📚 Year 8 SQA Chemistry: Case Study Practice | 八年级 SQA 化学：案例分析实战演练
For SQA Year 8 Chemistry, being able to apply your knowledge to real‑life situations is just as important as learning the facts. Case studies provide the perfect opportunity to practise that skill. By working through everyday chemical scenarios, you will learn how to identify patterns, use scientific vocabulary, and suggest evidence‑based explanations. The following collection of case studies covers key topics from the Year 8 SQA course, including reactions of acids, rusting, separation techniques, and gas tests. Each case is broken down into steps so you can strengthen your problem‑solving abilities and feel fully prepared for written assessments.
在 SQA 八年级化学中，将知识运用到真实情境里和学习知识点本身同样重要。案例分析正是锻炼这一能力的绝佳方式。通过日常生活中的化学场景，你会学会如何发现规律、使用科学术语并提出有依据的解释。以下案例分析集涵盖 SQA 八年级课程的核心主题，包括酸碱反应、生锈、分离方法和气体检验。每个案例都拆分成步骤，帮助你增强解决问题的能力，为笔头测评做好充分准备。
1. Introduction to the Case Study Approach | 案例分析方法介绍
In a Chemistry case study, you are usually given a short story or a set of observations. Your task is to use your scientific knowledge to explain what is happening and to solve a problem. A strong answer will include the relevant chemical names, word equations or balanced symbol equations, and will link the observations to the science. Before diving into the examples, remember the three‑step approach: READ carefully, LINK each clue to a chemistry concept, and WRITE your explanation using accurate terms.
在化学案例分析中，你通常会读到一个小故事或一组观察记录。你的任务是运用科学知识解释现象并解决问题。一份出色的答案会包括正确的化学名称、文字方程式或配平的符号方程式，并把观察现象与科学原理联系起来。在进入案例之前，请记住三步法：仔细阅读，将每条线索与一个化学概念联系起来，然后用准确的术语写出解释。
2. Case Study 1: The Rusting Gate | 案例一：生锈的大门
Aiden noticed that the iron gate at the front of his house was covered with a flaky orange‑brown substance. The gate is exposed to rain and air, but a section under the porch, which stays dry, shows very little of this substance. Aiden wants to know what the substance is and why only the exposed part is affected.
艾登发现他家前院的铁门上覆盖着一种疏松的橙褐色物质。铁门暴露在雨水和空气中，但门廊下方保持干燥的一段却几乎没有这种物质。艾登想知道这种物质是什么，以及为什么只有暴露的部分受到了影响。
The orange‑brown substance is rust, chemically known as hydrated iron(III) oxide. Iron requires both oxygen and water to rust. The exposed part of the gate has access to both, while the dry section under the porch lacks water, so the reaction cannot proceed. To prevent further rusting, Aiden could apply paint, oil or a sacrificial zinc coating (galvanising).
这种橙褐色物质是铁锈，化学名为水合氧化铁(III)。铁生锈需要氧气和水同时存在。暴露的栅栏部分同时接触到了这两者，而门廊下干燥的部分缺少水，因此反应无法进行。为防止进一步生锈，艾登可以涂漆、上油或使用牺牲性锌镀层（镀锌）。
4Fe + 3O₂ + 2H₂O → 2Fe₂O₃·H₂O (simplified rust formation)
3. Case Study 2: Identifying an Unknown Solution | 案例二：鉴别未知溶液
Leah tests a colourless solution with red litmus paper and observes no colour change. When she adds a few drops of universal indicator, the solution turns green‑blue. She then evaporates a sample and obtains a white crystalline solid that does not conduct electricity when dry but does conduct when dissolved in water. Identify the type of substance.
莉娅用红色石蕊试纸检测一种无色溶液，发现颜色没有变化。她加入几滴通用指示剂后，溶液变为绿蓝色。接着她蒸发了一份样品，得到一种白色晶体固体，干燥时不导电，但溶于水后能导电。请判断该物质的类型。
The solution is neutral to slightly alkaline, as red litmus does not turn blue (so not strongly alkaline) and universal indicator gives a green‑blue shade corresponding to pH 7–8. The solid conducts electricity only when dissolved, which is typical of ionic compounds. The white crystalline appearance and solubility suggest it is a soluble salt, possibly sodium chloride. Ionic compounds conduct electricity when molten or in solution because their ions become free to move.
该溶液呈中性到弱碱性，因为红色石蕊没有变蓝（所以不是强碱），而通用指示剂呈现绿蓝色，对应 pH 7–8。固体仅在溶解时导电，这是离子化合物的典型特征。白色晶体外观和溶解度表明它是一种可溶性盐，可能是氯化钠。离子化合物在熔融或溶于水时能导电，因为其中的离子可以自由移动。
4. Case Study 3: The Fizzing Tablet | 案例三：泡腾片的反应
Kai drops an effervescent vitamin tablet into a glass of water. Bubbles of gas are produced rapidly. He collects the gas and finds that it turns limewater milky. He also notices the temperature of the water drops slightly. Explain these observations using chemical ideas.
凯把一片泡腾维生素片投入一杯水中，迅速产生气泡。他收集了这种气体，发现能使石灰水变浑浊。他还注意到水温略微下降。请用化学知识解释这些现象。
The tablet contains an acid, typically citric acid, and a carbonate or hydrogencarbonate, such as sodium hydrogencarbonate. When water is added, they dissolve and react, producing carbon dioxide gas: limewater turns milky because calcium carbonate is formed. The temperature drop indicates an endothermic reaction, where heat energy is absorbed from the surroundings.
该片剂含有酸（通常是柠檬酸）和碳酸盐或碳酸氢盐（例如碳酸氢钠）。加水后，它们溶解并发生反应，产生二氧化碳气体：石灰水变浑浊是因为生成了碳酸钙。温度下降表明这是一个吸热反应，反应从周围环境中吸收了热量。
C₆H₈O₇ + 3NaHCO₃ → Na₃C₆H₅O₇ + 3H₂O + 3CO₂
5. Case Study 4: Acid Rain and Marble Statues | 案例四：酸雨与大理石雕像
A historic marble statue in the city centre has been losing its fine details over the last fifty years. Marble is mainly calcium carbonate. Scientists have measured a decrease in the pH of local rainwater from 5.6 to 4.2 due to air pollution. Explain how acid rain damages the statue.
市中心一座历史悠久的的大理石雕像在近五十年里精细的纹饰逐渐模糊。大理石的主要成分是碳酸钙。科学家测量到由于空气污染，当地雨水的 pH 值从 5.6 降到了 4.2。请解释酸雨是如何破坏雕像的。
ric acid (from nitrogen oxides). The marble (calcium carbonate) reacts with these acids to form soluble products that are washed away. For sulfuric acid, a solid– fate + water + carbon dioxide. Because the calcium sulfate layer is slightly soluble and flaky, the surface continually erodes.
酸雨中含有硫酸（来自二氧化硫排放）和硝酸（来自氮氧化物）。大理石（碳酸钙）与这些酸反应，生成的可溶性产物被雨水冲走。对硫酸而言，发生了一种类似固体‑气体反应的类型：碳酸钙 + 硫酸 → 硫酸钙 + 水 + 二氧化碳。由于硫酸钙层微溶且疏松，表面会持续被侵蚀。
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
6. Case Study 5: Separating a Mixture | 案例五：混合物的分离
An accidental spill in the lab has created a mixture of iron filings, sand and salt. Design a step‑by‑step method to obtain a dry sample of each substance separately. Name the techniques you would use and explain the physical property that makes each separation possible.
实验室里一次意外洒落产生了铁屑、沙子和食盐的混合物。请设计一套逐步分离并得到每种物质干燥样品的方法。说出你将使用的技术名称，并解释使每种分离成为可能的物理性质。
First, use a magnet wrapped in paper to attract the iron filings — this exploits magnetism. Then add water to the remaining sand‑salt mixture and stir to dissolve the salt. Filter the mixture; the residue on the filter paper is sand, relying on its insolubility. Finally, evaporate the filtrate by gentle heating to obtain salt crystals, which works because salt is soluble but water evaporates leaving the solid behind.
首先，用纸包裹的磁铁吸出铁屑——这是利用铁能被磁化的性质。接着向剩余的沙子‑食盐混合物中加水并搅拌，使盐溶解。过滤混合物；滤纸上残留的固体是沙子，利用的是沙子的不溶性。最后，温和加热蒸发滤液即可得到食盐晶体，因为盐可溶而水蒸发后留下固体。
7. Case Study 6: Neutralising Stomach Acid | 案例六：中和胃酸
Josh sometimes suffers from indigestion caused by excess stomach acid (hydrochloric acid, HCl). He takes an antacid tablet containing calcium carbonate. Write the word and symbol equations for the neutralisation reaction and explain why this relieves his symptoms.
乔希有时因胃酸（盐酸，HCl）过多而引起消化不良。他服用了含碳酸钙的抗酸片。请写出该中和反应的字方程式和符号方程式，并解释为什么这能缓解他的症状。
The calcium carbonate neutralises the excess hydrochloric acid to form calcium chloride, water and carbon dioxide. The reaction reduces the concentration of acid in the stomach, easing discomfort. Burping is a side effect of the carbon dioxide released.
碳酸钙中和了过量的盐酸，生成氯化钙、水和二氧化碳。这个反应降低了胃酸浓度，从而缓解不适。打嗝则是释放出的二氧化碳带来的副作用。
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
8. Case Study 7: Choosing the Right Fire Extinguisher | 案例七：选择合适的灭火器
A warehouse stores flammable liquids such as petrol and alcohol. The safety officer must select a fire extinguisher. Water extinguishers are unsuitable because petrol floats on water. Carbon dioxide extinguishers, on the other hand, work by smothering the fire and cooling. Explain how CO₂ extinguishers stop a fire and why they are a better choice for this scenario.
一个仓库储存着汽油和酒精等易燃液体。安全员必须选择灭火器。水基灭火器不适用，因为汽油会浮在水面上。而二氧化碳灭火器通过窒息和降温来灭火。请解释 CO₂ 灭火器如何阻止燃烧，以及为什么它对这种情况是更好的选择。
For combustion, a fire needs fuel, heat and oxygen (the fire triangle). Carbon dioxide gas is denser than air, so it blankets the flammable liquid, cutting off the oxygen supply. Additionally, the expanding CO₂ cools the fuel. Water would not work because petrol is less dense and continues to burn on the surface, potentially spreading the fire.
燃烧需要燃料、热量和氧气（火三角）。二氧化碳气密度比空气大，能覆盖在易燃液体表面，隔绝氧气。同时，膨胀的 CO₂ 会使燃料降温。水不适用是因为汽油密度更小，会继续在水面燃烧，甚至使火势蔓延。
9. Case Study 8: Growing Copper Sulfate Crystals | 案例八：制取硫酸铜晶体
Amelia wants to prepare large, pure crystals of copper(II) sulfate from copper(II) oxide and sulfuric acid. She reacts excess black copper(II) oxide with warm dilute sulfuric acid, filters the mixture, and then leaves the blue filtrate in an evaporating dish at room temperature. After a few days, large blue crystals appear. Explain each step and the purpose of the technique used.
阿梅莉亚想用氧化铜和硫酸制取大块纯净的硫酸铜晶体。她将过量的黑色氧化铜与温热的稀硫酸反应，过滤混合物，然后把蓝色滤液放在蒸发皿中室温静置。几天后，出现了大块蓝色晶体。请解释每一步以及所用技术的目的。
The reaction produces soluble copper(II) sulfate and water. Excess copper(II) oxide ensures all the acid is used up, and the unreacted solid is removed by filtration. The filtrate is a saturated solution of copper(II) sulfate. Slow evaporation at room temperature allows large crystals to form because the particles have time to arrange into an ordered lattice. Rapid evaporation or heating would produce smaller, less regular crystals.
反应生成可溶性硫酸铜和水。过量的氧化铜确保所有酸被消耗完，未反应的固体通过过滤除去。滤液是硫酸铜的饱和溶液。室温下缓慢蒸发使得大块晶体能够形成，因为粒子有时间排列成有序的晶格。快速蒸发或加热则会产生较小且不规整的晶体。
CuO + H₂SO₄ → CuSO₄ + H₂O
10. Case Study 9: Preventing Food Spoilage | 案例九：防止食物变质
Supermarkets store some fruits under a modified atmosphere where the oxygen level is reduced and nitrogen is increased. Strawberries kept in air at 5 °C develop mould within three days, while those in the modified atmosphere last over a week. Use your knowledge of oxidation and microbial activity to explain this difference.
超市将一些水果储存在低氧高氮的改良环境下。在 5 °C 的空气中储存的草莓三天内就会发霉，而在改良气氛中则可以保存一周以上。请利用你对氧化和微生物活动的了解解释这一差异。
Mould and bacteria require oxygen for respiration. By lowering the oxygen concentration, the growth of spoilage microorganisms is slowed down. At the same time, many chemical spoilage reactions, such as the oxidation of vitamins and browning of fruit, are minimised. Nitrogen is inert and does not react with the food, acting as a filler gas. The low temperature further slows down all rate processes.
霉菌和细菌进行呼吸作用需要氧气。通过降低氧气浓度，腐败微生物的生长就被减缓了。同时，许多化学腐败反应，如维生素的氧化和水果褐变，也被抑制到最低程度。氮气是惰性气体，不与食物反应，起着填充气体的作用。低温还进一步减慢了所有反应速率。
11. Case Study 10: Testing Unknown Gases | 案例十：未知气体的检验
During a practical session, three unlabelled gas jars contain hydrogen, oxygen and carbon dioxide. Describe the tests you would carry out to positively identify each gas. Include the expected results and a safety note for the test involving hydrogen.
在一次实验课上，三个未贴标签的集气瓶中分别装有氢气、氧气和二氧化碳。请描述你将如何用实验来确认每种气体，包括预期的现象，并针对氢气的检验写一条安全注意事项。
To test for hydrogen, hold a lit splint at the mouth of the jar: a squeaky pop indicates hydrogen (safety note: point the jar away from people and flames, because hydrogen is explosive in air). Oxygen relights a glowing splint. Carbon dioxide turns limewater milky when bubbled through it. Always record the original appearance and the change observed to reach a clear conclusion.
检验氢气：将一根点燃的木条靠近瓶口，有“嘶嘶”爆鸣声则说明是氢气（安全提示：瓶口远离人员和明火，因为氢气在空气中具有爆炸性）。氧气能使带火星的木条复燃。将二氧化碳通入石灰水中，石灰水变浑浊。始终记录气体原有的外观和观察到的变化，以得出明确的结论。
12. Summary and Top Tips | 总结与成功诀窍
Case studies reward students who can join the dots between observation and theory. Always name the substances involved, use correct terminology (e.g. 'neutralisation' not 'cancelling out'), and, where possible, include a balanced symbol equation. Practise describing separation steps in a logical order and state the property being exploited. Finally, read the question twice — the clue to the answer is often hidden in the scenario details.
案例分析能奖励那些善于把观察和理论联系起来的学生。始终指出涉及的具体物质，使用正确的术语（例如“中和”而不是“抵消”），并且尽可能写出配平的符号方程式。练习按逻辑顺序描述分离步骤并说明所利用的性质。最后，把题目读两遍——答案的线索往往就隐藏在情景细节中。
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