniffing unknown chemicals), Sample A has a metallic, slightly sour smell. Clarity tests by shining a torch through the liquid show that Sample A scatters the light beam noticeably, meaning it contains suspended particles. These observations point toward a mixture — a combination of insoluble solids and a soluble coloured compound.
在动用任何仪器之前，我们先检查所采集样品的物理性质。物理性质包括颜色、清澈度和气味——这些特征不会改变物质的化学身份。样品 A 呈半透明状，明亮的蓝绿色让你想起在实验室见过的硫酸铜溶液。样品 B 颜色相同但较浅，而样品 C 仅呈现微弱的朦胧感。当你用手将每个瓶口的气体轻轻扇向鼻子时（切勿直接嗅闻未知化学品），样品 A 带有金属般微酸的气味。用手电筒照射液体进行清澈度测试时，样品 A 明显散射光束，说明含有悬浮颗粒。这些观察指向一种混合物——由不溶性固体和一种可溶的有色化合物共同组成。
4. Separation Challenge: Filtration First | 分离挑战：首先过滤
To isolate the insoluble particles, we set up a filtration apparatus. A filter funnel is placed in a conical flask and lined with a folded filter paper. We pour Sample A carefully so that it runs along a glass rod into the funnel; this prevents splashing. Almost immediately, solid greyish-brown material collects on the filter paper. The liquid that passes through (the filtrate) is still blue-green but now perfectly clear, confirming that the colour is due to a dissolved substance rather than suspended solids. Filtration is a physical separation technique that works based on particle size: the large insoluble particles cannot pass through the tiny pores of the filter paper. This step leaves us with two parts to investigate further — the residue (what stayed on the paper) and the filtrate (the liquid that passed through).
为了分离出不溶性颗粒，我们搭建了一套过滤装置。将过滤漏斗放在锥形瓶上，垫好折叠的滤纸。我们小心地将样品 A 沿玻璃棒倒入漏斗，以防飞溅。几乎在同时，灰褐色的固体物质就积聚在滤纸上。穿过滤纸的液体（滤液）仍然呈蓝绿色，但现在完全清澈了，这证实颜色是由溶解性物质而非悬浮固体造成的。过滤是一种基于颗粒大小的物理分离技术：大块不溶性颗粒无法通过滤纸的微小孔隙。这一步骤留给我们两部分需要进一步调查——残渣（留在滤纸上的物质）和滤液（穿过滤纸的液体）。
5. Residue Investigation: What Remains on the Filter Paper? | 残渣调查：滤纸上留下了什么？
The residue on the filter paper looks like damp soil mixed with tiny dark specks. We carefully remove the filter paper and place it in a warm oven at 60 °C to dry. After drying, the solid becomes crumbly and can be weighed. Its appearance is typical of silt, clay and perhaps small fragments of plant material — all naturally insoluble in water. We can confirm that this residue is not the main pollutant because Sample C (upstream) contains similar insoluble particles, just in smaller amounts. This tells us that while the river naturally carries some suspended matter, the real chemical threat is dissolved and passes through the filter. The residue is a non-hazardous component of the mixture.
滤纸上的残渣看起来就像潮湿的土壤夹杂着微小的黑色斑点。我们小心地取出滤纸，将其放入 60 °C 的烘箱中烘干。干燥后，固体变得易碎，可以称重。它的外观是典型的泥沙、黏土以及可能还有细小的植物残片——所有这些物质天然不溶于水。我们可以确认这些残渣并非主要污染物，因为上游的样品 C 也含有类似的不溶性颗粒，只是数量较少。这告诉我们，虽然河水天然携带着一些悬浮物，但真正的化学威胁已经溶解并穿过了滤纸。残渣是混合物中的非危险成分。
The following table summarises the comparison between the residue and the filtrate after this physical separation:
下表总结了这次物理分离后残渣与滤液的对比：
| Property | Residue | Filtrate |
|---|---|---|
| Phase at room temperature | Solid (after drying) | Liquid |
| Colour | Grey-brown | Blue-green, transparent |
| Solubility in water | Insoluble | Contains dissolved solute(s) |
| Toxicity indication | None (natural sediment) | Likely contains pollutant |
6. Evaporation: Seeking Dissolved Solids | 蒸发：寻找溶解的固体
To find out what is dissolved in the blue-green filtrate, we pour a small volume into an evaporating dish and heat it gently over a water bath. As the water warms up, steam rises, and the volume of liquid decreases. Soon, a crust begins to form at the edges of the dish. After all the water has evaporated, a bright blue crystalline solid remains. This solid looks strikingly similar to hydrated copper(II) sulfate (CuSO₄·5H₂O) crystals you have grown in class. Evaporation is effective here because the dissolved solid has a much higher boiling point than water; when the water turns into vapour, the solid is left behind. This confirms that the river contains a soluble metal salt, and its colour strongly suggests copper ions.
为了查明蓝绿色滤液中溶解了什么，我们将少量滤液倒入蒸发皿中，在水浴上缓缓加热。随着水温升高，水蒸气升起，液体的体积逐渐减少。很快，蒸发皿边缘开始结出一层硬壳。待所有水分蒸发后，留下了一种亮蓝色的晶体固体。这种固体与你在课堂上培养的硫酸铜含水晶体（CuSO₄·5H₂O）惊人地相似。在此蒸发之所以有效，是因为溶解固体的沸点远高于水；当水变成蒸汽时，固体便留了下来。这证实河水含有一种可溶性金属盐，其颜色强烈暗示铜离子的存在。
7. Dissolved Solids Analysis: Crystals Appear | 溶解固体分析：晶体出现
The blue crystals are carefully scraped from the dish and compared with known laboratory samples. They form triclinic shapes, are transparent when small, and crumble into a pale blue powder when ground. When we place a few crystals on a watch glass and add a drop of water, they dissolve instantly, reproducing the familiar blue-green solution. A simple heating test in a hard-glass tube shows that the crystals release water vapour and turn white, a classic test for hydrated salts. These properties match copper(II) sulfate pentahydrate almost exactly. This evidence suggests that a factory is likely discharging wastewater containing copper compounds into the river. Copper salts at high concentrations are toxic to aquatic life, explaining the dead fish.
小心地从蒸发皿上刮下蓝色晶体，并与实验室已知样品进行比对。它们呈三斜晶形，薄片时透明，研磨后变成淡蓝色粉末。将几粒晶体放在表面皿上，滴加一小滴水，它们立即溶解，再现了熟悉的蓝绿色溶液。在硬质玻璃管中简单加热，晶体会释放水蒸气并变为白色，这是含水盐的经典测试。这些性质几乎与五水合硫酸铜完美吻合。这一证据表明，一家工厂很可能正在向河中排放含有铜化合物的废水。高浓度的铜盐对水生生物有毒，这便解释了死鱼的原因。
8. pH Testing: Acidic or Alkaline? | pH测试：酸性还是碱性？
Many industrial effluents change the acidity of water, so we test the pH of the filtrate using universal indicator solution. A few drops turn it orange-yellow, indicating a pH around 5–6. We confirm this with a pH meter, which reads 5.5. This is moderately acidic compared with neutral water (pH 7). A solution of copper(II) sulfate is naturally slightly acidic because copper ions react with water to release hydrogen ions. The factory discharge not only introduces toxic copper but also lowers the river pH, which can harm plants and animals that are sensitive to acid conditions. The upstream Sample C gives a pH of 6.8, almost neutral, proving that the acidity comes from the pollution source.
许多工业废水会改变水的酸碱性，因此我们使用通用指示剂测试滤液的 pH。滴入几滴后，颜色变为橙黄色，表明 pH 在 5–6 左右。我们用 pH 计确认，读数为 5.5。与中性水（pH 7）相比，这属于中等酸性。硫酸铜溶液天然呈弱酸性，因为铜离子与水反应会释放氢离子。工厂排放不仅引入有毒的铜，还降低了河水的 pH，这可能危害对酸性环境敏感的动植物。上游的样品 C 测得的 pH 为 6.8，几乎中性，这证明酸性来自污染源。
The universal indicator gave a range of colours depending on pH. Our result fell in the acidic range:
通用指示剂根据 pH 显示一系列颜色。我们的结果落在酸性范围内：
- pH 1–2: red (strongly acidic)
- pH 3–4: orange (moderately acidic)
- pH 5–6: yellow (weakly acidic)
- pH 7: green (neutral)
- pH 8–9: blue (weakly alkaline)
- pH 10–11: purple (strongly alkaline)
- pH 1–2：红色（强酸性）
- pH 3–4：橙色（中等酸性）
- pH 5–6：黄色（弱酸性）
- pH 7：绿色（中性）
- pH 8–9：蓝色（弱碱性）
- pH 10–11：紫色（强碱性）
9. Distillation for Pure Water: Recovering the Solvent | 蒸馏获取纯水：回收溶剂
To demonstrate that clean water can be reclaimed from the polluted sample, we set up a simple distillation apparatus using a round-bottom flask, a delivery tube and a condenser. The filtrate is heated until it boils at about 100 °C. Steam travels through the condenser where cold water circulates around it, causing the vapour to condense back into liquid water. The distillate collected is completely colourless and has a neutral pH. No blue residue remains in the receiving flask. The dissolved copper salt stays behind in the boiling flask because it has a far higher boiling point. Distillation is a powerful separation method that can purify water from soluble contaminants, but it requires energy. This step proves that if a pollution incident occurs, distillation could in principle produce safe drinking water from the river, though on a large scale other methods such as chemical precipitation might be more practical.
为了证明可以从污染样品中回收清水，我们搭起一套简单的蒸馏装置，包括圆底烧瓶、导气管和冷凝器。将滤液加热至约 100 °C 至沸腾。蒸汽通过冷凝器，冷凝器外有冷水循环，使蒸汽冷凝回液态水。收集到的馏出液完全无色，且 pH 为中性。接收瓶中没有蓝色残留物。溶解的铜盐由于沸点远高于水而留在蒸馏烧瓶中。蒸馏是一种强大的分离方法，可以去除可溶性污染物从而净化水，但它需要消耗能源。这一步证明，如果发生污染事件，蒸馏原则上可以从河水中制取安全的饮用水，尽管在大规模应用中，化学沉淀等方法可能更实际。
10. Solving the Pollution Crisis: Applying Chemistry | 解决污染危机：应用化学
Putting all the evidence together, we can now picture the entire pollution scenario. A factory upstream has been releasing wastewater containing dissolved copper(II) sulfate, along with some suspended solid waste. This explains the blue-green colour, the metallic odour, the acidic pH, the blue crystals from evaporation, and the
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