📚 Year 7 OCR Chemistry: Case Study Practice | Year 7 OCR 化学：案例分析实战演练
Welcome to this case study practice guide for Year 7 OCR Chemistry. In this article, we work through real-world scenarios to strengthen your understanding of key concepts such as physical and chemical changes, separation techniques, signs of reactions, and practical investigation skills. Each case study is followed by thorough explanations in both English and Chinese, helping you build confidence for your end-of-topic tests and practical assessments.
欢迎来到 Year 7 OCR 化学案例分析实战演练。本文将通过真实情境的案例，帮你巩固物理变化与化学变化、分离技术、反应现象以及实验探究技能等核心概念。每个案例都配有详细的中英文解释，助你自信应对单元测试和实践评估。
1. Case Study: The Burning Candle | 案例一：燃烧的蜡烛
A student lights a candle and observes wax melting near the wick, then hardening again on the side. The student also notices a flame producing heat, light, soot, and water vapour on a cold beaker held above the flame. Which observations show a physical change and which show a chemical change?
一位学生点燃蜡烛，观察到烛芯附近的蜡熔化后又重新凝固在侧面。他还注意到火焰产生光和热，用冷烧杯罩在火焰上方时出现水蒸气和炭黑。哪些现象属于物理变化？哪些属于化学变化？
osition (mostly long-chain hydrocarbons like C₂₀H₄₂) remains the same. The flame, however, indicates a chemical change: the wax reacts with oxygen in the air to produce carbon dioxide, water vapour, soot (unburnt carbon particles), and energy. Evidence includes the production of gas (CO₂ and H₂O vapour), a colour change (soot), and energy release as light and heat. A chemical equation for the reaction can be simplified as: wax + O₂ → CO₂ + H₂O.
石蜡的熔化和凝固属于物理变化，因为石蜡仅发生状态变化（固态⇌液态），其化学成分（主要是长链烃如C₂₀H₄₂）保持不变。而火焰则表明发生了化学变化：石蜡与空气中的氧气反应，生成二氧化碳、水蒸气、炭黑（未燃烧的碳颗粒）和能量。证据包括气体产生（CO₂和水蒸气）、颜色变化（炭黑）以及以光和热的形式释放能量。该反应可用简式表示：石蜡 + O₂ → CO₂ + H₂O。
2. Case Study: Extracting Salt from Rock Salt | 案例二：从岩盐中提取食盐
A Year 7 class is given a sample of rock salt, which is a mixture of common salt (sodium chloride) and gritty sand. They must design a method to obtain pure salt crystals. The suggested steps: crush the rock salt, add warm water and stir, filter, then heat the filtrate. Explain the purpose of each step and identify the substances separated.
一个七年级班级拿到一份岩盐样品，它是食盐（氯化钠）和沙砾的混合物。学生们需要设计方法获得纯净的食盐晶体。建议步骤是：粉碎岩盐，加入温水搅拌，过滤，然后加热滤液。请解释每一步的目的，并指出分离出的物质。
Crushing increases the surface area of the rock salt, speeding up dissolving. Warm water dissolves the sodium chloride but not the sand, which is insoluble. Filtration then separates the insoluble sand (residue) from the salt solution (filtrate). Finally, heating the filtrate evaporates the water, leaving behind pure sodium chloride crystals. This process uses the key separation techniques of dissolving, filtration, and evaporation – all based on differences in physical properties. A diagram of the setup typically includes a filter funnel with filter paper, a beaker, and an evaporating dish over a Bunsen burner.
粉碎可增大表面积，加快溶解速度。温水能溶解氯化钠，而沙子不溶。过滤将不溶的沙子（残渣）与食盐溶液（滤液）分离。最后加热滤液蒸发水分，得到纯净氯化钠晶体。整个过程利用了溶解、过滤和蒸发三种分离技术，均基于物质物理性质的差异。典型装置图包括带滤纸的过滤漏斗、烧杯以及放在本生灯上的蒸发皿。
3. Case Study: Chromatography of Inks | 案例三：墨水色谱分析
A forensic scientist suspects that a note was written with a black marker containing a mixture of dyes. She places a spot of the ink on chromatography paper and dips the paper in water. After 10 minutes, coloured spots appear at different heights. Explain how chromatography separates the dyes and what the resulting pattern tells us.
一位法医科学家怀疑一份便条是用含有混合染料的黑色记号笔书写的。她在色谱纸上点样，将纸浸入水中。10分钟后，不同高度出现了彩色斑点。请解释色谱法如何分离染料，以及产生的图谱能告诉我们什么。
Chromatography separates substances based on their different solubilities in a solvent (the mobile phase). As water travels up the paper, the more soluble dyes dissolve better and travel further, while less soluble dyes lag behind. The pattern, called a chromatogram, shows the number of components in the ink and can be compared with known samples. If a sample produces spots at the same heights and colours as a reference, they likely contain the same dyes. Key terms include stationary phase (the paper), mobile phase (water), and Rf value (distance moved by spot ÷ distance moved by solvent front).
色谱法根据不同物质在溶剂（流动相）中的溶解度差异来实现分离。随着水在纸上爬升，溶解度高的染料移动得更远，溶解度低的则落后。所得图谱（色谱图）显示了墨水中的成分数量，可以与已知样品比对。如果样品的斑点高度和颜色与参照物相同，很可能含有相同的染料。关键术语包括固定相（纸）、流动相（水）和Rf值（斑点移动距离 ÷ 溶剂前沿移动距离）。
4. Case Study: Testing for Carbon Dioxide | 案例四：检验二氧化碳气体
During a reaction between vinegar (acetic acid) and baking soda (sodium bicarbonate), a colourless gas is produced. A student bubbles the gas through limewater (calcium hydroxide solution), which turns milky. Identify the gas and write a word equation for the limewater test. Why is this test considered a chemical change?
白醋（乙酸）与小苏打（碳酸氢钠）反应时生成一种无色气体。学生将气体通入石灰水（氢氧化钙溶液），石灰水变浑浊。请确定该气体，并写出石灰水检验的文字方程式。为什么此检验被视为化学变化？
ch appears as a white precipitate. The word equation is: calcium hydroxide + carbon dioxide → calcium carbonate + water. This is unmistakably a chemical change because a new substance (solid calcium carbonate) is formed, accompanied by a visible change from clear to cloudy. The balanced symbol equation is: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. This test is a classic method in key stage 3 chemistry for identifying CO₂.
该气体是二氧化碳（CO₂）。石灰水变浑浊是因为氢氧化钙与CO₂反应生成不溶于水的碳酸钙（白垩），它以白色沉淀形式出现。文字方程式为：氢氧化钙 + 二氧化碳 → 碳酸钙 + 水。这明显是化学变化，因为有新物质（固体碳酸钙）生成，并伴随从澄清到浑浊的可见变化。该反应的化学方程式为：Ca(OH)₂ + CO₂ → CaCO₃ + H₂O。这是KS3化学中验证CO₂的经典方法。
5. Case Study: Iron and Sulfur – Mixture or Compound? | 案例五：铁与硫——混合物还是化合物？
A student mixes iron filings and yellow sulfur powder. A magnet easily separates the iron from the sulfur. Then the student heats a fresh sample of the same mixture strongly until a dark grey solid forms. This new solid is not attracted to a magnet, and a gas was released during heating. Compare the mixture and the product to decide if the heating caused a chemical change.
一名学生将铁屑与黄色硫粉混合。磁铁可以轻松地将铁从硫中分离。接着，他将另一份同样的混合物强热至形成深灰色固体。这种新固体不被磁铁吸引，加热过程中有气体产生。请比较混合物与产物，判断加热是否引起了化学变化。
gnet can separate them, confirming a physical mixture. After heating, the grey solid is iron sulfide (FeS), a compound formed by a chemical reaction. The loss of magnetism, colour change, and gas release (sulfur dioxide) are evidence of a chemical change. The reaction is: Fe + S → FeS. In a compound, the constituent elements are chemically bonded and cannot be separated by physical means like magnets. This case also highlights the importance of working in a fume cupboard due to toxic SO₂ gas.
在原始混合物中，铁和硫只是物理混合，各元素保持原有性质（铁有磁性，硫呈黄色）。磁铁可将它们分开，证实这是物理混合物。加热后得到的灰色固体是硫化亚铁（FeS），是化学反应生成的化合物。磁性消失、颜色变化以及气体（二氧化硫）释放都表明发生了化学变化。反应式为：Fe + S → FeS。在化合物中，组成元素通过化学键结合，无法用磁铁等物理方法分离。本案例也强调了因产生有毒SO₂气体，需在通风橱中操作的重要性。
6. Case Study: Acid and Carbonate Reaction | 案例六：酸与碳酸盐反应
A student adds dilute hydrochloric acid to crushed eggshells (mainly calcium carbonate). Bubbles are seen immediately, and the eggshells gradually disappear. A test with limewater confirms the gas. Write a word equation and explain why this reaction is useful for investigating rate of reaction.
一位学生将稀盐酸加入碾碎的蛋壳（主要成分碳酸钙）。立即看到气泡产生，蛋壳逐渐消失。用石灰水检验气体得到确认。请写出文字方程式，并解释该反应为何可用于研究反应速率。
e + water + carbon dioxide. The rapid production of CO₂ bubbles makes it easy to measure reaction speed by collecting gas over water in a measuring cylinder or by timing how long it takes for the mass to decrease (using a balance). Factors such as concentration of acid, temperature, and particle size (whole eggshell vs crushed) can be varied to investigate rates. This practical is a common OCR required activity and must include safety precautions like wearing goggles because acid is corrosive.
反应为：碳酸钙 + 盐酸 → 氯化钙 + 水 + 二氧化碳。迅速产生的CO₂气泡使得测量反应速率十分方便，可以通过排水集气法用量筒收集气体，或记录质量减少所需时间（使用天平）。可改变酸的浓度、温度或颗粒大小（完整蛋壳与碾碎）来探究反应速率影响因素。该实验是OCR常见活动，必须佩戴护目镜，因为酸有腐蚀性。
7. Case Study: Distillation of Seawater | 案例七：海水蒸馏
In a survival scenario, a chemist obtains fresh water from seawater using simple distillation. The apparatus includes a flask with seawater, a delivery tube, a water-cooled condenser, and a receiving beaker. Explain how distillation works and identify the two key changes of state involved.
在求生场景中，一位化学家通过简单蒸馏从海水中获取淡水。装置包括盛有海水的烧瓶、导管、水冷冷凝器和接收烧杯。请解释蒸馏原理，并指出涉及的两种主要状态变化。
Distillation separates a liquid from dissolved solids based on differences in boiling points. The seawater is heated until water boils at 100°C and turns into steam (evaporation – liquid to gas). The water vapour then travels through the condenser, where cold water circulating in the outer jacket cools the steam back to liquid water (condensation – gas to liquid). The pure distillate is collected, leaving salt and other impurities behind in the flask. This demonstrates that substances in a solution can be separated by using their different boiling points. Simple distillation is ideal for separating a solvent from a solute when only the solvent is required.
蒸馏利用沸点差异将液体与溶解的固体分离。加热海水至100°C，水沸腾变为水蒸气（蒸发——液态变为气态）。水蒸气随后进入冷凝器，外层夹套中的冷水使蒸气冷却重新变为液态水（冷凝——气态变为液态）。收集到的纯蒸馏水就是淡水，而盐和其他杂质则留在烧瓶中。这说明可利用沸点不同分离溶液中的物质。当只需要溶剂时，简单蒸馏是理想选择。
8. Case Study: Investigating Rusting Conditions | 案例八：探究生锈的条件
A student sets up four test tubes with identical iron nails. Tube A: nail in air only. Tube B: nail in boiled water with a layer of oil to exclude air. Tube C: nail in water exposed to air. Tube D: nail in a desiccator (dry air). After a week, only the nail in Tube C shows significant rust. What does this tell us about the requirements for rusting?
一名学生准备四支试管，各放入相同的铁钉。A管：铁钉只接触空气。B管：铁钉浸在煮沸过的水中，上层用油封住隔绝空气。C管：铁钉在水中但暴露于空气。D管：铁钉放在干燥器（干燥空气）中。一周后，只有C管中的铁钉明显生锈。这告诉我们生锈需要什么条件？
Rusting requires both oxygen and water. Tube A lacks water, so no rust forms. Tube B has water but lacks oxygen (boiling removes dissolved air, and oil prevents re-entry), so no rust. Tube D has oxygen but no moisture, so again no rust. Only Tube C provides both elements, resulting in the formation of hydrated iron(III) oxide (rust). Rusting is a slow chemical reaction, and its formula can be approximated as Fe₂O₃·xH₂O. This investigation highlights the importance of controlling variables and using comparison tests to deduce necessary conditions.
生锈需要氧气和水同时存在。A管缺水，不生锈；B管有水但缺少氧气（煮沸除去溶解的空气，油封防止再溶），不生锈；D管有氧气但无水分，也不生锈。只有C管同时提供氧气和水，生成水合氧化铁（铁锈），化学式可近似为Fe₂O₃·xH₂O。生锈是一种缓慢的化学变化。该探究体现了控制变量和对比实验在推断必要条件时的重要性。
9. Case Study: Purifying a Solid by Recrystallization | 案例九：重结晶提纯固体
A student accidentally mixes copper sulfate crystals with a small amount of sand. How can they recover pure copper sulfate? The student dissolves the mixture in hot water, filters hot, then cools the filtrate. Blue crystals form. Explain the process.
一位学生意外将硫酸铜晶体与少量沙子混合在一起。如何回收纯净的硫酸铜？学生将混合物溶于热水，趁热过滤，然后冷却滤液，得到蓝色晶体。请解释该过程。
Hot water dissolves copper sulfate completely but leaves sand undissolved. Hot filtration removes the sand while the solution is still hot, preventing premature crystallization. As the solution cools, copper sulfate becomes less soluble, forming pure crystals. This technique, called recrystallization, relies on the change in solubility with temperature. It works well for solids with steep solubility curves, like copper sulfate (CuSO₄·5H₂O). The sand stays on the filter paper, and the final crystals are separated by filtration and dried. This is more advanced than simple evaporation and yields purer crystals because soluble impurities stay dissolved in the mother liquor.
热水完全溶解硫酸铜而沙子不溶。趁热过滤可在溶液冷却前除去沙子，避免过早结晶。随着溶液冷却，硫酸铜溶解度下降，形成纯净晶体。这一技术称为重结晶，利用溶解度随温度变化的性质。它适用于溶解度曲线陡峭的固体，如硫酸铜（CuSO₄·5H₂O）。沙子留在滤纸上，最终晶体经过滤和干燥获得。这比简单蒸发更精细，因可溶性杂质留在母液中，所以晶体纯度更高。
10. Case Study: Melting and Boiling Points as Purity Indicators | 案例十：熔沸点作为纯净度指标
A student measures the melting point of a white solid claimed to be pure benzoic acid. The literature value is 122°C. The sample melts over a range of 118–125°C. What can the student conclude about the purity? Also, explain why boiling points of liquids can be used similarly.
一名学生测量了据称是纯苯甲酸的白色固体的熔点。文献值为122°C。该样品在118–125°C范围内熔化。关于纯度，该学生能得出什么结论？另请解释为何液体的沸点也可类似使用。
A pure solid has a sharp, fixed melting point. The observed melting range of 118–125°C, which is broad and lower than the true melting point, indicates that the sample is impure. Impurities disrupt the regular crystal lattice, causing melting to start at a lower temperature and finish over a range. Similarly, a pure liquid boils at a constant temperature, while an impure liquid boils over a range, often starting above the pure boiling point for non-volatile impurities. These tests help assess purity in materials like pharmaceuticals and food additives.
纯固体具有敏锐固定的熔点。观察到的118–125°C范围过宽且低于真实熔点，表明样品不纯。杂质会破坏规则晶格，使熔化在较低温度开始并在一个范围内完成。类似地，纯液体在恒定温度沸腾，而不纯液体（含非挥发性杂质）则在一个区间内沸腾，起始温度往往高于纯物质沸点。这些测试帮助评估药物和食品添加剂等物质的纯度。
11. Case Study: Identifying Unknown Solutions | 案例十一：鉴定未知溶液
Three colourless liquids labelled P, Q, and R are thought to be water, dilute hydrochloric acid, and dilute sodium hydroxide (an alkali). A student uses litmus paper and evaporation to identify each. Describe a logical series of tests and expected observations.
标有P、Q、R的三种无色液体可能是水、稀盐酸和稀氢氧化钠（碱）。学生用石蕊试纸和蒸发来进行鉴定。描述一系列合理的测试及预期现象。
Step 1: Test each liquid with blue litmus paper. The one that turns red is acidic – thus hydrochloric acid. Step 2: Test the remaining two with red litmus paper; the one that turns blue is alkaline – sodium hydroxide. The unchanged one is neutral – water. Step 3: Evaporate small samples of each. The water will evaporate leaving no residue; the hydrochloric acid may smell and release fumes with no solid residue; sodium hydroxide solution will leave a white solid residue (NaOH). This procedure demonstrates the use of acid-base indicators and the difference between a pure liquid and solutions. Safety: wear goggles and use small volumes.
步骤1：用蓝色石蕊试纸检测各液体。变红的为酸性——即稀盐酸。步骤2：用红色石蕊试纸检测剩余两种，变蓝的是碱性——氢氧化钠溶液。不变色的为中性——水。步骤3：蒸发少量样品。水完全蒸发不留残余物；盐酸可能冒烟且无固体残留；氢氧化钠溶液留下白色固体（NaOH）。该过程演示了酸碱指示剂的应用以及纯液体与溶液的区别。安全要求：佩戴护目镜，用量要少。
12. Wrap-up: Skills and Common Pitfalls | 总结：技能与常见错误
Through these case studies, you have practised observing changes, planning separation sequences, using indicator tests, and interpreting data. Common mistakes to avoid: confusing melting and dissolving; forgetting to state that filtration separates insoluble solids from liquids; assuming that all colour changes are chemical changes (some, like mixing coloured dyes, are physical); and not specifying control variables in rate experiments. Always link the method to the scientific principle, and label apparatus clearly in diagrams.
通过这些案例，你练习了观察变化、设计分离流程、使用指示剂检测以及解释数据。需要避免的常见错误包括：混淆熔化与溶解；忘记说明过滤是分离不溶性固体与液体；误认为所有颜色变化都是化学变化（如混合彩色染料是物理变化）；在速率实验中未指明控制变量。务必将实验方法与科学原理挂钩，并在装置图中清晰标注仪器名称。
Remember to revise hazard symbols, learn common word equations, and practise writing methods in a logical step-by-step format. These case studies reflect the style of OCR Year 7 assessments and will help you think like a chemist.
请记住复习危险品标志，熟记常见文字方程式，并练习按逻辑步骤书写实验方法。这些案例反映了OCR七年级评估的风格，将帮助你像化学家一样思考。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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