📚 Year 7 AQA Chemistry: Case Study Practice | 七年级 AQA 化学：案例分析实战演练
Case studies are a fantastic way to see how the chemistry you learn in the classroom applies to real experiments and everyday situations. Each case study presents a problem or observation, and by applying your knowledge of particles, separation techniques, chemical reactions, acids and alkalis, you can figure out what is happening and why. This article will guide you through ten practical scenarios, helping you build confidence in explaining results and choosing the right scientific method. Remember to always think about safety, fair testing, and the underlying particle model when you work through each case.
案例分析是让你看到课堂上学到的化学知识如何应用到真实实验和日常情境中的绝佳方式。每个案例都提出一个问题或观察到现象，通过运用你对粒子、分离技术、化学反应、酸和碱的理解，你就能弄清楚发生了什么及其原因。本文将通过十个实际情景，帮助你建立解释结果和选择正确科学方法的信心。处理每个案例时，请始终牢记安全、公平测试和基础的粒子模型。
1. Separating Sand and Salt | 分离沙子和盐
During a beach clean-up, a student collected a mixture of dry sand and table salt. The student wants to obtain pure, dry salt from the mixture and return clean sand to the beach. The mixture is placed in a beaker, and the student adds water and stirs. A cloudy liquid forms with sand settling at the bottom. The student then pours the liquid through filter paper into an evaporating basin. The sand stays on the filter paper, and a clear solution passes through. The basin is heated gently until crystals appear.
在一次海滩清理活动中，一位学生收集了干沙和食盐的混合物。该学生想从混合物中得到纯净干燥的盐，并将干净的沙子归还海滩。将混合物放入烧杯中，学生加水搅拌。形成了浑浊的液体，沙子沉在底部。然后学生将液体通过滤纸倒入蒸发皿。沙子留在滤纸上，清澈的溶液滤了过去。将蒸发皿缓缓加热，直至出现晶体。
This case uses two separation methods: filtration and evaporation. Salt dissolves in water because the water particles pull apart the salt particles, forming a solution. Sand does not dissolve; its particles are too large and stay as solid grains. Filtration separates the insoluble sand from the salt solution. The filter paper has tiny holes that let water and dissolved salt through but block sand particles. Heating the salt solution evaporates the water, leaving solid salt crystals behind. To get dry sand, you can wash the residue on the filter paper with a little water and then let it dry in the air.
这个案例使用了两种分离方法：过滤和蒸发。盐溶于水是因为水粒子拉开了盐粒子，形成了溶液。而沙不溶于水，沙的粒子太大，仍以固态颗粒形式存在。过滤能将不溶的沙子和盐溶液分离开。滤纸上有细小的孔洞，让水和溶解的盐通过，但阻挡沙粒。加热盐溶液会使水蒸发，留下固态盐晶体。要获得干燥的沙子，可以用少量水冲洗滤纸上的残留物，然后于空气中晾干。
2. Identifying Pure Water | 鉴别纯水
A teacher gives three colourless liquids in unlabelled bottles: pure water, salty water, and sugar water. Students are asked to find out which bottle contains pure water without tasting them. One group decides to place a few drops of each liquid onto separate watch glasses and heat them gently over a water bath until all liquid disappears. Another group suggests measuring the boiling point of each liquid using a thermometer and a heating apparatus.
老师提供了三个未贴标签的瓶子，装有三种无色液体：纯水、盐水和糖水。要求学生在不品尝的情况下找出哪一瓶是纯水。一组学生决定将每种液体分别取几滴放在不同的表面皿上，在水浴上缓缓加热直至所有液体消失。另一组建议使用温度计和加热装置测量每种液体的沸点。
Pure water has a fixed boiling point of 100 °C at normal atmospheric pressure. Dissolved substances like salt or sugar raise the boiling point, so the salty water and sugar water will boil at temperatures slightly above 100 °C. By heating each liquid and recording the temperature when it boils steadily, the one that boils at 100 °C is pure water. The evaporation method also works: pure water will leave no solid residue, whereas the other two liquids will leave white crystals (salt or sugar). However, sugar tends to char and go brown when overheated, giving an additional clue. Both approaches rely on the fact that pure substances have specific, sharp melting and boiling points, while mixtures melt or boil over a range of temperatures.
在标准大气压下，纯水的沸点为 100 °C。溶解的物质如盐或糖会升高沸点，因此盐水和糖水会在略高于 100 °C 的温度沸腾。通过加热每种液体并记录稳定沸腾时的温度，在 100 °C 沸腾的就是纯水。蒸发的方法同样可行：纯水不会留下任何固体残留物，而另外两种液体会留下白色晶体（盐或糖）。然而，糖在过热时会焦化变褐，这提供了额外的线索。两种方法都利用了纯物质具有固定、尖锐的熔点和沸点，而混合物则在一个温度区间内熔化或沸腾这一事实。
3. Testing for Carbon Dioxide | 检验二氧化碳
A student adds hydrochloric acid to some marble chips (calcium carbonate) in a test tube. Bubbles of a colourless gas are produced rapidly. The student holds a burning splint near the mouth of the tube, but the flame goes out. The student then uses a delivery tube to bubble the gas into a test tube containing limewater (calcium hydroxide solution). The limewater turns cloudy, or milky.
一名学生向盛有大理石碎块（碳酸钙）的试管中加入盐酸。试管中迅速产生无色气泡。学生将燃着的木条靠近管口，火焰熄灭了。然后学生用导管将气体通入装有石灰水（氢氧化钙溶液）的试管中。石灰水变浑浊，或者呈乳白色。
g tiny solid particles of calcium carbonate that are insoluble, making the solution appear milky. The equation is: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). The fact that the burning splint is extinguished is also evidence, because CO₂ does not support combustion. However, other gases such as nitrogen also put out a flame, so the limewater test is more specific. In this case study, the reaction between hydrochloric acid and calcium carbonate (marble) produces calcium chloride, water and carbon dioxide: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Identifying CO₂ helps students recognise the products of a carbonate–acid reaction.
石灰水变浑浊是检验二氧化碳的经典方法。当二氧化碳（CO₂）鼓入石灰水时，会发生反应，生成不溶于水的细小碳酸钙固体颗粒，使溶液呈乳白色。方程式为：Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)。燃着的木条熄灭也是证据，因为 CO₂ 不支持燃烧。然而，氮气等其他气体也能使火焰熄灭，所以石灰水检验法更具专一性。在此案例中，盐酸与碳酸钙（大理石）的反应生成氯化钙、水和二氧化碳：CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)。识别 CO₂ 帮助学生认识碳酸盐与酸反应的产物。
4. Investigating Hard Water | 探究硬水
A family moves to a new house and notices that soap does not lather easily in the bathroom. A scum forms instead, and a white scale builds up inside the kettle. The family sends a water sample to a laboratory. The scientist tests the water by shaking it with soap solution in a measuring cylinder, counting how many drops of soap are needed to produce a lather that lasts for 30 seconds. The test is repeated with the same water after boiling and filtering it.
一家人搬进新居，发现浴室里的肥皂不容易起泡，反而生成浮渣，水壶内部也结上了一层白垢。这家人将水样送至实验室。科学家在一个量筒中用肥皂溶液与水样混合振荡，记录需要多少滴肥皂才能产生持续 30 秒的泡沫。将同一份水样煮沸并过滤后，再次重复该测试。
Hard water contains dissolved calcium and magnesium ions, usually from rocks such as limestone and chalk. These ions react with soap to form an insoluble scum before any lather can be produced, so more soap is needed. Inside a kettle, heating causes temporary hardness to break down, forming solid calcium carbonate (limescale). In the laboratory, the boiling and filtering test helps decide the type of hardness. If the boiled water needs fewer drops of soap, the original water contained temporary hardness (removable by boiling). If it still needs many drops, permanent hardness is present. This case study applies the knowledge of solubility, chemical reactions and the practical technique of comparative testing to solve a real household problem.
硬水含有溶解的钙离子和镁离子，通常来自石灰岩和白垩等岩石。这些离子会与肥皂反应生成不溶性浮渣，阻碍泡沫的形成，因此需要更多肥皂。在水壶内部，加热会导致暂时硬度分解，生成固态碳酸钙（水垢）。在实验室中，煮沸并过滤的测试有助于判断硬度的类型。如果煮沸后的水需要的肥皂滴数减少，则原水含有暂时硬度（可通过煮沸去除）。如果仍需很多滴肥皂，则存在永久硬度。该案例运用了溶解度、化学反应的知识以及比较测试的实践技术，来解决真正的家庭问题。
5. Chromatography of Ink | 墨水的色谱分析
A teacher finds a note written in black ink, but the pen used is unknown. Suspecting the note may have been written with a specific brand of pen, the teacher cuts out a small piece of the ink mark and uses paper chromatography. A spot of the ink is placed on a pencil line near the bottom of chromatography paper. The paper is suspended in a beaker with a small amount of water, so the water level is below the spot. Over ten minutes, the water travels up the paper and separates the ink into several coloured bands: blue, yellow and a tiny red band.
一位老师发现一张用黑色墨水写的便条，但不知道用的是哪种笔。由于怀疑这张便条可能是用某个特定品牌的笔写的，老师剪下了一小块墨水痕迹，并使用纸色谱法进行分析。将墨水点样在色谱纸靠近底部的铅笔线上。把纸悬挂在一个盛有少量水的烧杯中，水面低于样点。十分钟后，水沿着纸上行，将墨水分离成几条色带：蓝色、黄色和一条微小的红色带。
Chromatography separates the different pigments in a mixture. The water (the solvent) dissolves each pigment and carries them up the paper. Different pigments have different solubilities and stick to the paper to different extents, so they travel at different speeds. The more soluble a pigment, the further it moves. The resulting pattern of coloured spots is called a chromatogram. By comparing the colours and their heights with those from known pens tested under the same conditions, we can identify the ink. In this case, the black ink is a mixture of at least three dyes. The pencil line is drawn in pencil because pencil lead (graphite) does not dissolve in water, so it stays in place. This technique is widely used in forensic science.
色谱法可将混合物中的不同色素分离。水（溶剂）溶解每种色素并带着它们在纸上移动。不同色素具有不同的溶解度，与纸张的吸附程度也不同，因此它们以不同的速度向上移动。溶解度越大的色素移动得越远。形成的色斑图案称为色谱图。通过将颜色和高度与在相同条件下测试的已知笔样进行比较，我们可以鉴定墨水。在这个案例中，黑色墨水是由至少三种染料组成的混合物。铅笔线之所以用铅笔绘制，是因为铅笔芯（石墨）不溶于水，因此会停留在原位。该技术被广泛应用于法医学。
6. Acid or Alkali? | 酸性还是碱性？
A gardener finds that the soil in one part of the garden is not growing plants well. She tests the soil by mixing a sample with distilled water, letting it settle, and then dipping red and blue litmus paper into the liquid. Blue litmus paper turns red, and red litmus paper stays red. She also adds a few drops of universal indicator to the mixture and observes a colour change to orange.
一位园丁发现花园中某片区域的土壤植物生长不佳。她取了一份土壤样品与蒸馏水混合，静置后，将红色和蓝色石蕊试纸浸入液体中。蓝色石蕊试纸变红，红色石蕊试纸保持红色。她又往混合物中加了几滴通用指示剂，观察到颜色变为橙色。
Litmus paper shows whether a solution is acidic or alkaline. Acid turns blue litmus red, while alkali turns red litmus blue. In this test, blue litmus turned red, confirming the soil water is acidic. The red litmus staying red rules out an alkali. Universal indicator gives a more precise pH value; orange corresponds roughly to pH 3–4, which is quite acidic. Most plants grow best in slightly acidic to neutral soil (pH 6–7). The gardener could add lime (calcium oxide or calcium hydroxide) to neutralise the acidity. The neutralisation reaction between an acid and an alkali produces a salt and water. By understanding indicators and pH, we can monitor and adjust conditions in gardening, farming and even in our own bodies.
石蕊试纸可以显示溶液是酸性还是碱性。酸使蓝色石蕊试纸变红，碱使红色石蕊试纸变蓝。本测试中蓝色石蕊试纸变红，证实土壤水呈酸性。红色石蕊试纸不变红，排除了碱的可能。通用指示剂给出更精确的 pH 值；橙色大致对应 pH 3–4，酸性相当强。大多数植物在微酸性至中性土壤（pH 6–7）中生长最佳。园丁可以添加石灰（氧化钙或氢氧化钙）来中和酸性。酸与碱之间的中和反应生成盐和水。通过了解指示剂和 pH，我们可以监测并调整园艺、农业甚至我们自身体内的条件。
7. Metal and Acid Reaction | 金属与酸的反应
In a science lesson, students add small pieces of magnesium ribbon to dilute hydrochloric acid in a test tube. The mixture fizzes vigorously, and the test tube becomes warm. A student holds another test tube upside down over the mouth of the reacting tube to collect some of the gas produced. When a lit splint is applied to the collected gas, a squeaky pop sound is heard. The solution left behind is clear and colourless.
在一堂科学课上，学生们将小段镁条加入盛有稀盐酸的试管中。混合物剧烈冒泡，试管变热。一名学生将另一支试管倒置在反应试管的管口，收集一些生成的气体。当用点燃的木条接触收集到的气体时，听到了尖锐的爆鸣声。留下的溶液清澈无色。
The 'squeaky pop' agnesium atoms displace hydrogen ions in the acid, forming magnesium chloride (which stays dissolved) and hydrogen gas bubbles. The test tube feeling warm shows the reaction is exothermic – he rate of bubbling depends on factors like acid concentration, temperature and surface area of the metal. This case helps students link observations to chemical equations and recognise practical techniques for gas collection and testing.
“尖锐爆鸣声”是氢气的检验方法。镁与盐酸发生置换反应：Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)。镁原子置换出酸中的氢离子，生成氯化镁（留在溶液中）和氢气气泡。试管温热说明该反应是放热的——它释放了热能。氢气密度比空气小，所以可以通过向下排空气法或用倒置的试管在反应上方收集。气泡产生的速率取决于酸的浓度、温度和金属的表面积等因素。该案例帮助学生将观察到的现象与化学方程式联系起来，并掌握气体收集和检验的实践操作。
8. Conservation of Mass in Burning | 燃烧中的质量守恒
A student carefully weighs a piece of magnesium ribbon in a crucible with a lid. The crucible is heated strongly with the lid slightly open. The magnesium burns with a brilliant white light. After cooling, the student weighs the crucible, lid and the white ash inside. The mass appears to have increased. The student repeats the experiment but this time keeps the lid tightly closed after adding air through a small hole. The mass change is slightly different. Another student burns a candle inside a large sealed glass jar placed on a balance. They observe the reading on the balance throughout the burning and after it extinguishes.
一名学生仔细称量了坩埚和盖子中一段镁条的质量。强烈加热坩埚，盖子略微打开。镁条燃烧发出耀眼的白光。冷却后，学生称量了坩埚、盖子和里面的白色灰烬。质量似乎增加了。该学生重复实验，但这次在通过小孔加入空气后紧紧盖上盖子。质量变化略有不同。另一名学生在一个放在天平上的大型密封玻璃罐内燃烧蜡烛。他们观察了天平读数在燃烧过程中及熄灭后的变化。
g) → 2MgO(s). The oxygen atoms add to the mass, so the product is heavier than the original metal. When the crucible is kept closed, the total mass of the container and contents stays the same (if no gas escapes) demonstrating the law of conservation of mass. However, if gas escapes (for example, when opening the lid while hot), the mass might change slightly. For the candle in a sealed jar, the total mass remains constant, even though the candle wax and oxygen react to form carbon dioxide and water vapour, both of which stay trapped. In an open system, mass may seem to change, but that is because gases enter or leave. Students learn that in a chemical reaction, atoms are not created or destroyed, only rearranged.
镁在空气中燃烧质量增加，是因为镁与空气中的氧气结合生成了氧化镁：2Mg(s) + O₂(g) → 2MgO(s)。氧原子增加了质量，所以生成物比原来的金属重。当坩埚保持密闭时，容器及内部物质的总质量保持不变（如果没有气体逸出），这证明了质量守恒定律。然而，如果气体逸出（例如在炽热时打开盖子），质量可能会略有变化。对于密封罐中的蜡烛，总质量保持不变，尽管蜡烛蜡和氧气反应生成了二氧化碳和水蒸气，但都被封闭在罐内。在开放体系中，质量看似变化了，其实是因为有气体进入或离开。学生们从中了解到，在化学反应中，原子既不能被创造也不能被消灭，只能重新排列。
9. Making a Soluble Salt | 制备可溶性盐
A student wants to make pure copper sulfate crystals. She starts by warming dilute sulfuric acid in a beaker. She adds black copper oxide powder a little at a time, stirring until no more dissolves and some powder remains at the bottom. The mixture is filtered to remove the excess copper oxide. The blue filtrate is transferred to an evaporating basin and heated gently until crystals begin to form at the edge. The basin is then left to cool slowly. Large, well-shaped blue crystals appear the next day.
一名学生想制备纯硫酸铜晶体。她首先加热烧杯中稀硫酸。然后少量多次地加入黑色氧化铜粉末，搅拌直至不再溶解，且烧杯底部留有一些粉末。将混合物过滤以去除过量的氧化铜。把蓝色滤液转移到蒸发皿中，缓缓加热，直到边缘开始有晶体析出。然后把蒸发皿静置缓慢冷却。第二天出现了大而形状完好的蓝色晶体。
aq) + H₂O(l). The copper oxide neutralises the acid. By adding excess copper oxide, we ensure all the acid has reacted, leaving only neutral solution and unreacted solid. Filtration removes the excess solid, giving pure copper sulfate solution. Heating evaporates some water to concentrate the solution. Slow cooling allows large crystals to form because particles have time to arrange themselves in a regular pattern. Quick cooling would produce many tiny crystals. The crystals can be dried by pressing between filter papers. Understanding this process reinforces concepts of neutralisation, solubility, filtration and crystallisation.
这是由不溶性碱（或金属氧化物）和酸制备可溶性盐的经典方法。反应为：CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)。氧化铜中和了酸。通过加入过量氧化铜，确保所有酸都反应完全，只留下中性溶液和未反应的固体。过滤去除过量固体，得到纯净的硫酸铜溶液。加热蒸发部分水分以浓缩溶液。缓慢冷却可使晶体长得很大，因为粒子有足够时间排列成规则的结构。快速冷却则会产生许多微小的晶体。晶体可夹在滤纸间压干。理解该过程可巩固中和、溶解度、过滤和结晶的概念。
10. The Water Cycle and Distillation | 水循环与蒸馏
A group of students is tasked with obtaining fresh water from salty seawater using simple laboratory equipment. They set up a distillation apparatus: a round-bottom flask containing seawater is heated, and the vapour passes through a condenser with cold water running through the outer jacket. The condensed liquid is collected in a beaker. The liquid tastes fresh, and no salt residue is present. The students also draw a diagram of the water cycle, linking evaporation, condensation and precipitation to their experiment.
一组学生利用简单的实验室设备从咸海水中获取淡水。他们搭建了一套蒸馏装置：将盛有海水的圆底烧瓶加热，蒸汽通过冷凝管，管的外夹套中有冷水流过。冷凝后的液体收集在烧杯中。这液体尝起来是淡的，且无盐类残留。学生们还绘制了水循环的示意图，将蒸发、冷凝和降水与他们的实验联系起来。
Simple distillation separates a solvent (water) from a solution by boiling and condensing. When seawater is heated, water particles gain enough energy to escape as vapour, while the dissolved salt particles are left behind because they are not volatile. The water vapour then cools in the condenser, turning back into liquid water. This pure water is called distillate. The process mimics the natural water cycle: energy from the sun causes evaporation from oceans, water vapour rises and cools to form clouds (condensation), and then returns as rain (precipitation). However, the laboratory method requires careful temperature control to avoid boiling over and to ensure only water evaporates. This case study connects a practical separation technique to a global environmental cycle, reinforcing the idea that matter is constantly recycled on Earth.
简单蒸馏通过煮沸和冷凝的方法从溶液中分离出溶剂（水）。海水被加热时，水粒子获得足够能量以蒸汽形式逸出，而溶解的盐粒子因不挥发而留在瓶内。然后水蒸汽在冷凝管中冷却，变回液态水。这种纯水称为馏出液。该过程模拟了自然界的水循环：太阳的能量引起海洋蒸发，水蒸气上升、冷却形成云（冷凝），然后以雨的形式返回地面（降水）。不过，实验室方法需要小心控制温度，以免暴沸，并确保只有水被蒸出。本案例将一项实用的分离技术同全球环境循环联系起来，强化了物质在地球上不断循环的观念。
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