📚 Case Study: Desalination and Water Quality Testing | 案例分析：海水淡化与水质检测
In this case study, you will step into the role of a scientist helping a coastal community facing a freshwater shortage. You must design a method to obtain clean drinking water from seawater and test its purity using simple chemical techniques. Along the way, you will apply your knowledge of mixtures, evaporation, distillation, and chemical analysis, all key topics in the KS3 Edexcel Chemistry curriculum.
在这个案例研究中，你将扮演一名科学家的角色，帮助一个沿海社区解决淡水短缺的问题。你必须设计一种从海水中获取清洁饮用水的方法，并使用简单的化学技术检测其纯度。在此过程中，你将运用混合物、蒸发、蒸馏和化学分析等知识，这些都是 KS3 Edexcel 化学课程的核心内容。
1. The Problem: A Community Needs Fresh Water | 问题：社区需要淡水
A small village by the sea has been experiencing drought for months. Their only available water source is the ocean, but seawater is too salty to drink. The villagers ask you to produce potable water from the sea. The first step is to understand exactly what seawater contains and why it is not safe for direct consumption. Scientists always begin by defining the problem and gathering background information before designing an investigation.
海边的一个小村庄已经遭受了几个月的干旱。他们唯一可用的水源是海洋，但海水太咸，无法饮用。村民们请你从海水中生产出可饮用的水。第一步是准确了解海水的成分以及为什么它不能直接饮用。科学家们在设计调查方案之前，总是先明确问题并收集背景信息。
2. Investigating Seawater Composition | 调查海水成分
Seawater is a mixture. It contains water (the solvent), dissolved salts such as sodium chloride (NaCl), and small amounts of other minerals like magnesium chloride. It also contains suspended solid particles like sand and microorganisms. When you taste seawater, the salty flavour comes mainly from dissolved sodium chloride. If you simply boil seawater, the water evaporates but the salt remains. This is a physical change because the substances are not chemically altered. The particles are simply separated, not rearranged into new substances.
海水是一种混合物。它含有水（溶剂）、溶解的盐类，如氯化钠（NaCl），以及少量的其他矿物质，如氯化镁。还含有悬浮固体颗粒，如沙子和微生物。当你品尝海水时，咸味主要来自溶解的氯化钠。如果只是将海水煮沸，水会蒸发，但盐会留在原处。这是一种物理变化，因为物质没有发生化学变化。微粒只是被分离开来，并没有重新组合成新的物质。
3. Choosing a Separation Technique | 选择分离技术
To separate the water from the dissolved salts, we need a technique that takes advantage of the difference in boiling points. Water boils at 100 °C, while common salts have much higher boiling points (over 1400 °C). Therefore, distillation is the ideal method. In distillation, the mixture is heated, the water turns into steam, and the steam is then cooled and condensed back into liquid water in a separate container. The salt is left behind. This produces pure water, also called distilled water. Distillation is a powerful separation technique for obtaining a pure liquid solvent from a solution.
为了将水与溶解的盐分离，我们需要利用沸点的差异。水的沸点是 100 °C，而常见盐类的沸点要高得多（超过 1400 °C）。因此，蒸馏是理想的方法。在蒸馏过程中，混合物被加热，水变成水蒸气，然后水蒸气被冷却并在另一个容器中凝结回液态水。盐被留下。这就产生了纯净水，也称为蒸馏水。蒸馏是一种从溶液中获取纯液态溶剂的强大分离技术。
4. Laboratory Setup for Simple Distillation | 简单蒸馏的实验室装置
You set up the following apparatus: a round-bottom flask containing seawater, a delivery tube connected to a condenser, and a receiving beaker to collect the distillate. A thermometer is placed at the top of the flask to monitor the temperature of the vapour. The condenser has cold water running through its outer jacket to cool the hot steam. The key components are:
- Round-bottom flask: holds the seawater and withstands heating.
- Thermometer: measures temperature near the side arm to ensure vapour is at 100 °C.
- Condenser: cools the steam into liquid; cold water enters at the bottom and leaves at the top for efficient cooling.
- Receiving beaker: collects the distilled water.
As the seawater boils, pure water vapour travels through the condenser, condenses, and drips into the beaker as a clear liquid.
你搭建了以下装置：一个盛有海水的圆底烧瓶，一根连接冷凝管的导管，以及一个收集蒸馏液的接收烧杯。温度计放在烧瓶顶部，用于监测蒸气的温度。冷凝管的外套管中有冷水流动，用于冷却热蒸汽。关键组件包括：
- 圆底烧瓶：盛放海水并能承受加热。
- 温度计：测量支管附近的温度，确保蒸气温度在 100 °C。
- 冷凝管：将水蒸气冷却成液体；冷水从下端进入、上端流出，以达到高效冷却。
- 接收烧杯：收集蒸馏水。
当海水沸腾时，纯水蒸气穿过冷凝管，冷凝后以清澈液体的形式滴入烧杯中。
5. Observing the Distillation Process | 观察蒸馏过程
As heating continues, you notice bubbles forming in the flask at around 100 °C. The water level drops gradually, while a white crust of salt becomes visible on the inner walls of the flask. The liquid collected in the beaker looks colourless and clear. You smell it and find no odour. This suggests the distillate is pure water, but observations alone are not enough to confirm purity. In science, we must always back up our observations with quantitative or qualitative tests. You decide to carry out a series of purity checks on the collected liquid.
随着持续加热，你注意到在 100 °C 左右烧瓶中形成气泡。水位逐渐下降，而烧瓶内壁上出现一层白色的盐壳。收集在烧杯中的液体看起来无色透明。你闻了一下，没有气味。这表明蒸馏液是纯净的水，但仅靠观察不足以确认纯度。在科学中，我们必须始终通过定量或定性测试来支持我们的观察。你决定对收集的液体进行一系列纯度检查。
6. Testing the Purity of the Distilled Water | 检验蒸馏水的纯度
To test whether the distillate contains any dissolved salts, you perform an evaporation test. The procedure is simple but effective:
- Pipette 2 cm³ of distilled water into a clean, dry evaporating dish.
- Place the dish on a water bath or a low tripod with gentle flame.
- Allow the water to evaporate completely — this may take several minutes.
- Inspect the dish carefully: if no solid residue is visible, the water contained no dissolved solids.
For comparison, repeat the same test with a sample of the original seawater. That dish will show a prominent white crust of salt. Another quality indicator is the boiling point. Fill a test tube with the distillate, insert a thermometer, and heat gently in a water bath; the temperature should plateau at exactly 100 °C (at standard pressure). Any dissolved impurities would elevate the boiling point, a phenomenon known as boiling point elevation. Together, these tests provide strong evidence that distillation successfully removed the salt.
为了检验蒸馏液中是否含有溶解的盐，你进行蒸发测试。操作步骤简单但有效：
- 用移液管取 2 cm³ 蒸馏水放入一个干净、干燥的蒸发皿中。
- 将蒸发皿放在水浴或带有温和火焰的三脚架上。
- 让水完全蒸发——这可能需要几分钟。
- 仔细检查蒸发皿：如果看不到固体残留物，说明水中不含溶解的固体。
作为对比，用原始海水样品重复相同的测试。那个蒸发皿上会出现明显的白色盐壳。另一个质量指标是沸点。在试管中装入蒸馏液，插入温度计，在水浴中缓慢加热；温度应该稳定在恰好 100 °C（标准压强下）。任何溶解的杂质都会使沸点升高，这种现象称为沸点升高。这些测试共同提供了有力证据，表明蒸馏成功去除了盐分。
7. Checking for Other Contaminants | 检查其他污染物
Distillation removes dissolved salts and also kills most microorganisms because of the high temperature. However, some volatile impurities with boiling points near 100 °C could still be present. To check the water quality further, you test the pH using universal indicator paper. Pure water should have a neutral pH of 7. Dip a strip of indicator paper into the distillate and compare the colour to the chart; it should match green. If the water were acidic or alkaline, the colour would differ.
An optional advanced test for chlorides can be done with silver nitrate solution (AgNO₃), if available. Add a few drops of AgNO₃ to a sample of the distillate. No change should occur. In contrast, adding AgNO₃ to seawater produces a thick white precipitate of silver chloride (AgCl), confirming the presence of chloride ions (Cl⁻). For KS3 level, the evaporation residue and boiling point tests already provide sufficient evidence of purity.
蒸馏可以去除溶解的盐，又因为高温而杀死大多数微生物。但是，一些沸点在 100 °C 附近的挥发性杂质可能仍然存在。为了进一步检查水质，你使用广泛 pH 试纸检测 pH 值。纯净水的 pH 应为中性的 7。将一条 pH 试纸浸入蒸馏液中，与比色卡对比；颜色应该对应绿色。如果水呈酸性或碱性，颜色会不同。
如果有条件，还可以用硝酸银溶液（AgNO₃）进行一种高级氯化物测试。向蒸馏液样品中滴加几滴 AgNO₃，不应发生变化。相反，向海水中加入 AgNO₃ 会产生浓厚的白色氯化银（AgCl）沉淀，证实氯离子（Cl⁻）的存在。对于 KS3 阶段，蒸发残渣测试和沸点测试已经提供了足够的纯度证据。
8. Energy Considerations and Practicality | 能源考虑与实际可行性
While distillation effectively produces pure water, it requires a lot of heat energy. Boiling one litre of water takes a long time and uses fuel or electricity. This might not be practical for providing large amounts of freshwater to a whole village. To illustrate, consider the energy needed:
- To boil 1 L of seawater (from 20 °C to 100 °C) and then evaporate it, approximately 2.6 × 10⁶ J of energy is required.
- This is equivalent to running a 1000 W electric kettle for about 43 minutes.
- Published by TutorHao | KS3 Chemistry Revision Series | aleveler.com
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