📚 Year 8 WJEC Chemistry: Case Study Practical Drills | 八年级WJEC化学：案例分析实战演练
Welcome to a series of real-world chemistry case studies designed to sharpen your problem-solving skills for Year 8 WJEC chemistry. Each scenario is built around key topics from the curriculum, including states of matter, separation techniques, chemical reactions and the rock cycle. You will be guided through the thinking steps needed to analyse data, identify patterns and apply scientific knowledge. Remember that in WJEC assessments, being able to explain the 'why' behind an observation is just as important as giving the right answer.
欢迎来到一系列真实世界的化学案例分析，这些案例旨在提升你八年级WJEC化学的解题能力。每个场景都围绕课程的核心主题构建，包括物质状态、分离技术、化学反应和岩石循环。你将跟随引导的思维步骤分析数据、识别规律并应用科学知识。记住，在WJEC测评中，能够解释观察现象背后的‘为什么’与给出正确答案同样重要。
1. The Disappearing Puddle | 消失的水坑
After a summer rain shower, a puddle on the pavement gradually shrinks and eventually disappears, even though the temperature is well below boiling point. Your task is to explain what happens to the water particles during this process, and why the puddle vanishes faster on a hot, windy day.
夏日阵雨后，人行道上的水坑逐渐缩小并最终消失，即使温度远低于沸点。你的任务是解释在这个过程中水粒子发生了什么，以及为什么在炎热有风的日子里水坑消失得更快。
In the liquid state, water particles are close together but can move around each other. Particles near the surface gain enough energy from the surroundings to overcome the attractive forces holding them in the liquid. They escape into the air as a gas – this is evaporation. On a hot day, more particles have sufficient energy to escape. Wind removes the water vapour from just above the surface, allowing more particles to leave according to the particle model.
在液态下，水粒子紧密排列但能相互移动。表面附近的粒子从环境中获得足够能量，克服了将其束缚在液体中的吸引力。它们以气体形式逸出到空气中——这就是蒸发。在热天，更多的粒子有足够能量逃逸。风将水蒸气从表面上方带走，根据粒子模型，这使得更多粒子能够离开。
No new substance is formed during evaporation; it is a physical change. The particle model helps us understand why liquids evaporate faster when the temperature is higher and air movement is greater.
蒸发过程中没有新物质生成；这是一种物理变化。粒子模型帮助我们理解为什么在温度更高、空气流动更大时液体蒸发得更快。
2. Separating Sand and Salt | 分离沙子和盐
You are given a mixture of sand and table salt. The sand is insoluble in water, while salt dissolves easily. Devise a plan to obtain pure, dry samples of both sand and salt using common laboratory equipment. Explain the scientific principles behind each step.
给你一份沙子和食盐的混合物。沙子不溶于水，而盐容易溶解。设计一个方案，使用普通实验室设备获得纯净干燥的沙子和盐样品。解释每一步背后的科学原理。
First, add water to the mixture and stir. The salt dissolves to form a salt solution, while the sand settles at the bottom because it does not dissolve. This relies on the difference in solubility. Next, pour the mixture through filter paper in a funnel. The sand is trapped as a residue, and the salt solution passes through as the filtrate. Filtration works because the filter paper has tiny pores that allow water and dissolved salt particles through but block the larger sand grains.
首先，向混合物中加水并搅拌。盐溶解形成盐水溶液，而沙子沉在底部，因为它不溶解。这依靠的是溶解度的差异。接着，将混合物通过漏斗中的滤纸倾倒过滤。沙子作为残渣被截留，盐溶液作为滤液通过。过滤的原理是滤纸有微小的孔隙，允许水和溶解的盐粒子通过，但阻挡了较大的沙粒。
To reclaim the salt, heat the filtrate in an evaporating dish. The water evaporates, leaving the salt behind as solid crystals. By using gentle heating near the end, you avoid spitting and obtain dry salt. Rinse the sand with a little distilled water and dry it in an oven or warm place. This sequence – dissolving, filtration and evaporation – is a classic method for separating a soluble solid from an insoluble one.
要回收盐，将滤液放在蒸发皿中加热。水蒸发后，盐以固态晶体形式留下。在接近结束时使用温和加热，可以避免飞溅并获得干燥的盐。用少量蒸馏水冲洗沙子，然后在烘箱或温暖处干燥。这个顺序——溶解、过滤和蒸发——是分离可溶性固体与不溶性固体的经典方法。
3. Making Pure Copper Sulfate Crystals | 制备纯硫酸铜晶体
Your teacher provides a beaker of blue copper sulfate solution that contains some insoluble impurities. You need to produce a sample of large, pure copper sulfate crystals. Explain how you would do this and why cooling is crucial for crystal formation.
老师提供了一杯含有一些不溶性杂质的蓝色硫酸铜溶液。你需要制备一份大颗粒的纯净硫酸铜晶体。解释你会如何操作，以及为什么冷却对晶体形成至关重要。
First, filter the solution to remove any undissolved dirt. Then heat the clear blue filtrate to evaporate some of the water until you have a hot, saturated solution – you can tell it is saturated when a small crystal dipped on a glass rod no longer dissolves. At this point, allow the solution to cool slowly. As the temperature drops, the solubility of copper sulfate decreases, so the dissolved particles come together in a regular, repeating pattern to form crystals.
首先，过滤溶液以除去所有未溶解的污垢。然后加热清澈的蓝色滤液，蒸发掉一部分水，直到得到一个热饱和溶液——当用玻璃棒蘸取的少量溶液冷却后有小晶体不再溶解时，就表明已饱和。此时，让溶液缓慢冷却。随着温度下降，硫酸铜的溶解度降低，溶解的粒子以规则、重复的模式聚集在一起形成晶体。
Slow cooling yields larger crystals because particles have time to arrange themselves neatly. If you cool the solution too quickly, small crystals form. Once crystallisation is complete, filter the crystals, wash with a little cold distilled water and dry them between filter papers. This technique is called crystallisation, and it works especially well for solids whose solubility changes significantly with temperature.
缓慢冷却能得到较大的晶体，因为粒子有时间整齐地排列。如果冷却过快，会形成小晶体。结晶完成后，过滤出晶体，用少量冷蒸馏水洗涤，在滤纸间干燥。这种方法称为结晶，特别适用于溶解度随温度变化显著的固体。
4. Ink Chromatography Mystery | 墨水色谱谜题
A crime scene note has been written with a black pen. The forensic scientist suspects the ink is a mixture of several coloured dyes. Using paper chromatography, how can you identify which dyes are present? You are given water as the solvent and a strip of chromatography paper.
一份犯罪现场的字条是用黑色墨水笔写的。法医科学家怀疑这种墨水是几种有色染料的混合物。使用纸色谱法，你如何鉴定存在哪些染料？你得到水作为溶剂和一张色谱纸条。
Draw a pencil baseline near one end of the paper and place a small dot of the black ink on it. Pencil is used because it is insoluble and will not move with the solvent. Dip the bottom of the paper into water, making sure the ink spot is above the water surface. As water travels up the paper by capillary action, it carries the ink components at different speeds. A dye that is more soluble in water and less attracted to the paper will travel further.
在纸的一端附近画一条铅笔基线，并在其上点一小点黑色墨水。使用铅笔是因为它不溶，不会随溶剂移动。将纸的底部浸入水中，确保墨点在水面之上。当水通过毛细作用上升时，它以不同的速度携带着墨水成分。一种在水中溶解度更高、对纸的吸引力较小的染料会移动得更远。
After the solvent front has risen near the top, remove the paper and mark the final position of the water. You will see several coloured spots at different heights. A black ink that separates into blue, red and yellow spots proves the ink is a mixture. This method, chromatography, separates substances based on their different solubilities and attractions to the stationary phase. It works for any coloured mixture that dissolves in the chosen solvent.
当溶剂前沿上升到接近顶部时，取出纸并标记水的最终位置。你会看到在不同高度有几个彩色斑点。一种分离出蓝色、红色和黄色斑点的黑色墨水证明该墨水是混合物。这种方法，色谱法，基于物质在溶剂中的溶解度和对固定相的吸引力不同来分离物质。它适用于任何能溶解在选定溶剂中的有色混合物。
5. Testing for Carbon Dioxide | 二氧化碳检验
When a student drops a piece of seashell (calcium carbonate) into a test tube of dilute hydrochloric acid, vigorous bubbling occurs. The gas produced is bubbled into limewater, and the clear limewater turns milky. What does this tell you about the gas, and what is the word equation for the reaction taking place?
当一名学生将一块贝壳（碳酸钙）放入盛有稀盐酸的试管中时，剧烈冒泡。将产生的气体通入石灰水中，澄清的石灰水变浑浊。这说明了气体的什么性质，以及发生反应的文字方程式是什么？
The production of bubbles indicates a gas is being released. The limewater test is specific for carbon dioxide (CO₂). When CO₂ reacts with calcium hydroxide dissolved in limewater, a solid precipitate of calcium carbonate forms, which appears as a milky suspension. This is a positive test for carbon dioxide. The roar of bubbles and milky limewater together prove that the carbonate in the shell reacts with acid to produce CO₂.
气泡的产生表明有气体释放。石灰水测试是二氧化碳（CO₂）的特征检验。当CO₂与溶解在石灰水中的氢氧化钙反应时，生成了碳酸钙的固体沉淀，表现为浑浊的悬浊液。这是二氧化碳的阳性检验。气泡和浑浊的石灰水共同证明贝壳中的碳酸盐与酸反应生成了CO₂。
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
碳酸钙 + 盐酸 → 氯化钙 + 水 + 二氧化碳
This type of reaction is called an acid-carbonate reaction. All carbonates fizz with acids and produce carbon dioxide gas, which can be confirmed using limewater. Remember that water is also formed, though it is not visible.
这种反应类型称为酸与碳酸盐的反应。所有碳酸盐遇酸会起泡并产生二氧化碳气体，这可以用石灰水来确认。记住同时也生成水，尽管水不可见。
6. Reaction of Magnesium with Acid | 镁与酸反应
A strip of shiny magnesium ribbon is added to a test tube containing dilute hydrochloric acid. The metal quickly disappears, and the tube feels warm. A lighted splint held to the mouth produces a squeaky pop sound. Identify the gas and write a word equation for the reaction.
将一条光亮的镁带加入盛有稀盐酸的试管中。金属迅速消失，试管摸起来热。将一根点燃的木条放在管口，发出‘噗’的一声爆鸣。鉴定这种气体并写出该反应的文字方程式。
The squeaky pop sound is the classic test for hydrogen gas. Hydrogen is a flammable gas that burns with oxygen in the air, producing a mini explosion and a pop. The warmth of the tube tells you the reaction is exothermic – it gives out energy to the surroundings.
‘噗’的一声爆鸣是氢气的经典检验。氢气是一种可燃气体，与空气中的氧气燃烧，产生小型爆炸和爆鸣声。试管发热告诉你该反应是放热的——它向周围释放能量。
Magnesium + hydrochloric acid → magnesium chloride + hydrogen
镁 + 盐酸 → 氯化镁 + 氢气
ctive metals with acids: metal + acid → salt + hydrogen. Not all metals react with acids; copper, for example, will not produce hydrogen because it is less reactive than hydrogen.
在该反应中，镁取代了酸中的氢，生成盐（氯化镁）和氢气。这是一般活泼金属与酸的反应模式：金属 + 酸 → 盐 + 氢气。并非所有金属都与酸反应；比如铜就不会产生氢气，因为它不如氢活泼。
7. Candle in a Jar | 罐中蜡烛
A small candle is fixed to a piece of cork floating in a tray of water. When a gas jar is lowered over the lit candle, the flame soon goes out, and water rises up inside the jar to about one‑fifth of its volume. Explain these two observations and what they reveal about the air.
一支小蜡烛固定在一块浮在水槽中的软木塞上。当一个集气瓶罩住点燃的蜡烛时，火焰很快熄灭，瓶内的水位上升至约瓶子体积的五分之一。解释这两个观察现象以及它们揭示了空气的什么性质。
The candle uses oxygen from the air in the jar to burn. When the oxygen is used up, the flame dies. Burning is a chemical reaction between the wax (hydrocarbons) and oxygen, producing carbon dioxide and water vapour. As the oxygen is consumed and converted into carbon dioxide, which dissolves slightly in water, the overall gas pressure inside drops. The external atmospheric pressure pushes water up into the jar to equalise the pressure.
蜡烛燃烧消耗了瓶中空气中的氧气。当氧气耗尽时，火焰熄灭。燃烧是蜡（碳氢化合物）与氧气之间的化学反应，生成二氧化碳和水蒸气。当氧气被消耗并转化为二氧化碳，而二氧化碳又少量溶于水时，瓶内总气压下降。外部大气压将水推入瓶中使压力平衡。
The water rises to roughly one‑fifth of the jar's volume, which corresponds to the proportion of oxygen in the air. This classic experiment indicates that air is approximately 20% oxygen and 80% other gases such as nitrogen. It shows that oxygen is necessary for combustion.
水位大约上升到瓶子体积的五分之一，这与空气中氧气的比例相对应。这个经典实验表明空气由大约20%的氧气和80%的其他气体（如氮气）组成。它表明氧气是燃烧所必需的。
8. Rock Cycle Story | 岩石循环故事
Follow the journey of a grain of sand from a weathered granite mountain. Over millions of years it becomes part of a sedimentary rock, and much later it is found in a metamorphic rock. Explain the processes that transformed it and name the rock types at each stage.
跟随一粒沙子从风化花岗岩山脉的旅程。经过数百万年，它成为沉积岩的一部分，很久以后又在一变质岩中被发现。解释转化过程，并命名每个阶段的岩石类型。
Granite is an igneous rock formed from cooled magma. Weathering breaks up the granite, and rivers carry the sand grains to the sea where they settle in layers. Over time, layers of sediment build up, and the pressure from above squeezes out water, while minerals crystallise between grains to cement them together. This compaction and cementation form sandstone, a sedimentary rock.
花岗岩是一种由冷却岩浆形成的火成岩。风化破坏了花岗岩，河流将沙粒搬运到海洋，在那里它们层层沉积。随着时间的推移，沉积层堆积，上方的压力挤出水分，同时矿物质在颗粒间结晶，将它们胶结在一起。这种压实和胶结作用形成了砂岩，一种沉积岩。
If the sandstone is buried deeper, heat and pressure cause the minerals to recrystallise without melting. The sand grains fuse together into interlocking crystals, producing quartzite, a metamorphic rock. If subjected to even greater heat and melting, it could become magma and eventually cool into new igneous rock, completing the cycle.
如果砂岩被埋得更深，热和压力会使矿物在未熔化的情况下重新结晶。沙粒融合成交锁的晶体，生成石英岩，一种变质岩。如果受到更大的热量并熔化，它可能变成岩浆，最终冷却成新的火成岩，完成循环。
9. Smelling Perfume Across the Room | 在房间另一头闻到香水
When someone opens a bottle of perfume at the front of the classroom, students at the back soon smell the fragrance. Explain this observation in terms of the particle model. Also suggest why the smell spreads faster if the room is warm.
当有人在教室前面打开一瓶香水时，后排的学生很快闻到香味。用粒子模型解释这一观察。同时说明为什么如果房间温暖，气味扩散得更快。
Perfume particles are in the liquid state inside the bottle, but some escape and become gas particles. These gas particles move in all directions at high speed, colliding with air particles and spreading out through the room. This random movement and mixing of particles is called diffusion. It does not require a draught; it happens because particles are constantly in motion.
香水粒子在瓶中处于液态，但有些逸出成为气态粒子。这些气态粒子以高速向各个方向运动，与空气粒子碰撞，并扩散到整个房间。这种粒子的随机运动和混合称为扩散。它不需要风；它之所以发生是因为粒子在不停地运动。
At a higher temperature, the perfume particles have more kinetic energy and move faster. They also collide more frequently with air particles, speeding up the spreading process. This is why diffusion is quicker on a warm day. The particle model explains smell, pollution spreading and even how tea infuses from a tea bag.
在较高温度下，香水粒子具有更多的动能，运动更快。它们也更容易与空气粒子碰撞，从而加快了扩散过程。这就是为什么在温暖的日子里扩散更快。粒子模型能解释气味的传播、污染的扩散，甚至茶叶如何从茶包中浸泡出来。
10. Pure or Impure? | 纯净还是混合物？
Two white solids, A and B, look identical. When heated, solid A melts sharply at exactly 80°C, while solid B softens over a range between 70°C and 85°C. Based on these measurements, which solid is most likely to be a pure compound, and why?
两种白色固体A和B外观相同。加热时，固体A在精确80°C时敏锐熔化，而固体B在70°C到85°C的范围内软化。根据这些测量，哪种固体最可能是一种纯净化合物，为什么？
A pure substance has a specific, fixed melting point because all its particles are identical and need the same amount of energy to overcome the forces holding them in the solid lattice. Solid A melts exactly at 80°C, behaving as a pure compound. In contrast, a mixture contains different particles that disrupt the regular arrangement, so melting occurs gradually over a range of temperatures. Solid B's behaviour indicates it is most likely impure – a mixture of substances.
纯净物质具有特定的固定熔点，因为其所有粒子都相同，需要相同的能量来克服将其束缚在固态晶格中的力。固体A在恰好80°C时熔化，表现为一种纯净化合物。相比之下，混合物含有不同的粒子，扰乱了规则排列，因此熔化发生在一个温度范围内。固体B的行为表明它很可能是不纯的——是一种混合物。
This principle applies to boiling points as well: pure water boils at 100°C at standard pressure, while salt water boils over a range above 100°C. Checking melting and boiling points is a useful way to assess purity in the laboratory.
这一原理同样适用于沸点：纯净水在标准压力下在100°C沸腾，而盐水在高于100°C的范围内沸腾。检查熔点和沸点是实验室中评估纯度的一种有用方法。
11. Cooling Curves and State Changes | 冷却曲线与状态变化
A student pours liquid stearic acid into a test tube and records its temperature every minute as it cools. The graph shows the temperature dropping, then remaining constant at 69°C for several minutes, before dropping again. Explain the flat section of the cooling curve using the particle model.
一名学生将液态硬脂酸倒入试管，并每分钟记录其冷却温度。曲线图显示温度下降，然后在69°C恒定了数分钟，之后再次下降。用粒子模型解释冷却曲线的平坦部分。
Stearic acid is a pure substance that changes from liquid to solid at its freezing point. During the flat part of the curve, the particles are losing energy but the temperature stays constant. The energy being removed is not lowering the kinetic energy of the particles; instead, it is being released as the particles arrange themselves into a fixed, regular solid lattice. This is the latent heat of freezing.
硬脂酸是一种纯净物质，在凝固点由液态变为固态。在曲线的平坦部分，粒子在失去能量，但温度保持不变。散失的能量并没有降低粒子的动能；相反，它在粒子排列成固定的、规则的固体晶格时释放出来。这是凝固潜热。
Once all the liquid has solidified, the temperature begins to fall again as the solid cools further. Flat plateaus on cooling or heating curves always indicate a change of state, where energy is used to break or form bonds rather than change temperature.
一旦所有液体都凝固，温度再次开始下降，固体进一步冷却。冷却或加热曲线上的平坦台阶总是表明状态变化，此时能量用于断裂或形成键，而非改变温度。
12. Acid, Alkali or Neutral? | 酸性、碱性还是中性？
Three colourless liquids are labelled X, Y and Z. You test each with red and blue litmus paper and universal indicator solution. The results are: X turns blue litmus red and gives a red‑orange colour with universal indicator; Y has no effect on litmus and gives green; Z turns red litmus blue and gives a violet‑purple colour. Identify each liquid as acidic, alkaline or neutral, and name a safety precaution when handling Y if you do not know what it is.
三种无色液体分别标记为X、Y和Z。你用红色和蓝色石蕊试纸以及通用指示剂溶液检测它们。结果如下：X使蓝色石蕊试纸变红，通用指示剂呈红橙色；Y对石蕊试纸无影响，呈绿色；Z使红色石蕊试纸变蓝，通用指示剂呈紫罗兰色。鉴定每种液体是酸性、碱性还是中性，
Published by TutorHao | Year 8 Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图