📚 Year 7 AQA Chemistry: Case Study Practice Exercises | Year 7 AQA 化学：案例分析实战演练
Case studies bring chemistry to life by showing how scientific ideas solve real‑world problems. In Year 7, you explore particles, mixtures, chemical reactions, acids and alkalis, and the Earth’s resources. This collection of practice exercises encourages you to apply your knowledge to everyday situations – from purifying water to understanding why metal objects rust. Read each scenario carefully, think about the key chemical concepts, and then work through the analysis. Model answers are provided in both English and Chinese to help you check your understanding and build confidence for assessments.
案例分析将化学知识带入现实生活，展示科学知识如何解决实际问题。在七年级，大家会学习粒子模型、混合物、化学反应、酸与碱以及地球资源等内容。这套实战演练旨在引导你将所学知识应用到日常情境中去——从净化饮用水到理解金属为何生锈。请仔细阅读每个场景，思考涉及的关键化学概念，然后进行分析。每个案例都配有中英对照的参考答案，帮助你检验理解程度，并提升应对考试的信心。
1. Purifying Muddy Water | 净化泥水
After heavy rain, a hiking group collects water from a stream. The water is brown, contains bits of leaves and a few grains of sand. They have a plastic bottle, some clean sand, gravel, a piece of cloth, and charcoal. The group needs to produce clear, drinkable water.
大雨过后，一群徒步者从小溪中取水。水体呈棕色，含有树叶碎片和一些沙粒。他们有一个塑料瓶、一些干净的沙子、砾石、一块布和木炭。这群人需要制取清澈、可饮用的水。
Which separation techniques should they use, and in what order? Explain the role of each layer of the filter.
他们应该使用哪些分离技术？顺序如何？解释过滤器中每一层的作用。
- Filtration is the main technique, followed by the use of charcoal to improve taste. The cloth first removes large debris such as leaves and twigs. A layer of gravel traps larger solid particles; sand removes finer suspended solids; charcoal adsorbs some dissolved impurities and improves the taste and odour. The clear liquid collected after filtration is the filtrate, and the solids retained are the residue. 过滤是主要分离技术，之后用木炭改善口感。 布首先去除树叶和树枝等大块杂物。砾石层截留较大的固体颗粒；沙子去除更细的悬浮物；木炭吸附部分溶解性杂质并改善味道和气味。过滤后收集的清澈液体是滤液，被截留的固体是滤渣。
- Filtration cannot remove dissolved salt or make muddy water safe to drink. Harmful microorganisms can still be present. To obtain truly drinkable water, they would need to boil it or use disinfection methods. 过滤无法去除溶解的盐，也不能让泥水变得安全可饮。 有害微生物可能仍然存在。要获得真正可饮用的水，他们还需要煮沸或使用消毒方法。
2. The Rusty Gate | 生锈的大门
A farm gate made of iron has developed a flaky orange‑brown layer over the years. In winter, the gate is often covered with rainwater and mud. The farmer wonders why the rust appears mainly in areas where the paint has peeled off.
一扇铁制农场大门经过多年表面出现了片状的橙褐色物质。冬季，大门经常被雨水和泥浆覆盖。农民想知道为什么铁锈主要出现在油漆剥落的地方。
Explain the chemical change taking place and suggest two ways the farmer could prevent further rusting.
解释发生的化学变化，并给出两种防止进一步锈蚀的方法。
- Rusting is a chemical reaction between iron, oxygen and water. The orange‑brown solid, hydrated iron(III) oxide (Fe₂O₃·xH₂O), forms when iron atoms lose electrons and react with oxygen in the presence of water. Where paint has peeled, both air and moisture can reach the iron surface directly, speeding up rusting. 铁锈是铁、氧气和水之间发生的化学反应。 当铁原子失去电子并在有水存在的条件下与氧气反应，就生成橙褐色固体水合氧化铁（Fe₂O₃·xH₂O）。油漆剥落的地方，空气和水分能直接接触铁表面，加速锈蚀。
- Rusting can be prevented by keeping water or oxygen away from iron. The farmer could repaint the gate to create a barrier. Another method is to apply oil or grease to metal parts. Galvanising (coating with zinc) also works because zinc acts as a sacrificial metal. 防止锈蚀的方法是隔离水或氧气。 农民可以重新刷漆形成保护层。另一种方法是给金属部位上油或涂润滑脂。镀锌（表面覆盖锌层）也有效，因为锌可作为牺牲金属。
3. Baking Powder Reaction | 发酵粉的反应
A baker mixes baking powder into cake batter. Baking powder contains sodium hydrogencarbonate and a weak acid. When the batter is heated in the oven, bubbles appear throughout the cake, making it light and fluffy.
一位烘焙师将发酵粉混入蛋糕面糊中。发酵粉含有碳酸氢钠和一种弱酸。当面糊在烤箱中被加热时，整个蛋糕中出现大量气泡，使蛋糕变得松软。
What type of chemical reaction produces the bubbles? Write a word equation and explain why the reaction only occurs when the mixture is wet or heated.
产生气泡的是哪种化学反应？写一个文字方程，并解释为什么这个反应只在混合物湿润或加热时发生。
- A neutralisation reaction between an acid and a carbonate produces carbon dioxide gas. n: sodium hydrogencarbonate + acid → sodium salt + water + carbon dioxide. The gas bubbles become trapped in the batter, causing the cake to rise. 酸和碳酸盐之间的中和反应产生二氧化碳气体。 碳酸氢钠（一种温和的碱）与酸（通常是塔塔粉或磷酸）反应，释放CO₂、水和一种盐。文字方程：碳酸氢钠 + 酸 → 钠盐 + 水 + 二氧化碳。气体气泡被困在面糊中，使蛋糕膨胀。
- The reaction only takes place when the acid and sodium hydrogencarbonate are dissolved or heated. In the dry powder, the particles are mixed but cannot react. Water in the batter dissolves the reactants, allowing their particles to collide and react. Heating further increases the rate of reaction, producing more bubbles rapidly. 该反应仅在酸和碳酸氢钠溶解或加热时发生。 干粉状态下，粒子混合但无法反应。面糊中的水溶解了反应物，使其粒子能够碰撞并发生反应。加热进一步加快反应速率，迅速产生更多气泡。
4. Soil pH Testing | 土壤pH测试
A gardener notices that blueberry plants are growing poorly. The leaves have turned yellow, even though the soil is well watered and fertilised. A test shows the soil pH is 7.5. Blueberries need acidic soil, ideally pH 4.5–5.5.
一位园丁发现蓝莓植物生长不良，叶片变黄，尽管土壤水分和肥料充足。测试显示土壤pH为7.5。蓝莓需要酸性土壤，理想的pH范围是4.5–5.5。
Describe a safe method the gardener could use to measure soil pH at home, and suggest an amendment to make the soil more acidic.
描述园丁可在家庭中安全测量土壤pH的方法，并建议一种使土壤更酸的改良方案。
- Soil pH can be tested using universal indicator or a pH testing kit. Mix a small amount of soil with distilled water, let it settle, then dip a piece of universal indicator paper into the liquid. Compare the colour to a pH chart. A pH of 7.5 is slightly alkaline. 可以使用通用指示剂或pH测试套件来检测土壤pH。 取少量土壤与蒸馏水混合，静置后，将通用指示纸浸入液体中。将颜色与pH比色卡进行对比。pH 7.5为弱碱性。
- To lower the soil pH, an acidic substance can be added. The gardener could mix elemental sulfur (which oxidises to sulfuric acid by soil bacteria) or use sphagnum peat. Compost made from pine needles or coffee grounds can also gradually acidify the soil. This is an application of neutralisation: an acid is added to neutralise the alkaline components. 要降低土壤pH，可以添加酸性物质。 园丁可以混入元素硫（硫磺）（被土壤细菌氧化成硫酸）或使用泥炭苔。松针堆肥或咖啡渣也能缓慢酸化土壤。这是中和反应的应用：通过加酸来中和碱性物质。
5. Carbonated Drinks – The Fizz | 碳酸饮料——嘶嘶声
A bottle of sparkling water is sealed under pressure. When the cap is opened, bubbles rise rapidly. If the bottle is left open overnight, the water tastes flat. The label lists only two ingredients: water and carbon dioxide.
一瓶气泡水在加压下密封。打开瓶盖时，气泡迅速上涌。如果瓶子敞开放置一整夜，水尝起来不再有气泡感。标签上只列出了两种成分：水和二氧化碳。
Use the particle model to explain why bubbles form when the bottle is opened, and why the drink eventually goes flat.
运用粒子模型解释为什么开瓶时会产生气泡，以及为什么饮料最终会跑气。
- Carbon dioxide gas is dissolved in water under high pressure. When the bottle is sealed, many CO₂ particles are forced between the water particles. Opening the bottle releases the pressure, so more gas particles escape from the liquid than are being dissolved. The bubbles you see are carbon dioxide leaving the solution. 二氧化碳气体在高压下溶解于水中。 当瓶子密封时，许多CO₂粒子被压入水粒子之间。打开瓶盖释放了压力，于是从液体中逸出的气体粒子多于被溶解的。所看到的气泡就是二氧化碳从溶液中释放出来。
- Over time, the concentration of dissolved CO₂ decreases. Gas particles continue to escape until equilibrium is reached with the low partial pressure of CO₂ in the air. The drink goes flat because very little dissolved gas remains; the bubbles stop forming. This illustrates the dynamic nature of dissolving. 随着时间的推移，溶解的CO₂浓度降低。 气体粒子不断逸出，直到与空气中CO₂的低分压达成平衡。饮料跑气是因为溶解的气体所剩无几，气泡不再形成。这表明溶解过程的动态特性。
6. Swimming Pool Chlorination | 游泳池氯化
A public swimming pool maintains a safe environment by adding a small amount of chlorine. Chlorine kills bacteria but can also react with substances from swimmers to form by‑products. The pool operator tests the water daily to keep the pH between 7.2 and 7.6.
公共游泳池通过添加少量氯来维持安全环境。氯能杀灭细菌，但也可能与游泳者带来的物质反应生成副产物。泳池工作人员每天检测水质，将pH维持在7.2至7.6之间。
Suggest why controlling pH is important, and name one substance the operator might use if the pool water becomes too acidic.
解释为什么控制pH很重要，并说出如果池水酸性过强，工作人员可能会使用的一种物质。
- Chlorine is most effective as a disinfectant at a near‑neutral pH. If the water is too alkaline (pH > 8), chlorine reacts to form less effective chlorate ions. If it is too acidic, chlorine gas may be released, causing eye and skin irritation. Maintaining pH 7.2–7.6 ensures the chlorine remains in the active hypochlorous acid form, HOCl. 氯在接近中性的pH下杀菌效果最佳。 如果水碱性过强（pH > 8），氯会反应生成效果较差的氯酸根离子。如果酸性过强，可能释放氯气，刺激眼睛和皮肤。维持pH 7.2–7.6可确保氯主要以活性次氯酸（HOCl）形式存在。
- If the pool water becomes too acidic, an alkali should be added to raise the pH. Sodium carbonate (soda ash) or sodium hydrogencarbonate is commonly used. These substances neutralise excess acid. This is an important neutralisation reaction that keeps the water safe for swimmers. 如果池水酸性过强，应加入碱来提高pH。 常用碳酸钠（纯碱）或碳酸氢钠。这些物质能中和过量的酸。这一中和反应对保持泳池水对游泳者的安全性至关重要。
7. The Mystery White Powders | 神秘白色粉末
A kitchen cupboard contains three unlabelled white powders: table salt, sugar, and sodium hydrogencarbonate. A student adds a few drops of dilute hydrochloric acid to a sample of each powder, then tests with a glowing splint. She also tests the solubility of each in water.
厨房柜子里有三瓶没有标签的白色粉末：食盐、糖和碳酸氢钠。某学生向每种粉末的样品中滴加几滴稀盐酸，然后用带火星的木条测试。她还测试了每种粉末在水中的溶解度。
Describe how these tests can identify each substance. Which observation would confirm the presence of sodium hydrogencarbonate?
描述这些测试如何鉴别每种物质。哪种观察结果可以确认碳酸氢钠的存在？
- Adding dilute hydrochloric acid distinguishes sodium hydrogencarbonate from the other two. Sodium hydrogencarbonate (NaHCO₃) reacts with HCl to produce carbon dioxide gas with vigorous fizzing. A glowing splint placed near the mouth of the test tube will be extinguished, because CO₂ does not support combustion. Sugar and salt do not produce a gas with acid. 滴加稀盐酸可以区分碳酸氢钠与另外两种粉末。 碳酸氢钠与HCl反应产生二氧化碳气体，并剧烈冒泡。放在试管口附近的带火星木条会熄灭，因为CO₂不支持燃烧。糖和盐与酸不产生气体。
- Solubility tests can separate sugar and salt. Both sugar and salt dissolve in water, but salt (sodium chloride) produces a clear, colourless solution. By evaporating a few drops of the solution on a microscope slide, salt leaves cubic crystals, while sugar leaves more irregular crystals. Additionally, tasting a tiny pinch can help in a lab context (safety permitting). 溶解度测试可以区分糖和盐。 糖和盐都溶于水，但盐（氯化钠）形成无色透明溶液。通过在载玻片上蒸发几滴溶液，盐会留下立方晶体，而糖留下的晶体较不规则。另外，在实验室条件下尝极小的一撮也能帮助区分（在安全允许时）。
8. Fizzy Sherbet – Acid–Base Chemistry | 汽水粉——酸碱化学
Fizzy sherbet sweets contain citric acid, sodium hydrogencarbonate and sugar. When the powder touches moisture on the tongue, a tingling sensation occurs and bubbles form. A recipe book suggests storing sherbet away from humid air.
汽水粉糖果含有柠檬酸、碳酸氢钠和糖。当粉末接触到舌头上的水分时，会出现刺麻感并产生气泡。一份食谱建议将汽水粉储存在远离潮湿空气的地方。
Explain the chemical reaction that happens in the mouth. Why must the ingredients be kept dry before use?
解释在口中发生的化学反应。为什么原料在使用前必须保持干燥？
- An acid–base reaction between citric acid and sodium hydrogencarbonate releases carbon dioxide gas. te + water + carbon dioxide. The bubbles cause the tingling sensation on the tongue. This is the same type of reaction used in baking powder and effervescent tablets. 柠檬酸与碳酸氢钠之间的酸碱反应释放出二氧化碳气体。 文字方程：柠檬酸 + 碳酸氢钠 → 柠檬酸钠 + 水 + 二氧化碳。气泡导致舌头上的刺麻感。这与发酵粉和泡腾片中发生的反应属于同一类型。
- If moisture enters the mixture before it reaches the mouth, the reaction will occur prematurely. The citric acid and sodium hydrogencarbonate need water to dissolve and allow their particles to collide. Keeping the powder dry prevents the reaction from taking place in the packet, preserving the fizz for eating. 如果水分在入口前就进入混合物，反应会提前发生。 柠檬酸和碳酸氢钠需要水来溶解，使其粒子能够碰撞。保持粉末干燥可防止反应在包装内发生，从而将为食用保留嘶嘶的口感。
9. Separating Sand, Salt and Iron Filings | 分离沙子、食盐和铁屑
A student spills a mixture of sand, table salt and iron filings on the bench. The teacher asks her to separate the three substances using simple laboratory equipment and no dangerous chemicals.
一名学生把沙子、食盐和铁屑的混合物撒在了实验台上。老师要求她用简单的实验室器材且不使用危险化学品将三种物质分离。
Design a step‑by‑step method. Name each separation technique and the physical property it relies on.
设计一套分步方法。说出每一步分离技术的名称及其所依据的物理性质。
- Step 1: Use a magnet to remove the iron filings. Iron is magnetic, while sand and salt are not. A bar magnet wrapped in paper or inside a plastic bag can attract and lift the iron filings, leaving them easy to collect. Physical property: magnetism. 第一步：用磁铁去除铁屑。 铁具有磁性，而沙子和食盐没有。用纸包住或用塑料袋套着的条形磁铁可以吸引并吸起铁屑，方便收集。物理性质：磁性。
- Step 2: Add water to the remaining sand–salt mixture and stir. Salt dissolves in water; sand does not. This forms a salt solution with sand suspended. Physical property: solubility. 第二步：向剩下的沙-盐混合物中加水并搅拌。 食盐溶解于水，沙子不溶。这形成了食盐溶液和悬浮的沙子。物理性质：溶解度。
- Step 3: Filter the mixture. Pour the mixture through filter paper in a funnel. The sand is trapped as residue, and the salt solution passes through as filtrate. Physical property: particle size. 第三步：过滤混合物。 将混合物倒入有滤纸的漏斗中。沙子作为残渣被截留，食盐溶液作为滤液通过。物理性质：颗粒大小。
- Step 4: Evaporate the water from the salt solution. Heat the filtrate gently in an evaporating dish over a water bath or allow it to evaporate slowly at room temperature. Water turns into vapour, leaving solid salt crystals behind. Physical property: boiling point difference. 第四步：蒸发食盐溶液中的水。 在水浴上缓缓加热蒸发皿中的滤液，或在室温下让其缓慢蒸发。水变成水蒸气，留下食盐晶体。物理性质：沸点差异。
10. Global Warming and the Carbon Cycle | 全球变暖与碳循环
Year 7 students learn that carbon cycles between the atmosphere, oceans, living things and rocks. A newspaper article states that burning fossil fuels increases the amount of carbon dioxide in the air, which warms the planet. A student asks: 'If plants take in CO₂, why can't they just absorb all the extra we produce?'
七年级学生学到碳在大气、海洋、生物和岩石之间循环。一篇报纸文章说，燃烧化石燃料增加了空气中的二氧化碳含量，导致地球变暖。一名学生问道：“如果植物吸收CO₂，为什么它们不能把我们额外产生的所有CO₂都吸收掉？”
Using your knowledge of the carbon cycle, explain why forests alone cannot balance the extra CO₂ from fossil fuel emissions. Suggest two actions that can help reduce atmospheric CO₂.
运用你对碳循环的知识，解释为什么仅靠森林无法平衡燃烧化石燃料额外产生的CO₂。提出两种可以降低大气CO₂含量的行动。
- Plants absorb CO₂ through photosynthesis and store carbon in their biomass. However, the rate at which forests can remove CO₂ is limited by land area, growth rate, and deforestation. When trees die and decompose, much of the stored carbon returns to the air as CO₂ through respiration of decomposers. 植物通过光合作用吸收CO₂，并将碳储存在生物质中。 然而，森林移除CO₂的速率受限于土地面积、生长速度和森林砍伐。当树木死亡并分解时，储存的大部分碳通过分解者的呼吸作用以CO₂形式返回空气中。
- Burning fossil fuels releases carbon that has been locked underground for millions of years. This adds new CO₂ to the active carbon cycle much faster than natural sinks can absorb it. The imbalance leads to a net increase in atmospheric CO₂ concentration, enhancing the greenhouse effect. 燃烧化石燃料释放了原本在地下封存数百万年的碳。 这向活跃的碳循环中添加新的CO₂，其速度远超自然碳汇的吸收能力。这种不平衡导致大气CO₂浓度净增长，强化了温室效应。
- Actions to reduce atmospheric CO₂ include using less fossil fuels and protecting forests. Choosing renewable energy sources such as wind or solar reduces emissions. Reforestation and preventing deforestation help maintain the natural carbon sink. Individual lifestyle changes, such as walking or cycling instead of driving, also contribute. 降低大气CO₂的行动包括减少使用化石燃料和保护森林。 选择风能或太阳能等可再生能源可减少排放。植树造林和阻止森林砍伐有助于维持自然碳汇。个人的生活方式改变，例如以步行或骑车代替驾车，也能做出贡献。
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