📚 Year 7 Cambridge Science: Case Study Practical Workout | Year 7 Cambridge 科学：案例分析实战演练
Welcome to this case study workout designed for Year 7 Cambridge Science learners. Here we will walk through a complete scientific investigation just as you would in your own school laboratory. You will practise identifying variables, planning a fair test, recording results, drawing graphs, writing conclusions and evaluating an experiment. All of these skills are assessed in the Cambridge Lower Secondary Science Stage 7 curriculum and will build your confidence in tackling practical-based questions.
欢迎来到本次为 Year 7 Cambridge 科学学习者设计的案例分析实战演练。我们将一起走完一次完整的科学探究过程，就像你在学校实验室中要做的一样。你将练习识别变量、设计公平测试、记录结果、绘制图表、写作结论以及评价实验。所有这些技能都是 Cambridge 初中科学 Stage 7 课程考查的内容，并将帮助你建立应对实操类题目的信心。
1. The Investigation Scenario | 探究情境
Lily, a Year 7 student, wants to know how the temperature of water affects how quickly salt dissolves. She sets up an experiment using four beakers, each containing 200 cm³ of water at a different temperature: 20°C, 35°C, 50°C, and 65°C. Into each beaker she adds exactly 10 g of fine table salt. She stirs each mixture gently with a glass rod and uses a stopwatch to time how long it takes for the salt to disappear completely. She repeats the whole experiment three times for reliability.
Year 7 学生 Lily 想知道水温会如何影响食盐溶解的速率。她搭建了一个实验，使用了四个烧杯，每个烧杯装有 200 cm³ 温度不同的水：20°C、35°C、50°C 和 65°C。她在每个烧杯中加入正好 10 g 的精细食盐。她一边用玻璃棒轻轻搅拌，一边用秒表记录食盐完全消失所需的时间。为了获得可靠的数据，她将整个实验重复做了三次。
2. Identifying the Variables | 识别变量
Before analysing Lily’s results, we must be clear about which factor is changed, which is measured, and what must be kept the same. The independent variable is the one she deliberately changes – the temperature of the water. The dependent variable is the one she measures – the time taken for the salt to dissolve. Controlled variables include the volume of water (200 cm³), the mass of salt (10 g), the type and grain size of salt, and the stirring method. Keeping these constant ensures a fair test.
在分析 Lily 的结果之前，我们必须清楚哪一个因素是被改变的，哪一个是被测量的，以及哪些必须保持不变。自变量是她有意改变的量——水的温度。因变量是她测量的量——食盐溶解所需的时间。控制变量包括水的体积（200 cm³）、食盐的质量（10 g）、食盐的种类和颗粒大小，以及搅拌的方式。保持这些因素不变才能保证公平测试。
- Independent variable: temperature of water (°C) | 自变量：水的温度（°C）
- Dependent variable: time taken to dissolve (s) | 因变量：溶解所需的时间（秒）
- Controlled variables: volume of water, mass of salt, type of salt, stirring | 控制变量：水的体积、食盐质量、食盐种类、搅拌方式
3. Designing a Fair Test | 设计公平测试
A fair test means changing only the independent variable and keeping everything else identical. Lily used the same type of salt and the same volume of water in every beaker. She stirred each one in the same way, moving the glass rod in small circles at the same speed. She also repeated the experiment three times, which helps to spot anomalies and calculate an average. Without these precautions, the results could not be trusted to answer her question.
公平测试意味着只改变自变量，其余条件完全相同。Lily 在每一个烧杯中都使用了相同的食盐种类和水体积。她用相同的方式搅拌，即以相同的速度用玻璃棒做小圈搅动。她还把实验重复了三次，这有助于发现异常值并计算平均值。没有这些预防措施，结果就无法可靠地回答她的问题。
4. Collecting the Raw Data | 收集原始数据
Here is the data Lily recorded in her notebook. Notice that each trial is listed separately, and the units are always given in the column headings. The temperatures were checked with a thermometer before adding the salt.
下面是 Lily 在记录本上记录的数据。请注意每一次实验都被单独列出，单位总是写在表格的标题行中。在加入食盐前，所有温度都先用温度计检查过了。
| Temperature (°C) | Trial 1 (s) | Trial 2 (s) | Trial 3 (s) |
|---|---|---|---|
| 20 | 128 | 135 | 131 |
| 35 | 82 | 79 | 84 |
| 50 | 54 | 51 | 53 |
| 65 | 38 | 41 | 39 |
5. Calculating Averages and Spotting Anomalies | 计算平均值并发现异常
Now we need to calculate the mean time for each temperature. Add the three trial times and divide by 3. Lily has done this already. She also checked for any results that look very different from the others – anomalies. For example, at 20°C the three times are close together, so there is no obvious anomaly. At 35°C, all times are within 5 seconds of each other, which looks consistent. The most important reason for calculating an average is to make the results more reliable than any single trial.
现在我们需要计算每个温度下的平均时间。把三次实验的时间相加，再除以 3。Lily 已经做好了这些计算。她还检查了是否有些结果看上去和其他数据相差很大——也就是异常值。例如在 20°C 下，三次时间都很接近，因此没有明显的异常值。在 35°C 下，所有时间之间相差不超过 5 秒，看起来很一致。计算平均值最重要的理由是让结果比任何一次单独实验都更加可靠。
Mean time = (Trial 1 + Trial 2 + Trial 3) ÷ 3
Mean at 20°C: (128 + 135 + 131) ÷ 3 = 131.3 s (we can round to 131 s) | 20°C 下的平均值：(128 + 135 + 131) ÷ 3 = 131.3 秒 (约 131 秒)
Mean at 35°C: (82 + 79 + 84) ÷ 3 = 81.7 s ≈ 82 s | 35°C 下的平均值：(82 + 79 + 84) ÷ 3 = 81.7 秒 ≈ 82 秒
Mean at 50°C: (54 + 51 + 53) ÷ 3 = 52.7 s ≈ 53 s | 50°C 下的平均值：(54 + 51 + 53) ÷ 3 = 52.7 秒 ≈ 53 秒
Mean at 65°C: (38 + 41 + 39) ÷ 3 = 39.3 s ≈ 39 s | 65°C 下的平均值：(38 + 41 + 39) ÷ 3 = 39.3 秒 ≈ 39 秒
6. Drawing a Line Graph | 绘制线形图
The best way to display a pattern in this type of data is a line graph. The independent variable (temperature) goes on the x-axis, and the dependent variable (average time) goes on the y-axis. Lily plotted four points and joined them with a smooth curve. She remembered to label both axes with the quantity and unit, and gave the graph a clear title.
要展示这类数据中的规律，最好的方法是绘制线形图。自变量（温度）放在 x 轴，因变量（平均时间）放在 y 轴。Lily 绘制了四个点，并用平滑曲线将它们连接起来。她记得在两条坐标轴上标明了物理量和单位，还为图表加上了清楚的标题。
Even without seeing the actual graph, we can describe the pattern: as temperature increases, the time taken for the salt to dissolve decreases. The line falls steeply at first and then begins to flatten, suggesting that at higher temperatures the rate of dissolving becomes less sensitive to further temperature increases.
即使没有看到图形的实际样子，我们也可以描述出其中的规律：随着温度升高，食盐溶解所需的时间减少。曲线一开始下降得很快，然后逐渐变得平缓，这表明在较高的温度下，溶解速率对温度的进一步升高不再那么敏感了。
7. Writing a Scientific Conclusion | 撰写科学结论
A strong conclusion states what you have found, supports it with data, and links back to the original question. Lily wrote: ‘The higher the temperature of the water, the faster the salt dissolved. At 20°C it took an average of 131 s, while at 65°C it took only 39 s. This supports my hypothesis that temperature affects the rate of dissolving.’ Notice that she used the word ‘rate’, which refers to how much solute dissolves per second, and that she compared actual numbers instead of just writing ‘it got faster’.
一个完美的结论需要说明你发现了什么、用数据支持、并回应最初的问题。Lily 写道：“水温越高，食盐溶解得越快。在 20°C 时，溶解平均需要 131 秒，而在 65°C 时只需 39 秒。这支持了我的假设，即温度会影响溶解速率。”注意她使用了“速率”这个词，指的是每秒溶解多少溶质，而且她比较了实际数字，而不仅仅是写“它变快了”。
In Cambridge Science, always refer back to the independent and dependent variables. Do not just say ‘the results proved it correct’, because science rarely ‘proves’ anything – it provides evidence.
在 Cambridge 科学课程中，一定要回扣自变量和因变量。不要只说“结果证明它正确”，因为科学几乎没有“证明”什么——科学提供的是证据。
8. Explaining the Science | 解释科学原理
When water is heated, its particles gain kinetic energy and move faster. Salt crystals are held together by ionic bonds between sodium (Na⁺) and chloride (Cl⁻) ions. The faster-moving water particles collide with the surface of the salt more frequently and with more energy, pulling the ions away into the solution more quickly. This is why at 65°C the dissolving time is much shorter than at 20°C. However, stirring and crushing the salt would also increase the rate because they increase the surface area exposed to water particles.
当水被加热时，水分子获得动能，运动得更快。食盐晶体由钠离子 (Na⁺) 和氯离子 (Cl⁻) 之间的离子键结合在一起。运动更快的水分子会更频繁、更有力地与食盐表面碰撞，从而更快地将离子拉入溶液。这就是为什么在 65°C 时溶解时间比在 20°C 时短得多的原因。然而，搅拌和把盐压碎也能增加速率，因为它们增大了与水分子接触的表面积。
9. Evaluating the Experiment | 评价实验
Lily’s method was reasonable, but there are always improvements to be made. She used the same stirring technique, but it was difficult to keep the speed exactly the same each time. A magnetic stirrer would be more reliable. She also only used four temperature points; more temperatures, especially between 20°C and 35°C, would give a more detailed trend. The thermometer might have had a reading error of ±0.5°C, adding a small uncertainty. In a full evaluation you must also discuss whether the repeats were close together, which they were, confirming good precision.
Lily 的方法是合理的，但总有可以改进的地方。她使用了相同的搅拌手法，但要使每次搅拌的速度完全一样是比较困难的。使用磁力搅拌器会更加可靠。她还只选用了四个温度点；如果能多几个温度，特别是在 20°C 与 35°C 之间多测几个点，就能得到更加细致的趋势。温度计可能会有 ±0.5°C 的读数误差，这增加了一点点不确定性。在完整的评价中，你还必须讨论重复实验的数据是否接近——它们确实是接近的，这证实了良好的精密度。
10. Handling Anomalous Results and Reliability | 处理异常结果与信度
Imagine that Lily had recorded a dissolving time of 160 s in one trial at 35°C. This would clearly not fit the pattern and could be an anomaly. She should have repeated that trial to check. Anomalies can occur because salt grains were not fully separated before adding, or because the stopwatch was started too late. Repeating the whole experiment three times improves reliability. To increase reliability further, more repeats or a different investigator could be used.
想象一下，如果 Lily 在 35°C 的一次试验中记录到 160 秒的溶解时间，这显然不符合整体规律，就可能是一个异常值。她应该重做那次试验以作检查。异常值的出现可能是因为食盐颗粒在加入前没有完全散开，或者是秒表启动得晚了。将整个实验重复三次提高了信度。为了进一步提高信度，可以增加重复次数，或请另一位研究者操作。
11. Presenting Results in a Scientific Report | 在科学报告中展示结果
A full investigation report should include a title, aim, hypothesis, materials list, method, results table, graph, conclusion and evaluation. In your Cambridge checkpoints and progression tests, you might be asked to write parts of this report, identify mistakes in someone else’s method, or suggest improvements to their data presentation. Always use clear headings, correct units, and avoid personal language like ‘I saw’ – instead write ‘It was observed’.
一份完整的探究报告应该包含标题、目的、假设、材料清单、方法、结果表格、图表、结论和评价。在你的 Cambridge checkpoint 和 progression 考试中，你可能需要撰写这份报告的部分内容，找出别人方法中的错误，或对数据呈现方式提出改进建议。一定要使用清晰的标题和正确的单位，避免使用“我看到”这样的个人化语言——而要写成“观察到”。
12. Applying the Skills to a New Case | 将技能应用于新案例
Now try applying these skills to a fresh scenario: Jamie wants to investigate how the length of a simple pendulum affects the time for one complete swing (the period). He plans to change the length of the string while keeping the mass of the bob and the angle of release the same. Identify the independent, dependent and controlled variables. Describe how you would make sure the experiment is a fair test, what safety precautions you would take, and how you would record and display the data. Practising with different contexts will sharpen your scientific reasoning and prepare you for any practical-based exam question.
现在试着把这些技能应用到一个新的情境中：Jamie 想研究单摆的长度如何影响其完成一次完整摆动所需的时间（周期）。他打算改变绳子的长度，但保持摆锤的质量和释放角度不变。请你识别出这里的自变量、因变量和控制变量。请描述你将如何确保实验是一个公平测试，你会采取哪些安全防范措施，以及你将如何记录和展示数据。在不同的情境中练习可以磨炼你的科学推理能力，并为你应对任何实操类考题做好准备。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导