📚 Case Study Practice for Year 7 Chemistry | 七年级化学案例分析实战演练
Welcome to a special revision session where we apply your Year 7 Cambridge Chemistry knowledge to real-world-style case studies. Instead of just recalling facts, you will learn how to think like a scientist by solving problems step by step. Each case study is designed around core topics: mixtures and separation, acids and alkalis, chemical reactions, and the particle model. Read each scenario carefully, follow the reasoning, and check your understanding against the paired explanations.
欢迎来到一场特别的复习课，我们将把七年级剑桥化学知识应用到真实情境的案例分析中。不再是简单地背诵事实，你将学习如何像科学家一样逐步解决问题。每个案例都围绕核心主题设计：混合物与分离、酸与碱、化学反应以及粒子模型。请仔细阅读每个场景，跟随推理过程，并对照配对的解释来检验自己的理解。
1. Why Case Studies? | 为什么需要案例研究？
Case studies bridge the gap between textbook theory and practical experimentation. They encourage you to observe, hypothesise, and evaluate – exactly the skills examined in Cambridge Checkpoint and beyond. By working through these examples, you will strengthen your ability to explain observations using scientific vocabulary and to predict outcomes based on chemical principles.
案例分析在课本理论和动手实验之间架起了桥梁。它鼓励你去观察、提出假设并进行评估——这正是剑桥 Checkpoint 及更高阶段考试所考查的技能。通过练习这些例子，你将提升用科学词汇解释观察结果的能力，以及根据化学原理预测结果的能力。
In each case, we start with a problem, analyse the available data, suggest a method, and then reflect on what the results tell us about the nature of matter. This mirrors how chemists work in research and industry.
每个案例中，我们都从一个问题出发，分析现有数据，提出方法，然后反思结果能告诉我们关于物质性质的哪些信息。这正反映了化学家们在研究和工业中工作的方式。
2. Case 1: Separating a Mixture | 案例一：分离混合物
A student is given a beaker containing a mixture of sand, table salt (sodium chloride), and iron filings. The challenge is to obtain each component as a separate, pure substance. All three are mixed together as a greyish powder with visible shiny bits.
一名学生拿到一个烧杯，里面装满了沙子、食盐（氯化钠）和铁屑的混合物。挑战是要将每种组分作为单独、纯净的物质分离出来。这三种物质混合在一起，呈灰色粉末状，其中可见闪亮的碎屑。
What physical properties would help you design a separation plan? Remember, no chemical change should occur – we only use physical methods.
哪些物理性质能帮助你设计分离方案？请记住，不应发生化学变化——我们只使用物理方法。
3. Identifying the Problem | 识别问题
Iron is magnetic, whereas sand and salt are not. Salt dissolves in water, but sand does not. Sand is denser than water and insoluble. These differences allow us to target each substance selectively. A good strategy uses the least number of steps and avoids contamination.
铁具有磁性，而沙子和食盐没有。食盐溶于水，沙子不溶。沙子密度大于水且不溶。这些差异使我们能够选择性地分离每种物质。一个好的策略应使用最少的步骤并且避免污染。
We can summarise the properties in a table to compare them before planning:
在规划前，我们可以用一张表格汇总它们的性质进行比较：
| Substance | Magnetic? | Soluble in water? | State after dissolving |
|---|---|---|---|
| Iron filings | Yes | No | Solid remains |
| Sand | No | No | Settles at bottom |
| Salt | No | Yes | Dissolves completely |
From this table, the order of separation becomes clear: first remove iron, then remove sand, finally recover salt from solution.
从这张表格可以清楚看出分离的顺序：首先移除铁，然后移除沙子，最后从溶液中回收食盐。
4. Planning the Separation | 规划分离步骤
Step 1: Use a magnet wrapped in a piece of paper to attract the iron filings. Lifting the magnet pulls iron out of the mixture without picking up sand or salt. The iron can then be scraped off and collected.
步骤一：用一张纸包裹磁铁，来吸引铁屑。提起磁铁即可将铁从混合物中分离出来，而不会带上沙子或食盐。随后可将铁屑刮下并收集。
Step 2: Add water to the remaining sand-salt mixture and stir. The salt dissolves, forming a salt solution. The sand stays solid and sinks.
步骤二：向剩下的沙子-食盐混合物中加水并搅拌。食盐溶解，形成食盐水溶液。沙子保持固态并沉到杯底。
Step 3: Filter the mixture using a filter funnel and filter paper. The sand is trapped as the residue, while the salt solution passes through as the filtrate.
步骤三：使用漏斗和滤纸过滤混合物。沙子在滤纸上成为滤渣，而食盐溶液则作为滤液通过。
Step 4: Pour the filtrate into an evaporating basin and heat gently using a Bunsen burner or water bath. The water evaporates, leaving behind pure salt crystals.
步骤四：将滤液倒入蒸发皿中，用本生灯或水浴缓缓加热。水分蒸发，留下纯净的食盐晶体。
This plan uses simple apparatus and exploits the unique properties of each material.
该方案使用简单的仪器，并利用了每种材料的独特性质。
5. Carrying Out the Experiment | 实验操作
When conducting the separation, always wear safety goggles. Heat solutions slowly to prevent spitting. The iron-filing removal should be done first because if we added water directly, the iron would rust slowly, but more importantly, the iron would be harder to retrieve from a wet mixture. However, magnetic separation still works in wet conditions if necessary.
进行分离时，务必佩戴护目镜。缓慢加热溶液以防液体飞溅。首先除去铁屑，因为如果直接加水，铁会缓慢生锈，但更重要的是铁屑更难从潮湿混合物中回收。不过如有必要，磁性分离在湿润条件下仍可操作。
A useful tip: when evaporating the salt solution, stop heating just as crystals begin to form, and leave the rest to cool – this yields larger, purer crystals.
一个有用的技巧：在蒸发食盐水溶液时，一到晶体开始析出就停止加热，让剩余溶液冷却——这样能得到更大、更纯净的晶体。
The entire process is a physical change because no new substances are made. The iron, sand, and salt retain their chemical identities throughout.
整个过程属于物理变化，因为没有生成新物质。铁、沙子和食盐在整个过程中都保持各自的化学身份。
6. Reflection and Key Concepts | 反思与关键概念
This case reinforces the idea that mixtures can be separated by exploiting differences in physical properties. The techniques used – magnetic attraction, filtration, evaporation – are core methods in the Year 7 Cambridge syllabus. You should be able to justify each step and explain why the order matters.
这个案例强化了一个理念：可以利用物理性质的差异来分离混合物。所用的技术——磁吸、过滤、蒸发——都是七年级剑桥课程的核心方法。你应该能解释每一步的理由以及为什么顺序很重要。
If the sand had been iron-contaminated sand from a beach, magnetic separation alone might be sufficient. But with three components, a sequenced approach is essential. Always ask: which property makes this component unique in the mixture?
如果沙子是来自海滩的含铁沙子，仅用磁分离可能就够了。但存在三种组分时，有序的方法至关重要。永远要问：哪种性质让该组分在混合物中独一无二？
7. Case 2: Identifying Unknown Substances | 案例二：鉴定未知物质
A lab technician gives you two white powders, labelled A and B. Both look identical. Your task is to determine whether each powder is an acid, an alkali, or a carbonate compound, using only indicator solutions, pH paper, and dilute hydrochloric acid.
一位实验室管理师给你两种白色粉末，标签为 A 和 B。两者外观完全相同。你的任务是仅使用指示剂溶液、pH 试纸和稀盐酸，判断每种粉末是酸性物质、碱性物质还是碳酸盐化合物。
You are told that one powder might be citric acid (found in lemons) and the other might be sodium hydrogencarbonate (baking soda). But you must confirm through tests rather than assumptions.
你被告知其中一种粉末可能是柠檬酸（存在于柠檬中），另一种可能是碳酸氢钠（小苏打）。但你必须通过测试来确认，而不能靠猜测。
8. Using Indicators and pH | 使用指示剂和pH
Dissolve a small amount of each powder in water separately. Test with red and blue litmus paper. Red litmus turning blue indicates an alkali; blue litmus turning red indicates an acid. You observe: for powder A, blue litmus turns red; for powder B, red litmus turns blue.
分别将少量每种粉末溶于水。用红色和蓝色石蕊试纸进行测试。红色石蕊试纸变蓝表明是碱性物质；蓝色石蕊试纸变红表明是酸性物质。你观察到：粉末 A 使蓝色石蕊试纸变红；粉末 B 使红色石蕊试纸变蓝。
Then dip universal indicator paper into each solution. The colour of powder A’s solution matches pH 2–3, confirming a strong acid. Powder B’s solution gives a colour corresponding to pH 8–9, confirming a weak alkali.
接着将通用指示纸浸入每种溶液中。粉末 A 溶液的颜色与 pH 2–3 匹配，证实为强酸。粉末 B 溶液的颜色对应 pH 8–9，证实为弱碱。
The pH scale ranges from 0 to 14. Values below 7 are acidic, 7 is neutral, and above 7 are alkaline. Both litmus and universal indicator provide quick, reliable evidence.
pH 标度范围从 0 到 14。低于 7 为酸性，7 为中性，高于 7 为碱性。石蕊和通用指示剂都能提供快速、可靠的证据。
9. Testing for Carbonates | 碳酸盐检验
To check if powder B contains a carbonate (like baking soda or limestone), add a few drops of dilute hydrochloric acid to a sample of the powder. Rapid bubbling and a fizzing sound are observed. The gas produced turns limewater milky, confirming carbon dioxide.
为了检验粉末 B 是否含碳酸盐（如小苏打或石灰石），向该粉末样品中加入几滴稀盐酸。观察到快速冒泡和嗞嗞声。产生的气体使石灰水变浑浊，确认为二氧化碳。
The word equation for the reaction with sodium hydrogencarbonate is:
sodium hydrogencarbonate + hydrochloric acid → sodium chloride + water + carbon dioxide
与碳酸氢钠反应的文字方程式为：
碳酸氢钠 + 盐酸 → 氯化钠 + 水 + 二氧化碳
Using chemical formulas, we can write the reaction as:
NaHCO₃ + HCl → NaCl + H₂O + CO₂
Powder A did not produce a gas with hydrochloric acid, meaning it is not a carbonate but a simple acid like citric acid. So we have successfully identified A as an acid and B as an alkali carbonate.
粉末 A 与盐酸不产生气体，说明它不是碳酸盐，而是一种像柠檬酸这样的简单酸。至此我们成功地鉴定出 A 为酸，B 为碱性碳酸盐。
This two-step testing – pH indication then carbonate test – is a standard analytical sequence in chemistry.
这种两步测试——先测 pH 指示再作碳酸盐检验——是化学中标准的分析顺序。
10. Case 3: Burning a Candle | 案例三：蜡烛燃烧
A candle is placed on a digital balance, and its mass is recorded as 50.0 g. The candle is lit and allowed to burn for ten minutes. The balance reading drops steadily to 47.5 g. Some students claim that mass has been destroyed. As a scientist, how would you investigate this claim?
一支蜡烛被放在数字天平上，其质量记录为 50.0 克。点燃蜡烛并让它燃烧十分钟。天平读数稳步下降至 47.5 克。有些学生声称质量被消灭了。作为一名科学家，你会如何探究这一说法？
This case study helps clarify the difference between open and closed systems and introduces the concept of gas products being invisible but having mass.
这个案例有助于澄清开放系统与封闭系统的区别，并引入产物气体虽然看不见但具有质量的概念。
11. Observing Mass Change | 观察质量变化
When wax (a hydrocarbon) burns, it reacts with oxygen from the air to produce carbon dioxide gas and water vapour. Both products are colourless gases that escape into the surroundings. The mass on the balance decreases because these gaseous products are not being collected.
当蜡（一种碳氢化合物）燃烧时，它与空气中的氧气反应，生成二氧化碳气体和水蒸气。这两种产物都是无色气体，会逸散到周围环境中。天平显示的质量下降是因为这些气体产物没有被称量在内。
If we were to capture all the gases produced – for example, by burning the candle inside a sealed container with a supply of oxygen – the total mass inside the container would stay the same. However, the experiment is tricky because oxygen is consumed and gases are produced.
如果我们能够捕获所有产生的气体——例如，在一个有氧气供应的密封容器内燃烧蜡烛——容器内的总质量将会保持不变。然而，这个实验有难度，因为氧气被消耗并且气体被产生。
The simple combustion reaction for candle wax can be represented as:
CₓHᵧ + O₂ → CO₂ + H₂O
where CₓHᵧ stands for the hydrocarbon molecules in wax. The actual composition is complex, but the principle holds.
其中 CₓHᵧ 代表蜡中的碳氢化合物分子。实际组成很复杂，但原理是成立的。
The decrease from 50.0 g to 47.5 g means 2.5 g of mass ‘disappeared’ from the balance; in reality, that mass left the system as gases. This is a demonstration of the need for careful accounting of all reactants and products.
从 50.0 克减少到 47.5 克，意味着有 2.5 克质量“消失”在天平上；但实际上，这些质量以气体形式离开了系统。这演示了仔细核算所有反应物和产物的必要性。
12. Linking to Conservation of Mass | 质量守恒的联系
The law of conservation of mass states that in a chemical reaction, mass is neither created nor destroyed. The burning candle appears to violate this rule only because we are not measuring the mass of the gases. When we include the escaped gases, the total mass before and after burning is exactly the same.
质量守恒定律指出，在化学反应中，质量既不会被创造也不会被消灭。燃烧的蜡烛看似违反这条规则，只是因为我们没有测量气体的质量。当我们把逸散的气体质量计算在内时，燃烧前后的总质量是完全相同的。
To prove this in a school laboratory, we can modify the experiment: set up a flask with soda lime to absorb the CO₂ and a drying agent to trap water vapour. As the candle burns inside, the total mass of the sealed system does not change. This reinforces the particle model – atoms are rearranged, not lost.
为了在学校实验室验证这一点，我们可以改进实验：设置一个烧瓶，放入碱石灰吸收二氧化碳，并用干燥剂捕集水蒸气。当蜡烛在内部燃烧时，密封系统的总质量不会改变。这巩固了粒子模型——原子只是被重新排列，而非丢失。
Understanding this concept also helps explain why rusting iron gains mass (it combines with oxygen) and why burning magnesium ribbon gains mass as it becomes magnesium oxide. The mass change simply reflects which substances we are including in our measurement.
理解这一概念也有助于解释为什么铁生锈后质量增加（铁与氧结合），以及为什么镁条燃烧后质量增加变成氧化镁。质量的变化仅仅反映了我们在测量中包含了哪些物质。
So, the mystery is solved: mass is conserved, and the balance reading alone does not tell the whole story. Always consider any invisible products that may escape or join during a reaction.
于是谜团解开了：质量是守恒的，单靠天平读数是不能说明全部情况的。永远要考虑反应中可能逸散或参与进来的不可见产物。
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