📚 Case Studies in Action | 案例分析实战演练
In Year 7 WJEC Chemistry, applying knowledge to real-world scenarios is crucial for understanding scientific concepts. This article presents several case studies that allow students to practise problem-solving and experimental design. Each case study is followed by an analysis that links observations to particle theory, chemical reactions, and separation techniques.
在WJEC七年级化学中，将知识应用于实际场景对于理解科学概念至关重要。本文展示了几个案例研究，让学生练习问题解决和实验设计。每个案例都配以分析，将观察结果与粒子理论、化学反应和分离技术联系起来。
1. Separating Sand and Salt – A Beach Cleanup Challenge | 分离沙子和盐——海滩清理挑战
Imagine you are a scientist helping to restore a beach after a cargo spill. The beach is covered with a mixture of dry sand and table salt. Your task is to recover both pure sand and pure salt for reuse. How would you design an experiment to achieve this?
假设你是一名科学家，帮助在一次货物泄漏后恢复海滩。海滩上覆盖着干沙和食盐的混合物。你的任务是回收纯沙和纯盐以供再利用。你将如何设计实验来实现这一目标？
Sand does not dissolve in water (insoluble), but salt does dissolve (soluble). This difference is key to separation. According to the particle model, salt particles break away and disperse evenly among water molecules, while sand particles remain as larger clumps.
沙不溶于水（不可溶），而盐可溶（可溶）。这一差异是分离的关键。根据粒子模型，盐粒子脱离并在水分子间均匀分散，而沙粒则保持较大的团块。
Step 1: Add plenty of water to the sand-salt mixture and stir well with a glass rod. The salt dissolves, forming a solution.
步骤1：向沙盐混合物中加入大量水并用玻璃棒充分搅拌。盐溶解，形成溶液。
Step 2: Filter the mixture using filter paper and a funnel. The sand is trapped as residue on the filter paper, while the salt solution passes through as filtrate.
步骤2：用滤纸和漏斗过滤混合物。沙作为残渣被截留在滤纸上，而盐溶液作为滤液通过。
Step 3: Transfer the filtrate to an evaporating dish and heat it gently over a water bath. Water evaporates, leaving behind white salt crystals.
步骤3：将滤液转移到蒸发皿中，在水浴上轻轻加热。水蒸发，留下白色盐晶体。
This entire process is a physical change; no new substances are formed. The salt particles were simply separated from water molecules by evaporation, and sand was removed by filtration. Both recovered substances can be reused.
整个过程是物理变化；没有新物质生成。盐粒子只是通过蒸发从水分子中分离出来，沙则通过过滤去除。两种回收的物质都可以再利用。
2. The Mystery of the Forged Letter – Paper Chromatography in Action | 伪造信件之谜——纸上色谱实战
A suspicious signed letter was found at a crime scene. Forensic scientists need to determine if the ink matches a suspect’s pen. Paper chromatography can separate the mixture of dyes in the ink to create a unique pattern, like a fingerprint.
在犯罪现场发现了一封可疑的签名信。法医科学家需要确定墨水是否与嫌疑人的笔匹配。纸上色谱可以分离墨水中的染料混合物，生成一个独特的图谱，就像指纹一样。
Using a small spot of the ink on chromatography paper, the edge is dipped into water. The water travels up by capillary action, carrying the dye components at different rates. Dyes that dissolve more easily travel further, while those that stick to the paper move less.
在色谱纸上点一小滴墨水，边缘浸入水中。水通过毛细作用上升，以不同速率携带染料成分。更容易溶解的染料移动得较远，而附着在纸上的染料移动较少。
After the water has risen, the paper is removed and dried. The separated coloured spots form a chromatogram. By comparing the pattern and colours with samples from a known pen, scientists can identify whether the ink matches. This method is a physical separation technique and relies on differences in solubility.
当水上升到一定高度后，取出纸并晾干。分离出的彩色斑点形成色谱图。通过将图谱和颜色与已知笔的样品进行比较，科学家可以鉴定墨水是否匹配。这种方法是一种物理分离技术，依赖于溶解度的差异。
In one case, the suspect’s pen produced a chromatogram with three spots – blue, red, and yellow – while the forged letter showed only blue and yellow. This proves the inks are different, and the letter is a forgery.
在一个案例中，嫌疑人笔的色谱图显示三个斑点——蓝色、红色和黄色——而伪造信件仅显示蓝色和黄色。这证明墨水不同，信件是伪造的。
3. Red Cabbage Detective – Making a pH Indicator | 红甘蓝侦探——制作pH指示剂
Red cabbage contains a natural pigment that changes colour when mixed with acids or bases. You can prepare your own pH indicator to test household substances and uncover their acidic or basic nature.
红甘蓝含有一种天然色素，当与酸或碱混合时会变色。你可以自制pH指示剂来测试家用物品，揭示它们的酸性或碱性。
To make the indicator: chop a few red cabbage leaves, place them in a beaker, and pour hot water over them. Leave to steep for 10 minutes. Filter the liquid to obtain a purple indicator solution.
制作指示剂：切碎几片红甘蓝叶，放入烧杯，倒入热水。浸泡10分钟。过滤液体得到紫色指示剂溶液。
Testing procedure: put a small amount of the substance to be tested in a dimple tray. Add a few drops of the cabbage indicator. If the colour turns pink or red, the substance is acidic (pH less than 7). If it turns blue or green, it is basic (pH greater than 7). Purple indicates neutral.
测试步骤：在点穴板上放少量待测物质。滴加几滴甘蓝指示剂。如果颜色变为粉红或红色，该物质是酸性的（pH小于7）。如果变为蓝色或绿色，则是碱性的（pH大于7）。紫色表示中性。
Case example: Testing lemon juice gave a vivid red, confirming its acidity. Baking soda solution turned the indicator bluish-green, showing it is a base. Tap water remained purple, indicating neutrality.
案例示例：测试柠檬汁得到鲜艳的红色，确认了其酸性。小苏打溶液使指示剂变为蓝绿色，表明它是碱。自来水保持紫色，表明中性。
The colour change happens because the pigment molecules change their structure when they gain or lose hydrogen ions (H⁺) from acids, or hydroxide ions (OH⁻) from bases. This simple home-made indicator is a valuable tool for spotting chemical properties.
颜色变化的发生是因为色素分子在从酸中获得或失去氢离子（H⁺），或从碱中获得或失去氢氧根离子（OH⁻）时会改变结构。这种简单的自制指示剂是发现化学性质的宝贵工具。
4. Why the Nail Rusted – Investigating Conditions for Rusting | 钉子为何生锈——探究生锈条件
A bicycle left outside in the rain soon develops reddish-brown rust on its iron parts. To investigate what causes rusting, you can design a controlled experiment using iron nails.
一辆停放在雨中的自行车，其铁制部件很快会出现红褐色的锈迹。为了探究生锈的原因，你可以设计一个使用铁钉的对照实验。
Place three clean iron nails in three separate test tubes. Tube A: add a pinch of calcium chloride (a drying agent) and seal with a bung – this provides dry air only. Tube B: add boiled water and a layer of oil to prevent air from dissolving – this gives water but no air. Tube C: add a little water and leave the tube open – this provides both water and air.
将三根洁净的铁钉分别放入三支试管中。试管A：加入一撮氯化钙（干燥剂）并塞上塞子——仅提供干燥空气。试管B：加入煮沸过的水并加一层油以防止空气溶解——提供水但没有空气。试管C：加入少量水并敞口放置——同时提供水和空气。
After one week, observe the nails. The nail in Tube A remains shiny. The nail in Tube B also shows little change. However, the nail in Tube C is covered in flaky, reddish-brown rust. This demonstrates that both water and oxygen are required for iron to rust.
一周后，观察钉子。试管A中的钉子保持光亮。试管B中的钉子也几乎没有变化。然而，试管C中的钉子覆盖着片状、红褐色的铁锈。这证明铁生锈需要水和氧气。
Chemically, iron reacts slowly with oxygen and water to form hydrated iron(III) oxide, commonly called rust. The process is a chemical change and can be represented simply as:
化学上，铁与氧气和水缓慢反应生成水合氧化铁，即常见的铁锈。该过程是化学变化，可简单表示为：
Iron + Oxygen + Water → Rust (Hydrated Iron Oxide)
铁 + 氧气 + 水 → 铁锈（水合氧化铁）
To prevent rusting, iron objects are often painted, oiled, or coated with a layer of zinc (galvanising), which stops air and water from reaching the surface.
为防止生锈，铁制品通常被涂漆、上油或镀锌层（镀锌），以阻止空气和水接触到表面。
5. Baking a Cake – Physical and Chemical Changes in the Kitchen | 烘烤蛋糕——厨房里的物理和化学变化
Baking a cake seems simple, but it involves a complex mix of physical and chemical changes. By identifying these, you can understand how ingredients transform into a delicious spongy treat.
烘烤蛋糕看似简单，但它涉及物理变化和化学变化的复杂混合。通过识别
Published by TutorHao | Year 7 Chemistry Revision Series | aleveler.com
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