📚 Case Study Practice for Year 7 OCR Chemistry | Year 7 OCR 化学案例分析实战演练
In Year 7 OCR Chemistry, case studies are a fantastic way to develop practical skills and connect theory to real-world situations. This article presents eight engaging case scenarios that test your understanding of mixtures, separation techniques, chemical reactions, acids and alkalis, and energy changes. Each case asks you to observe, predict, explain and evaluate — just like a scientist in the lab.
在 Year 7 OCR 化学中，案例分析是培养实践技能并将理论联系实际的好方法。本文提供了八个引人入胜的案例场景，考察你对混合物、分离技术、化学反应、酸碱和能量变化的理解。每一个案例都要求你观察、预测、解释并评估——就像在实验室里的科学家一样。
1. Case Study: Separating a Mixture of Sand, Salt and Iron Filings | 案例研究：分离沙子、盐和铁屑的混合物
A student spills a mixture of fine sand, table salt and iron filings on the bench. All three substances are mixed together evenly. The task is to recover all three materials separately, with the salt obtained as a dry solid.
一名学生将细沙、食盐和铁屑的混合物撒在了实验台上。三种物质均匀混合。任务是分别回收这三种材料，并将盐以干燥固体的形式得到。
First, pass a bar magnet wrapped in a plastic bag over the mixture. The iron filings will be attracted to the magnet, leaving sand and salt behind. Remove the magnet from the bag to release the iron filings into a separate container.
首先，用塑料袋包裹条形磁铁，在混合物上方移动。铁屑会被磁铁吸引，留下沙子和盐。从袋子中移开磁铁，使铁屑掉入单独的容器中。
Next, add the remaining sand–salt mixture to a beaker of warm water and stir well. Salt dissolves in water, forming a solution, while sand does not dissolve and stays as an insoluble solid.
接着，将剩余的沙子-盐混合物加入装有温水的烧杯中，充分搅拌。盐溶解于水，形成溶液，而沙子不溶解，保持为不溶性固体。
Pour the mixture through filter paper in a funnel. The sand is trapped as residue on the filter paper, and the salt solution (filtrate) passes through. Finally, heat the salt solution gently in an evaporating dish until all the water evaporates, leaving pure dry salt crystals.
将混合物倒入装有滤纸的漏斗中过滤。沙子作为滤渣留在滤纸上，盐溶液（滤液）通过。最后，在蒸发皿中缓慢加热盐溶液，直至所有水分蒸发，留下纯净干燥的盐晶体。
2. Case Study: Identifying Unknown White Solids — Sugar or Baking Soda? | 案例研究：鉴别未知白色固体——糖还是小苏打？
Two unlabelled jars each contain a white powder. One is sucrose (table sugar) and the other is sodium hydrogencarbonate (baking soda). You have access to dilute hydrochloric acid, a Bunsen burner, test tubes and limewater. How can you tell which is which?
两个未贴标签的罐子各装有一种白色粉末。一个是蔗糖（食糖），另一个是碳酸氢钠（小苏打）。你可以使用稀盐酸、本生灯、试管和石灰水。如何分辨哪个是哪个？
Add a few drops of dilute hydrochloric acid to separate samples of each powder. The baking soda will fizz immediately, producing colourless gas bubbles. Sugar will simply dissolve or show no visible reaction with the acid.
分别向两种粉末样品中滴加几滴稀盐酸。小苏打会立刻产生气泡，放出无色气体。食糖则只会溶解或与酸无可见反应。
Baking soda (NaHCO₃) reacts with acid to release carbon dioxide gas (CO₂). This can be confirmed by bubbling the gas through limewater, which turns milky. Sugar (C₁₂H₂₂O₁₁) does not react with acid in this way; however, if heated strongly, sugar melts, caramelises and eventually chars, giving a burnt smell, while baking soda decomposes but does not melt.
小苏打（NaHCO₃）与酸反应放出二氧化碳气体（CO₂）。将气体通入石灰水，石灰水变浑浊即可证实。食糖（C₁₂H₂₂O₁₁）不与酸发生此类反应；但若强热，食糖会熔化、焦糖化并最终碳化，产生焦糊味，而小苏打会分解但不熔化。
So, the acid test combined with limewater gives a clear chemical distinction. The baking soda sample will produce CO₂ and give a positive limewater result, while the sugar sample remains unreactive with acid.
因此，酸测试结合石灰水检验可给出明确的化学区分。小苏打样品会产生 CO₂并使石灰水变浑浊，而食糖样品与酸不反应。
3. Case Study: Testing Gases — Oxygen, Carbon Dioxide and Hydrogen | 案例研究：气体检验——氧气、二氧化碳和氢气
Three gas jars contain colourless, odourless gases labelled A, B and C. They could be oxygen, carbon dioxide or hydrogen. Describe the tests you would perform to identify each gas safely.
三个集气瓶分别装有标示为 A、B 和 C 的无色无味气体。它们可能是氧气、二氧化碳或氢气。描述你将安全地进行的检验，以鉴定每种气体。
To test for oxygen, hold a glowing splint just inside the mouth of the jar. If the splint relights brightly, the gas is oxygen. For hydrogen, bring a lighted splint to the mouth of the jar — a squeaky pop sound indicates hydrogen.
检验氧气时，将一根带火星的木条伸入瓶口。如果木条复燃并明亮燃烧，则气体是氧气。对于氢气，将点燃的木条靠近瓶口——听到尖锐的爆鸣声说明是氢气。
For carbon dioxide, bubble the gas through limewater using a delivery tube. If the limewater turns from colourless to milky, the gas is carbon dioxide. Alternatively, pour a small volume of limewater into the gas jar, stopper and shake — the limewater will turn milky if CO₂ is present.
检验二氧化碳时，用导管将气体通入石灰水中。如果石灰水从无色变为乳白色，则气体是二氧化碳。或者，在集气瓶中倒入少量石灰水，盖上瓶塞震荡——若存在 CO₂，石灰水变浑浊。
Always wear safety goggles and handle burning splints with care. These tests rely on the characteristic reactivity of each gas: oxygen supports combustion, hydrogen burns explosively in air, and carbon dioxide is acidic and reacts with limewater to form insoluble calcium carbonate.
务必始终佩戴安全护目镜，小心处理燃烧的木条。这些检验依赖每种气体特有的反应性：氧气助燃，氢气在空气中爆鸣燃烧，二氧化碳呈酸性并与石灰水反应生成不溶性碳酸钙。
4. Case Study: Investigating Acids and Alkalis with Universal Indicator | 案例研究：用通用指示剂研究酸和碱
You are given five colourless liquids: lemon juice, sodium hydroxide solution, distilled water, vinegar and ammonia solution. Using universal indicator solution and a spotting tile, determine whether each liquid is acidic, alkaline or neutral, and estimate its pH.
给你五种无色液体：柠檬汁、氢氧化钠溶液、蒸馏水、白醋和氨水。使用通用指示剂溶液和点滴板，判定每种液体是酸性、碱性还是中性，并估计其 pH 值。
Place one drop of each liquid into separate wells of the spotting tile. Add two drops of universal indicator to each well. Observe the colour change immediately. Compare the colours with a pH colour chart (1–14).
分别滴一滴每种液体于点滴板的不同凹槽中。向每个凹槽加入两滴通用指示剂。立即观察颜色变化，并与 pH 比色卡（1–14）对照。
| Liquid / 液体 | Colour with universal indicator / 通用指示剂颜色 | Approx. pH / 近似pH | Acid/Base/Neutral / 酸/碱/中性 |
|---|---|---|---|
| Lemon juice / 柠檬汁 | Red / 红 | 1–2 | Strong acid / 强酸 |
| Vinegar / 醋 | Orange–Red / 橙红 | 3 | Weak acid / 弱酸 |
| Distilled water / 蒸馏水 | Green / 绿 | 7 | Neutral / 中性 |
| Ammonia solution / 氨水 | Blue–Violet / 蓝紫 | 11 | Weak alkali / 弱碱 |
| Sodium hydroxide solution / 氢氧化钠溶液 | Violet / 紫 | 14 | Strong alkali / 强碱 |
Acids turn universal indicator red, orange or yellow (pH less than 7). Alkalis turn it blue or violet (pH greater than 7). Neutral solutions stay green (pH 7). Sodium hydroxide gives a violet colour indicating a very high pH, while lemon juice gives a red colour matching a very low pH.
酸使通用指示剂变为红色、橙色或黄色（pH 小于 7）。碱使其变为蓝色或紫色（pH 大于 7）。中性溶液保持绿色（pH 7）。氢氧化钠呈紫色表示 pH 很高，而柠檬汁呈红色对应很低的 pH。
5. Case Study: Exothermic Reactions — Magnesium and Hydrochloric Acid | 案例研究：放热反应——镁与盐酸
A student adds a strip of magnesium ribbon to a test tube containing dilute hydrochloric acid. Bubbles rise vigorously and the test tube feels warmer. The student records the temperature before and after mixing: 21°C rises to 48°C.
一名学生将一条镁带加入盛有稀盐酸的试管中。气泡剧烈冒出，试管感觉变热。学生记录混合前后的温度：21°C 升至 48°C。
This is an exothermic reaction — energy is released to the surroundings, causing the temperature to increase. The word equation is: magnesium + hydrochloric acid → magnesium chloride + hydrogen. The bubbles are hydrogen gas (H₂).
这是一个放热反应——能量释放到环境中，导致温度上升。文字方程式为：镁 + 盐酸 → 氯化镁 + 氢气。产生的气泡是氢气（H₂）。
The balanced symbol equation can be written as: Mg + 2HCl → MgCl₂ + H₂. The temperature rise of 27°C shows a large energy release. Exothermic reactions are used in hand warmers and self-heating cans.
配平的符号方程式可写为：Mg + 2HCl → MgCl₂ + H₂。温度升高 27°C 表明大量能量释放。放热反应被用于暖手宝和自热罐头。
To confirm the gas is hydrogen, the student holds a lighted splint at the mouth of the tube and hears a squeaky pop. Safety note: wear goggles and avoid leaning over the reaction.
为确认该气体是氢气，学生将点燃的木条放在试管口，听到尖锐爆鸣声。安全提示：佩戴护目镜，避免俯身在反应上方。
6. Case Study: Paper Chromatography of Ink | 案例研究：墨水纸层析
A crime scene note is written in black ink. Three suspect pens are collected. Use paper chromatography to find which pen matches the ink on the note. You have chromatography paper, a beaker, water and a pencil.
犯罪现场的一张便条是用黑墨水写的。收集了三支嫌疑笔。使用纸层析法找出哪支笔与便条上的墨水匹配。你拥有层析纸、烧杯、水和铅笔。
Draw a pencil line 2 cm from the bottom edge of the chromatography paper. Spot the ink from the note and the three pens at equal intervals along the line. Place the paper in a beaker with water just below the pencil line, so the spots do not touch the water.
在层析纸底边 2 cm 处画一条铅笔线。沿着该线等间距点上便条墨水和三支笔的墨水。将纸放入装有水的烧杯中，水面刚好在铅笔线以下，确保墨点不接触水。
As water travels up the paper, it separates the dyes in the ink. Black ink often contains several coloured components, such as blue, red and yellow. Each dye moves a different distance depending on its solubility and attraction to the paper.
随着水沿纸上升，它会分离墨水中的染料。黑墨水通常包含几种颜色组分，如蓝色、红色和黄色。每种染料移动的距离取决于其溶解度和对纸的亲和力。
When the water front nears the top, remove the paper and mark the solvent front with a pencil. Compare the pattern of coloured spots (chromatogram). The pen that produces a matching set of spots to the note is the one used to write the message.
当水流前沿接近顶端时，取出纸并用铅笔标出溶剂前沿。比较有色斑点图（色谱图）。产生与便条墨水匹配的一组斑点的那支笔，就是用来书写留言的笔。
Chromatography works because substances separate between a stationary phase (paper) and a mobile phase (water). It is also used to identify food colourings and analyse drugs.
层析法的原理是物质在固定相（纸）和流动相（水）之间进行分配分离。此法也用于鉴定食用色素和分析药物。
7. Case Study: Investigating Combustion — The Candle Experiment | 案例研究：探究燃烧——蜡烛实验
A burning candle is placed in a trough of water and a gas jar is lowered over it. After a few seconds the flame goes out and water rises up into the jar. Explain these observations in terms of oxygen consumption and product formation.
一支燃烧的蜡烛放在水槽中，用集气瓶罩住。几秒后火焰熄灭，水上升进入瓶中。请从氧气消耗和产物生成的角度解释这些观察结果。
Candle wax is a hydrocarbon. When it burns, it reacts with oxygen in the air to produce carbon dioxide and water vapour: wax + oxygen → carbon dioxide + water. The flame goes out when the oxygen inside the jar is used up.
蜡烛蜡是一种碳氢化合物。燃烧时，它与空气中的氧气反应生成二氧化碳和水蒸气：蜡 + 氧气 → 二氧化碳 + 水。当瓶内氧气耗尽时，火焰熄灭。
The water rises because the carbon dioxide produced dissolves partly in the water and the gases inside cool and contract, reducing pressure. The higher outside air pressure pushes water into the jar to equalise pressure.
水上升是因为产生的二氧化碳部分溶于水，且内部气体冷却收缩，压强减小。外部较高的气压将水压入瓶中以平衡压强。
Test the water droplets on the jar with blue cobalt chloride paper — it turns pink, confirming water. Carbon dioxide can be tested by shaking the remaining gas with limewater, which turns milky. This confirms the two combustion products.
用蓝色的氯化钴试纸检验瓶壁上的水珠——它会变成粉红色，确认为水。二氧化碳可通过将剩余气体与石灰水振荡检验，石灰水变浑浊。这证实了两种燃烧产物。
8. Case Study: Simple Distillation of Salty Water | 案例研究：含盐水的蒸馏
On a coastal field trip, a group collects a sample of sea water. They need pure water for a chemical test back in the lab. Explain how simple distillation can separate clean water from the dissolved salt.
在一次海边实地考察中，小组收集了一份海水样本。他们需要在实验室进行化学测试时使用纯水。解释如何通过简单蒸馏从溶解的盐中分离出干净的水。
Pour the sea water into a round-bottomed flask. Add a few anti-bumping granules to ensure smooth boiling. Set up a distillation apparatus with a thermometer at the flask neck, a condenser, and a receiving beaker.
将海水倒入圆底烧瓶。加入几粒防暴沸颗粒以确保持续平稳沸腾。搭建蒸馏装置，烧瓶颈部插入温度计，连接冷凝管和接收烧杯。
Heat the flask. Water boils at 100°C and turns into steam. The salt does not boil; it remains in the flask. The steam travels into the condenser, where cold water running through the outer jacket cools it, causing it to condense back to liquid water.
加热烧瓶。水在 100°C 沸腾并变为蒸汽。盐不沸腾，留在烧瓶内。蒸汽进入冷凝管，外层套管流动的冷水将其冷却，使其冷凝回液态水。
The pure liquid water (distillate) drips into the receiving beaker. The solid salt is left behind in the flask. This process separates a solvent from a solution by evaporation followed by condensation.
纯净的液态水（馏出液）滴入接收烧杯。固体盐留在烧瓶中。该过程利用蒸发和冷凝将溶剂从溶液中分离出来。
Distillation is used to obtain drinking water from sea water and to separate liquids with different boiling points, such as in the petroleum industry. It works because water and salt have vastly different boiling points.
蒸馏用于从海水制取饮用水以及分离不同沸点的液体，例如在石油工业中。其原理在于水和盐的沸点相差极大。
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