Variables and Controls in Scientific Experiments | 科学实验中的变量与控制

📚 Variables and Controls in Scientific Experiments | 科学实验中的变量与控制

In any scientific experiment, understanding the role of variables and controls is essential to obtaining valid and reliable results. This guide explains the different types of variables, how to control them, and how to design a fair test — a core skill assessed across the Edexcel IGCSE Science (Biology, Chemistry, and Physics) practical examinations.

在任何科学实验中，理解变量与控制的作用是获得有效且可靠结果的关键。本指南将解释不同类型的变量、如何控制它们以及如何设计公平测试——这是 Edexcel IGCSE 科学（生物、化学和物理）实验考试中评估的核心技能。

1. Types of Variables | 变量的类型

In an experiment, we deliberately change one thing to observe its effect on another thing. The variable we change is called the independent variable; the variable we measure is called the dependent variable. All other variables that could affect the result are called control variables.

在实验中，我们有意改变一个因素来观察它对另一个因素的影响。我们改变的量称为自变量；我们测量的量称为因变量。所有其他可能影响结果的变量统称为控制变量。

Independent variable (IV) – the variable you change deliberately in order to test its effect. Example: the concentration of acid added to a reaction.

自变量（IV）——为了检验其影响而有意改变的量。例如：加入反应中的酸浓度。
Dependent variable (DV) – the variable you measure or observe. Example: the volume of gas produced per minute.

因变量（DV）——你测量或观察的量。例如：每分钟产生的气体体积。
Control variables – the variables you keep the same to ensure a fair test. Example: temperature, pressure, and the mass of reactants.

控制变量——你保持相同以确保公平测试的量。例如：温度、压力以及反应物的质量。

Remember the key question: “What am I changing?” (IV), “What am I measuring?” (DV), “What am I keeping the same?” (CV).

记住关键问题：”我在改变什么？”（自变量），”我在测量什么？”（因变量），”我保持什么不变？”（控制变量）。

2. Recognising Variables in Exam Scenarios | 在考试场景中识别变量

Edexcel IGCSE exam questions often give you a brief description of an experiment and ask you to identify the independent, dependent, and control variables. Read carefully and look for verbs such as ‘change’, ‘measure’, and ‘keep constant’.

Edexcel IGCSE 考试问题通常会给出实验描述，要求你识别自变量、因变量和控制变量。仔细阅读并寻找”改变””测量””保持不变”等关键词。

Example question: “A student investigates how the rate of photosynthesis in pondweed changes with light intensity. She places a lamp at different distances from the plant and counts the bubbles produced per minute. She keeps the temperature of the water constant.”

示例问题：“一名学生研究水蕴草的光合作用速率如何随光照强度变化而变化。她把灯放在离植物不同距离处，并计算每分钟产生的气泡数量。她保持水温恒定。”

Here, the IV is the light intensity (or distance of the lamp), the DV is the rate of photosynthesis (or bubbles per minute), and the control variable is the temperature.

在这里，自变量是光照强度（或灯的距离），因变量是光合作用速率（或每分钟气泡数），控制变量是温度。

Independent variable	Dependent variable	Control variables
Light intensity (distance)	Number of bubbles per minute	Temperature, volume of water, mass of pondweed
光照强度（距离）	每分钟气泡数量	温度、水的体积、水蕴草的质量

In the exam, always write the variables down clearly, using the exact terminology from the question where possible.

在考试中，务必清晰写出各变量，并尽可能使用题目中的准确术语。

3. Controlling Variables | 控制变量

Control variables must be kept constant throughout the experiment. If they change, they may affect the dependent variable independently, making your results invalid. For example, if you are testing how the concentration of hydrochloric acid affects the rate of reaction with magnesium, you must keep the temperature, the surface area of the magnesium, and the volume of the acid the same.

控制变量必须在整个实验过程中保持不变。如果它们发生变化，就可能独立影响因变量，使你的结果失效。例如，如果你测试盐酸浓度如何影响与镁的反应速率，你必须保持温度、镁的表面积以及酸体积相同。

Use water baths to maintain a constant temperature in biology and chemistry experiments.

使用水浴锅在生物和化学实验中维持恒定温度。
Use the same equipment (e.g., same balance, same thermometer) to avoid measurement errors.

使用相同设备（例如同一台天平、同一支温度计）以避免测量误差。
Use the same volume and concentration of solutions when only one solution is being tested.

使用相同体积和浓度的溶液当只测试一种溶液时。

If a control variable is difficult to control exactly, you should record its value and state how it might have affected the results in your evaluation section.

如果某个控制变量难以精确控制，你应该记录其数值，并在评估部分说明它可能如何影响结果。

4. Control Groups and Controlled Experiments | 对照组与对照实验

A control group is a set of experimental units that are not subjected to the treatment under investigation. It provides a baseline against which the experimental group can be compared. For example, in a plant growth experiment, the control group might receive no fertiliser, while the experimental group receives varying amounts of fertiliser.

对照组是指不经过所研究处理的实验单元组。它提供了与实验组进行比较的基线。例如，在植物生长实验中，对照组可能不施肥，而实验组接受不同量的肥料。

In contrast, a controlled experiment means that all variables except the independent variable are kept constant. The term ‘control’ can therefore refer either to a group or to a procedure. In exam answers, be precise: say ‘control group’ when you mean the baseline group, and ‘controlled variable’ when you mean a factor kept constant.

相比之下，对照实验意味着除自变量外的所有变量都保持不变。因此”对照”一词既可以指对照组，也可以指控制措施。在考试作答中，请精确表达：当指基线组时说”对照组”，当指保持不变的因素时说”控制变量”。

Control group ≠ Controlled variable
对照组 ≠ 控制变量

This distinction is frequently tested in Edexcel multiple-choice and short-answer questions.

这一区别在 Edexcel 的选择题和简答题中经常被考到。

5. Fair Testing | 公平测试

A fair test is one in which only the independent variable is changed and all other variables are controlled. If two variables change at the same time, you cannot be sure which one caused the observed effect — your conclusion will be invalid.

公平测试是指只改变自变量，而所有其他变量均受控制的测试。如果同时有两个变量发生变化，你就无法确定是哪一个导致了观察到的效果——你的结论将是无效的。

Example: To test the effect of pH on enzyme activity, you use buffer solutions at pH 5, 6, 7, and 8. You must keep the temperature at 37 °C, the enzyme concentration the same, and the substrate concentration the same.

示例：为了测试 pH 对酶活性的影响，你使用 pH 为 5、6、7 和 8 的缓冲液。你必须保持温度在 37 °C，酶的浓度和底物浓度相同。

In your method, explicitly state: “All factors were kept the same except the independent variable, ensuring a fair test.”

在方法中明确说明：”除自变量外，所有因素都保持相同，以确保公平测试。”

6. Risk Assessment and Safety | 风险评估与安全

Controlling an experiment also involves managing safety risks. For each hazardous chemical, sharp object, or heat source, identify the risk, describe the precaution, and state the action to take if an accident occurs.

控制实验还涉及管理安全风险。对于每种危险化学品、尖锐物品或热源，识别风险，描述预防措施，并说明发生事故时应采取的行动。

Hazard	Risk	Precaution	Emergency action
Dilute hydrochloric acid	Skin irritation	Wear safety goggles and gloves	Rinse with water for 10 minutes
稀盐酸	皮肤刺激	佩戴护目镜和手套	用水冲洗 10 分钟

Learn common hazards: corrosive (acids/alkalis), flammable (ethanol), toxic (some metal salts), irritant, and hazardous to the environment. The exam may ask you to state the safety precaution for a given experiment.

学习常见危险标识：腐蚀性的（酸/碱）、易燃的（乙醇）、有毒的（某些金属盐）、刺激性的以及对环境有害的。考试可能要求你说明给定实验的安全预防措施。

7. Table of Results | 结果表格

When recording data, use a clear table with the independent variable in the left-hand column and the dependent variable in the right-hand column. Include units in the column heading, not in every cell.

记录数据时，使用清晰的表格，自变量放在左列，因变量放在右列。单位写在列标题中，而不是每个单元格中。

Distance of lamp / cm	Bubbles per minute (Trial 1)	Bubbles per minute (Trial 2)	Mean
10	42	44	43
20	31	33	32
30	21	23	22

Repeat measurements at least twice, calculate a mean, and check for any anomalous data points (outliers). If a value looks anomalous, investigate whether the equipment was faulty or whether a mistake was made.

至少重复测量两次，计算平均值，并检查是否有异常数据点（离群值）。如果某个数值看起来异常，应调查设备是否故障或操作是否有误。

8. Graphs and Data Interpretation | 图表与数据解读

When plotting a graph, place the independent variable on the x-axis and the dependent variable on the y-axis. Use a sharp pencil, choose an appropriate scale, and label both axes with quantities and units.

绘制图表时，将自变量放在 x 轴，因变量放在 y 轴。使用削尖的铅笔，选择适当的比例尺，并在两轴上标注物理量和单位。

Line of best fit – draw a smooth line (or a straight line) that follows the overall trend, ignoring minor scatter.

最佳拟合线——绘制一条跟随整体趋势的平滑线（或直线），忽略细微的散点偏离。
Curved graph – suggests a non-linear relationship, e.g., radiation intensity against distance.

曲线图——表明非线性关系，例如辐射强度与距离的关系。
Straight-line graph through the origin – indicates direct proportionality.

通过原点的直线图——表明正比例关系。

Rate = Δy ÷ Δx (gradient of the line)
速率 = Δy ÷ Δx（直线的斜率）

When interpreting a graph, always describe the trend in words before quoting any numerical values. Use phrases like “as the independent variable increases, the dependent variable increases then plateaus”.

解读图表时，先口头描述趋势，再引用数值。使用类似”随着自变量增加，因变量先增加后趋于平缓”的表述。

9. Making a Conclusion | 得出结论

A conclusion must link the dependent variable to the independent variable, using your data as evidence. State whether the results support any scientific theory, and quote specific data points from the table or graph.

结论必须将因变量与自变量联系起来，并以数据为证据。说明结果是否支持某个科学理论，并引用表格或图表中的具体数据点。

Weak conclusion: “The rate of reaction increases with temperature.”

较弱的结论：“反应速率随温度升高而增加。”

Strong conclusion: “The rate of reaction increases as the temperature is raised from 20 °C to 50 °C. For example, at 20 °C the mean rate was 0.12 g/s, while at 50 °C it was 0.87 g/s. This is because particles gain kinetic energy, leading to more frequent and more energetic collisions, so a higher proportion of collisions exceed the activation energy.”

较强的结论：“随着温度从 20 °C 升高到 50 °C，反应速率增加。例如，在 20 °C 时平均速率为 0.12 g/s，而在 50 °C 时为 0.87 g/s。这是因为颗粒获得动能，导致更频繁、更剧烈的碰撞，因此更高比例的碰撞超过活化能。”

Notice how the strong conclusion uses scientific reasoning and specific data. In the exam, include both biology/chemistry/physics theory and your experimental observations.

注意较强结论使用了科学推理和具体数据。在考试中，既要包括生物/化学/物理理论，也要包括你的实验观察。

10. Common Mistakes to Avoid | 常见错误与避坑指南

Students frequently lose marks in practical exams by confusing variables, failing to control them, or drawing graphs incorrectly. Here are the most common pitfalls:

学生在实验考试中经常因混淆变量、未能控制变量或绘图错误而失分。以下是最常见的陷阱：

Mistake 1: Writing “distance” instead of “distance of the lamp from the plant” – always state which quantity and between which objects.

错误 1：只写”距离”而不写”灯到植物之间的距离”——始终指明哪个量以及在哪两个物体之间。
Mistake 2: Changing the independent variable and a control variable at the same time. For example, changing both temperature and concentration simultaneously makes the test unfair.

错误 2：同时改变自变量和一个控制变量。例如，同时改变温度和浓度会使得测试不公平。
Mistake 3: Forgetting to include units in tables or on graph axes. Always write “cm³”, “g”, “°C”, “s”, or “min” in the heading.

错误 3：忘记在表格或坐标轴上写单位。始终在标题中写”cm³””g””°C””s”或”min”。
Mistake 4: Drawing a bar chart when a line graph is required. A line graph is appropriate for continuous data, e.g., temperature or time.

错误 4：当需要折线图时却画柱状图。折线图适用于连续数据，例如温度或时间。
Mistake 5: Using ‘heating’ as a general word when ‘temperature’ is the exact variable. Be specific in your scientific vocabulary.

错误 5：用泛泛的”加热”代替准确的”温度”变量。要使用准确的科学词汇。

Avoiding these mistakes will significantly improve your marks in the practical-based questions.

避免这些错误将显著提高你在实验类题目中的得分。

11. Exam-Style Question Walkthrough | 真题例题演练

Question (Edexcel IGCSE Biology-style): A student sets up four test tubes containing starch solution and amylase. She places tube A in an ice bath at 0 °C, tube B in water at 20 °C, tube C in water at 40 °C, and tube D in water at 60 °C. Every 30 seconds, she removes a drop of mixture and tests it with iodine solution.

题目（Edexcel IGCSE 生物风格）：一名学生设置四支装有淀粉溶液和淀粉酶的试管。她将试管 A 放入 0 °C 冰浴中，试管 B 放入 20 °C 水中，试管 C 放入 40 °C 水中，试管 D 放入 60 °C 水中。每 30 秒取出混合物一滴，用碘液测试。

(a) Identify the independent and dependent variables.
Independent: temperature. Dependent: time taken for the iodine test to stop turning blue-black (or the rate of starch digestion).

(a) 识别自变量与因变量。
自变量：温度。因变量：碘液不再变蓝黑色所需的时间（或淀粉消化速率）。

(b) State two control variables.
Control 1: the same volume of starch solution. Control 2: the same concentration of amylase.

(b) 写出两个控制变量。
控制 1：相同体积的淀粉溶液。控制 2：相同浓度的淀粉酶。

(c) Explain why the student repeats the experiment at each temperature.
To check for anomalous results and calculate a mean, which increases the reliability of the data.

(c) 解释为什么学生在每个温度下重复实验。
为了检查异常结果并计算平均值，从而提高数据的可靠性。

Practising with past papers will help you become familiar with the phrasing and expectations of the examiner.

多练习历年真题有助于你熟悉题型表述和考官的要求。

12. Summary Checklist | 总结清单

Before you submit any practical answer, quickly run through this checklist:

在提交任何实验题答案前，快速检查以下清单：

Have I stated the IV clearly, with enough detail?

我是否清楚且足够详细地陈述了自变量？
Have I stated the DV including the measuring instrument and unit?

我是否陈述了因变量及其测量仪器和单位？
Have I listed at least two relevant control variables?

我是否至少列出了两个相关的控制变量？
Have I repeated measurements and calculated a mean?

我是否重复了测量并计算平均值？
Is my graph axis labelled with quantity and unit?

我的图表坐标轴是否标注了物理量和单位？
Does my conclusion use data and scientific theory?

我的结论是否使用了数据和科学理论？

Mastering variables and controls is the foundation of all practical science at IGCSE. With regular practice, you can turn this area from a source of confusion into a reliable source of marks.

掌握变量与控制是 IGCSE 所有实验科学的基础。通过定期练习，你可以将这一部分从困惑之源转化为稳定的得分点。

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Variables and Controls in Scientific Experiments | 科学实验中的变量与控制