Case Study: Identifying Unknown Household Powders | 案例分析：鉴别未知家用粉末

📚 Case Study: Identifying Unknown Household Powders | 案例分析：鉴别未知家用粉末

In this case study, you will act as a junior forensic chemist to identify three unlabelled white powders commonly found in a home. Using your knowledge of chemical properties and practical investigation skills, you will design tests, record observations, and draw evidence-based conclusions.

在这个案例研究中，你将扮演一名初级法证化学家，鉴定家中常见的三种未标记白色粉末。运用你的化学性质知识和实践探究技能，设计测试、记录观察结果并得出基于证据的结论。

1. The Case Scenario | 案例情景

Mrs MacDonald has a problem: three containers in her kitchen have lost their labels. The powders look very similar – they are all white, fine-grained solids. She remembers they are table salt, granulated sugar, and baking soda, but she cannot tell which is which. She asks you to help identify each one safely.

麦克唐纳太太遇到了一个难题：厨房里有三个容器失去了标签。这些粉末看起来非常相似——都是白色的细颗粒固体。她记得它们分别是食盐、白砂糖和小苏打，但分不清哪个是哪个。她请求你帮忙安全地鉴别每一种。

2. Safety First | 安全第一

Before beginning any investigation, you must consider safety. Always wear safety goggles and a lab coat. Work in a well-ventilated area and tie back long hair. Although the materials are household items, some tests involve heating and mild acids (vinegar), which can cause burns or irritation. Never taste any chemical, and wash your hands after the experiment.

在开始任何探究之前，必须考虑安全。始终佩戴护目镜和实验服。在通风良好的地方操作，并将长发束起。尽管这些是家用物品，但有些测试涉及加热和弱酸（醋），可能导致灼伤或刺激。绝对不要品尝任何化学品，实验后要洗手。

3. Collecting Initial Observations | 收集初步观察

First, look closely at the three powders. Label them A, B, and C. Write down their appearance: are they crystalline or powdery? Do they glisten? All three are white, but you may notice that salt and sugar look slightly translucent or granular, while baking soda is a finer, softer powder. Record these qualitative observations as they may give initial clues.

首先，仔细观察这三种粉末。将它们标记为 A、B 和 C。记录它们的外观：是晶体状还是粉末状？它们是否闪亮？三者都是白色的，但你会发现盐和糖看起来略微半透明或呈颗粒状，而小苏打是更细、更软的粉末。记录这些定性观察，因为它们可能提供初步线索。

4. Planning the Investigation | 计划调查

You need a logical sequence of tests that will distinguish the three powders. Based on their known chemical and physical properties, you can plan the following steps: (1) test solubility in water; (2) test with an indicator to check acidity/alkalinity; (3) test with a weak acid (vinegar) to identify the carbonate; (4) test the remaining solids by heating. This plan uses simple apparatus and safe chemicals, and it minimises waste by only using small samples.

你需要一套逻辑有序的测试方法来区分这三种粉末。根据它们已知的化学和物理性质，你可以计划以下步骤：(1) 测试水溶性；(2) 用指示剂检验酸碱性；(3) 用弱酸（醋）测试以鉴别碳酸盐；(4) 对剩余固体进行加热测试。这个方案使用了简单仪器和安全化学品，且仅用少量样品以减少浪费。

5. Testing for Solubility | 溶解度测试

Place a small spatula-full of each powder into separate test tubes containing about 10 mL of water. Stir or shake gently. Observe what happens. All three powders – salt (sodium chloride, NaCl), sugar (sucrose, C₁₂H₂₂O₁₁), and baking soda (sodium hydrogen carbonate, NaHCO₃) – dissolve completely in water, forming colourless solutions. Therefore, this test alone cannot identify them; you must use further tests.

分别取一小刮勺的每种粉末放入含有约 10 毫升水的试管中。轻轻搅拌或振荡。观察现象。这三种粉末——盐（氯化钠，NaCl）、糖（蔗糖，C₁₂H₂₂O₁₁）和小苏打（碳酸氢钠，NaHCO₃）——都完全溶于水，形成无色溶液。因此，仅凭这个测试无法鉴别；你必须进行进一步测试。

6. Using Indicators | 使用指示剂

Prepare fresh solutions of A, B, and C. Dip a piece of red litmus paper into each solution. Baking soda dissolves to give a weakly alkaline solution because hydrogen carbonate ions (HCO₃⁻) can accept protons, turning red litmus paper blue. Salt and sugar solutions are neutral and produce no colour change. If you observe a powder turning red litmus blue, you have found the baking soda.

重新配制 A、B 和 C 的溶液。将一条红色石蕊试纸浸入每种溶液中。小苏打溶解后形成弱碱性溶液，因为碳酸氢根离子（HCO₃⁻）可以接受质子，使红色石蕊试纸变蓝。盐和糖的溶液是中性的，没有颜色变化。如果你观察到某种粉末使红色石蕊变蓝，那么它就是小苏打。

7. Testing with Acid (Vinegar) | 用酸（醋）测试

Take a small amount of each powder and add a few drops of vinegar (ethanoic acid, CH₃COOH). Watch for effervescence. Baking soda reacts vigorously with acids, producing carbon dioxide gas according to the equation:

取少量每种粉末，滴加几滴醋（乙酸，CH₃COOH）。观察是否有气泡冒出。小苏打与酸剧烈反应，产生二氧化碳气体，反应方程式如下：

NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂

Salt and sugar do not react with vinegar; no bubbles are seen. This test gives a clear positive identification for baking soda. If effervescence is observed, the powder is sodium hydrogen carbonate

Published by TutorHao | Year 7 Chemistry Revision Series | aleveler.com

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