📚 Year 7 Cambridge Chemistry: Case Study Practical Drills | Year 7 剑桥化学：案例分析实战演练
Applying chemistry knowledge to real-world scenarios is the best way to master the subject. This article provides ten case study drills covering key topics from the Year 7 Cambridge syllabus, including separation techniques, acids and alkalis, rusting, physical and chemical changes, gas tests, chromatography, purity, classification of matter, neutralisation and water purification.
将化学知识应用到实际情境中是掌握这门学科的最佳方法。本文提供了十个案例分析演练，涵盖 Year 7 剑桥课程大纲中的重点主题，包括分离技术、酸和碱、生锈、物理变化与化学变化、气体检验、色谱法、纯度、物质分类、中和反应以及水的净化。
1. How to Use These Case Study Drills | 如何使用这些案例分析演练
Every drill in this article follows the same pattern. A short scenario is given, you are asked to choose equipment, describe a method and predict a result. Chemistry is not mastered by memorising words such as filtrate or residue; it is mastered by using those words inside a real situation.
本文中的每一个演练都遵循相同的结构：先给出一个简短情境，然后要求你选择仪器、描述方法并预测结果。化学不是靠背下 filtrate（滤液）或 residue（残渣）这类单词就能掌握的，而是要在一个真实情境中把这些词用出来。
Work through each drill in three passes. First, read the scenario and underline every substance mentioned, noting whether it is a solid, liquid or gas. Second, write your full answer without looking at the notes. Third, mark your own work against the key points listed under each drill and rewrite only the parts you missed.
每个演练请分三轮完成。第一轮：阅读情境，划出其中提到的每一种物质，并标注它是固体、液体还是气体。第二轮：不看笔记，完整写出你的答案。第三轮：对照每个演练下方列出的要点自我批改，只重写你漏掉的部分。
- Pass 1 - Identify the substances and their properties | 第一轮：识别物质及其性质
- Pass 2 - Choose a technique and justify it | 第二轮：选择分离或检验方法并说明理由
- Pass 3 - Mark against the key points and fix gaps | 第三轮：按要点批改并补齐漏洞
| Drill | 演练 | Topic | 主题 | Main skill tested | 核心考查技能 |
| 1 | Separating sand, salt and water | 分离沙子、食盐和水 | Filtration and crystallisation | 过滤与结晶 |
| 2 | Acids and alkalis at home | 生活中的酸和碱 | pH and indicators | pH 与指示剂 |
| 3 | Rusting | 生锈 | Variables and fair testing | 变量与公平实验 |
| 4 | Physical and chemical changes | 物理变化与化学变化 | Evidence and classification | 证据与分类 |
| 5 | Gas tests | 气体检验 | Observation and conclusions | 观察与结论 |
| 6 | Chromatography | 色谱法 | Measuring and comparing | 测量与比较 |
| 7 | Purity | 纯度 | Interpreting melting and boiling points | 解读熔点与沸点 |
| 8 | Classification of matter | 物质的分类 | Elements, compounds, mixtures | 元素、化合物、混合物 |
| 9 | Neutralisation | 中和反应 | Word and symbol equations | 文字方程式与符号方程式 |
| 10 | Water purification | 水的净化 | Sequencing and purpose | 步骤排序与目的说明 |
2. Case 1: Separating Sand, Salt and Water | 案例一：分离沙子、食盐和水
You are given a beaker containing a mixture of sand, common salt (sodium chloride) and water. Sand is insoluble in water, while salt is soluble. Your goal is to obtain dry sand and pure salt crystals.
你得到一个盛有沙子、食盐（氯化钠）和水的烧杯。沙子不溶于水，而食盐可溶。你的目标是得到干燥的沙子和纯净的食盐晶体。
Step 1: Filtration - Pour the mixture through a filter funnel lined with filter paper. The insoluble sand is trapped on the paper as the residue, while the salt solution (filtrate) passes through.
步骤一：过滤——将混合物通过铺有滤纸的过滤漏斗。不溶的沙子作为残渣留在滤纸上，而食盐水溶液（滤液）则透过滤纸。
Step 2: Wash and dry the residue - Rinse the sand on the filter paper with a little distilled water to remove any salt solution still clinging to it, then leave it in a warm place to dry. If you skip the rinsing step, the sand will still be contaminated with salt when it dries.
步骤二：洗涤并干燥残渣——用少量蒸馏水冲洗滤纸上的沙子，除去仍然附着在上面的食盐水，然后放在温暖处晾干。如果省略洗涤这一步，沙子晾干后仍会被食盐污染。
Step 3: Crystallisation - Heat the filtrate gently in an evaporating basin over a water bath or a low flame. Water evaporates, the solution becomes more concentrated, and once it is saturated, salt crystals begin to form. Stop heating before the basin goes completely dry, because overheated crystals can spit out of the basin and the salt may decompose.
步骤三：结晶——把滤液放在蒸发皿中，用水浴或小火缓慢加热。水蒸发后溶液变得更浓，一旦达到饱和，食盐晶体便开始析出。在蒸发皿完全变干之前就要停止加热，因为过热的晶体会飞溅出蒸发皿，食盐也可能发生分解。
Why this order matters: filtration must come first because sand is the insoluble component. If you tried to crystallise the mixture directly, the sand would simply be mixed into the salt crystals and you would have to start again.
为什么顺序很重要：必须先过滤，因为沙子是不溶成分。如果你直接对混合物进行结晶，沙子只会混进食盐晶体中，你就得从头再来一次。
Key vocabulary for full marks: residue (the solid left on the filter paper), filtrate (the liquid that passes through), saturated solution (a solution that can dissolve no more solute at that temperature), crystallisation (forming solid crystals from a saturated solution).
拿满分需要的关键词汇：residue（滤纸上留下的固体）、filtrate（透过滤纸的液体）、saturated solution（在该温度下无法再溶解更多溶质的溶液）、crystallisation（从饱和溶液中析出固体晶体）。
3. Case 2: Acids and Alkalis Around the Home | 案例二：生活中的酸和碱
A student collects five liquids from the kitchen and tests them with universal indicator. The results are: lemon juice pH 2, vinegar pH 3, pure water pH 7, baking soda solution pH 9 and soap solution pH 10. The task is to sort them and explain what the numbers mean.
一名学生从厨房收集了五种液体并用通用指示剂检验。结果如下：柠檬汁 pH 2、食醋 pH 3、纯水 pH 7、小苏打溶液 pH 9、肥皂水 pH 10。任务是给它们分类并解释这些数字的含义。
The pH scale runs from 0 to 14. A pH below 7 means the substance is acidic, pH 7 means neutral, and a pH above 7 means alkaline. The further the number is from 7, the stronger the acid or alkali. So lemon juice (pH 2) is the most acidic of the five, and soap solution (pH 10) is the most alkaline.
pH 标度从 0 到 14。pH 小于 7 表示物质呈酸性，pH 等于 7 表示中性，pH 大于 7 表示碱性。数字离 7 越远，酸或碱就越强。因此在五种液体中，柠檬汁（pH 2）酸性最强，肥皂水（pH 10）碱性最强。
Litmus paper is a quick test that gives only two answers: red litmus turns blue in alkali, and blue litmus turns red in acid. Universal indicator is better when you need an actual pH number, because it produces a range of colours that can be matched to a pH colour chart.
石蕊试纸是一种快速检验方法，但只能给出两种答案：红色石蕊在碱中变蓝，蓝色石蕊在酸中变红。当你需要具体的 pH 数值时，通用指示剂更好，因为它会产生一系列颜色，可以与 pH 比色卡对照。
Acids react with metals such as magnesium and zinc to produce a salt and hydrogen gas. This is why acids must never be stored in reactive metal containers, and why hydrogen gas, which is flammable, is never allowed to build up near a flame.
酸会与镁、锌等金属反应，生成盐和氢气。这就是酸绝不能存放在活泼金属容器中的原因，也是易燃的氢气绝不允许在火焰附近积聚的原因。
- Acidic: lemon juice, vinegar, fizzy drinks, stomach acid | 酸性：柠檬汁、食醋、碳酸饮料、胃酸
- Neutral: pure water, salt solution, sugar solution | 中性：纯水、食盐水、糖水
- Alkaline: baking soda, soap, toothpaste, bleach | 碱性：小苏打、肥皂、牙膏、漂白剂
Safety point: when an acid is diluted with water, always add the acid to the water slowly, never water to concentrated acid, because the mixture gets hot and can spit.
安全要点：稀释酸时，一定要把酸缓慢加入水中，绝不能把水加入浓酸中，因为混合物会放热并可能飞溅。
4. Case 3: Rusting and Rust Prevention | 案例三：生锈与防锈
A bicycle is left outside for a term. The chain becomes covered in a brown flaky solid, but the plastic handlebar grips do not change. The question is: what conditions caused the rust, and how could it have been prevented?
一辆自行车被放在户外一个学期。车链上覆盖了一层棕色的片状固体，但塑料车把套没有变化。问题是：是什么条件导致了生锈？又该如何防止？
iron(III) oxide, which is the brown flaky substance we call rust. Remove either oxygen or water and the iron will not rust.
生锈需要同时满足两个条件：氧气和水。文字方程式为：铁 + 氧气 + 水 → 水合氧化铁(III)，也就是我们称为铁锈的棕色片状物质。只要除去氧气或水中的任意一个，铁就不会生锈。
A classic experiment uses three test tubes. Tube A contains an iron nail in dry air with a drying agent such as calcium chloride. Tube B contains an iron nail in boiled water covered with a layer of oil, so no new oxygen can dissolve in. Tube C contains an iron nail in ordinary tap water. Only tube C rusts, because only there are both oxygen and water present.
经典实验使用三支试管。试管 A 中放一枚铁钉，管内是干燥空气并加入氯化钙等干燥剂。试管 B 中放一枚铁钉，加水煮沸后覆盖一层油，使新的氧气无法溶入。试管 C 中放一枚铁钉，加普通自来水。只有试管 C 生锈，因为只有它同时具备氧气和水。
Salt water makes rusting much faster, which is why cars near the coast and bicycles ridden on salted winter roads corrode quickly. Salt does not cause rusting on its own; it acts as an electrolyte that speeds up the reaction. This is an important exam trap: salt is a rate factor, not a requirement.
盐水会大大加快生锈速度，这就是海边附近的汽车和冬天在撒盐路面上骑行的自行车腐蚀得很快的原因。盐本身不会引起生锈，它作为电解质加快反应速率。这是考试中的重要陷阱：盐是影响速率的因素，而不是必要条件。
Methods of prevention all work by creating a barrier or by blocking one of the two requirements: painting, oiling or greasing, plastic coating, galvanising with zinc, and making stainless steel by alloying iron with chromium and nickel. Galvanising also gives sacrificial protection, because zinc is more reactive than iron and corrodes in its place even when the coating is scratched.
各种防锈方法都是通过建立屏障或切断两个必要条件之一来实现的：刷漆、涂油或上油脂、塑料包覆、镀锌，以及把铁与铬、镍制成合金得到不锈钢。镀锌还具有牺牲阳极保护作用，因为锌比铁更活泼，即使镀层被刮伤，锌也会代替铁被腐蚀。
5. Case 4: Physical and Chemical Changes | 案例四：物理变化与化学变化
A teacher demonstrates six changes and asks the class to classify each one as physical or chemical: ice melting, salt dissolving, magnesium burning, a cake baking, an iron nail rusting and sugar dissolving in tea.
老师演示了六个变化，要求全班把它们分为物理变化或化学变化：冰融化、食盐溶解、镁燃烧、蛋糕烘烤、铁钉生锈、糖溶于茶。
A physical change does not produce a new substance. The particles are rearranged but the substance is still the same, and the change is usually easy to reverse. Melting ice, dissolving salt and dissolving sugar are all physical changes: ice is still H₂O, and dissolved salt can be recovered by evaporation.
物理变化不会产生新物质。粒子只是被重新排布，物质本身没有改变，而且这类变化通常容易逆转。冰融化、食盐溶解和糖溶解都属于物理变化：冰仍然是 H₂O，溶解的食盐也可以通过蒸发重新得到。
A chemical change produces at least one new substance, and it is usually hard or impossible to reverse. Burning magnesium gives a white powder, magnesium oxide, which has completely different properties from the shiny metal. Baking and rusting are also chemical changes.
化学变化至少会产生一种新物质，而且通常难以或无法逆转。镁燃烧生成白色粉末氧化镁，其性质与光亮的金属完全不同。烘烤和生锈同样属于化学变化。
Watch out for the evidence you quote. Bubbles, a colour change, a new smell, a temperature change, light given out or a solid appearing in a solution can all be signs of a chemical change. But a temperature change on its own is not proof: melting ice also absorbs heat, and that is a physical change.
注意你所引用的证据。气泡、颜色变化、新气味、温度变化、发光或在溶液中出现固体，都可能是化学变化的标志。但仅有温度变化并不能作为证据：冰融化也吸热，而那是物理变化。
| Change | 变化 | Type | 类型 | Reason | 原因 |
| Ice melting | 冰融化 | Physical | 物理 | Still H₂O | 仍是 H₂O |
| Magnesium burning | 镁燃烧 | Chemical | 化学 | New substance, MgO | 生成新物质 MgO |
| Nail rusting | 铁钉生锈 | Chemical | 化学 | New substance, hydrated iron(III) oxide | 生成新物质水合氧化铁(III) |
| Salt dissolving | 食盐溶解 | Physical | 物理 | Reversible by evaporation | 可通过蒸发逆转 |
6. Case 5: Gas Tests in the Laboratory | 案例五：实验室气体检验
Four unlabelled gas jars are on the bench. They contain hydrogen, oxygen, carbon dioxide and chlorine. You may only use a burning splint, a glowing splint, limewater and damp litmus paper. Identify each gas and explain the observation.
实验台上有四个未贴标签的集气瓶，分别装有氢气、氧气、二氧化碳和氯气。你只能使用燃着的木条、带火星的木条、石灰水和湿润的石蕊试纸。请鉴别每种气体并解释观察到的现象。
The burning splint test identifies hydrogen: place the splint at the mouth of the jar and you hear a squeaky pop as the hydrogen burns rapidly with the oxygen in the air to form water. The glowing splint test identifies oxygen: a splint that is glowing but not burning bursts back into flame in oxygen.
燃着的木条用于检验氢气：把木条放到瓶口，会听到尖锐的爆鸣声，因为氢气与空气中的氧气迅速燃烧生成水。带火星的木条用于检验氧气：只有余烬、并未燃烧的木条在氧气中会重新燃起火焰。
Carbon dioxide turns limewater milky or cloudy, forming a white precipitate of calcium carbonate. It also extinguishes a burning splint, but so do several other gases, so the limewater test is the confirmatory one because it is specific.
二氧化碳会使石灰水变浑浊，生成白色碳酸钙沉淀。它也会使燃着的木条熄灭，但其他几种气体同样可以做到这一点，因此石灰水检验才是确证性检验，因为它具有专一性。
Chlorine bleaches damp litmus paper, turning it white. Ammonia, if it were present instead, would turn damp red litmus paper blue. Always use damp litmus paper rather than dry, because the gas must dissolve in the water to form an acid or an alkali before it can change the colour.
氯气会漂白湿润的石蕊试纸，使其变成白色。如果换成氨气，则会使湿润的红色石蕊试纸变蓝。一定要用湿润的石蕊试纸而不是干燥的，因为气体必须先溶于水形成酸或碱，才能改变试纸的颜色。
- Hydrogen: burning splint, squeaky pop | 氢气：燃着的木条，尖锐爆鸣声
- Oxygen: glowing splint relights | 氧气：带火星的木条复燃
- Carbon dioxide: limewater turns milky | 二氧化碳：石灰水变浑浊
- Chlorine: damp litmus paper bleached white | 氯气：湿润石蕊试纸被漂白
7. Case 6: Chromatography and Dyes | 案例六：色谱法与色素分离
A student is given four black ink pens and one unknown mixture. Using paper chromatography, she must decide whether the unknown ink is a single dye or a mixture of dyes.
一名学生拿到四支黑色墨水笔和一种未知混合物。她要利用纸色谱法判断这种未知墨水是单一色素还是混合色素。
Draw a pencil baseline about 1 cm from the bottom of the chromatography paper and put a small spot of each ink on it. Pencil is used rather than pen because pencil marks do not dissolve in the solvent and would not travel up the paper. The paper is then lowered into a shallow layer of solvent, with the baseline above the solvent surface, and the solvent is allowed to rise.
在色谱纸底部约 1 厘米处用铅笔画一条基线，并在上面点上每一种墨水的小圆点。用铅笔而不是钢笔，是因为铅笔痕迹不溶于溶剂，不会随溶剂向上移动。然后把纸放入浅层溶剂中，基线要高于溶剂液面，让溶剂自由上升。
As the solvent travels up the paper, it carries the dyes with it. Dyes that are more soluble, or that stick less strongly to the paper, travel further. When the solvent front nears the top, remove the paper, mark the solvent front immediately in pencil and leave the paper to dry.
溶剂向上移动时会把色素一起带走。溶解度更大、或与纸结合更弱的色素移动得更远。当溶剂前沿接近顶端时取出纸，立刻用铅笔标出溶剂前沿位置，然后晾干。
The result is read from the number of spots. A pure dye gives a single spot, while a mixture separates into two or more spots at different heights. If the unknown ink shows three spots, and those spots line up at exactly the same heights as three of the known pens, the unknown ink is a mixture of those three dyes.
结果通过斑点的数量来判断。纯净的色素只显示一个斑点，而混合物会在不同高度分成两个或更多斑点。如果未知墨水显示三个斑点，且这些斑点与其中三支已知墨水的高度完全对齐，那么这种未知墨水就是这三种色素的混合物。
The Rf value describes how far a spot has travelled relative to the solvent:
Rf = distance moved by the spot ÷ distance moved by the solvent front
Rf 值表示斑点相对于溶剂移动的距离：
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