Primary Science: Tackling Control Set-up Questions like a Pro
- AGrader Learning Centre
- Sep 8
- 7 min read
If you’re a primary school student preparing for your science paper, chances are you’ve come across experimental design questions that ask you about control set-ups. For many students, this is one of the trickiest parts of experimental questions in a school examination. You might know the terms like independent and dependent variable, but when the paper asks, “What is the control set-up?” or “State the purpose of the control set-up,” panic often sets in.
Sound familiar? You’re not alone. Many students know the theory but struggle to answer the questions clearly and accurately. The good news is this: once you understand how to break down an experiment, identify the key variables, and design or explain a control set-up, you’ll be able to tackle these questions like a pro.
This article will walk you through everything you need to know, from understanding the aim of the experiment to spotting the control group and writing answers that earn you full marks.
Table of Contents:
Understanding Experimental Design Questions
Before we dive into control set-ups, let’s first revisit the basics of experimental design. In controlled experiments, your goal is to test how one factor (the changed variable) affects another (the measured variable). To do this properly, you need to know the key terms:
Changed variable (Independent variable): This is the factor you deliberately change in the experiment. For example, the number of fish in a bowl.
Measured variable (Dependent variable): This is the factor you observe or record. For example, the growth of water plants.
Constant variables: These are conditions you must keep the same across all set-ups to ensure fairness, such as the amount of water, sunlight, or type of plant used.
The aim of the experiment is often phrased as “To study how the changed variable affects the measured variable.” Understanding this simple structure will help you immediately spot what the experiment is trying to test.
What Is a Control Set-up?
Now, onto the heart of the matter. A control set-up is a set-up without the changed variable. It acts as a baseline or reference point, allowing you to compare and prove that any changes in the experimental results are only due to the changed variable and not to other factors.
Think of it as a “zero test.” If your experimental group shows a difference compared to the control, you can confidently say it was the independent variable that caused the change.
For example:
Aim: To study the effect of carbon dioxide given out by fish on the growth of water plants.
Changed variable: Number of fish (which affects the amount of carbon dioxide produced).
Measured variable: Growth of water plants.
Control set-up: A fishbowl with the same amount of water and plants, placed in the same spot, but without any fish.
Fair Test: In a fair test, only one variable is changed. Other variables are kept constant.
Without the control, you wouldn’t know if the plants grew due to the carbon dioxide from fish or simply from exposure to sunlight.
Why Is a Control Set-up Important?
Students often lose marks because they give vague answers like “To ensure a fair test.” While fairness is part of it, examiners want a specific explanation.
A good answer should clearly link the control to the purpose of the experiment. For instance:
“The control set-up is used to show that the growth of the water plants is only affected by the amount of carbon dioxide given out by the fish, and not by other factors.”
This kind of explanation not only secures the mark but also demonstrates that you understand the logic of scientific testing.
The Two Types of Control Set-up Questions
In most primary school science exams, control set-up questions fall into two categories:
Designing a Control Set-up
You’ll be asked to describe what the control set-up should look like.
To do this, you must first identify the changed variable. Then, design a similar set-up where everything is the same except the changed variable, which is removed.
Example:
Aim: To study the effect of fertilizer on plant growth.
Changed variable: Presence of fertilizer.
Control set-up: A plant with the same soil, sunlight, and water, but no fertilizer.
Stating the Purpose of a Control Set-up
You’ll be asked, “What is the purpose of Set-up C?” or “Why is this control set-up used?”
Here, you need to show that the control is used to compare results and confirm that changes are only due to the changed variable.
Example Answer: “The control set-up shows that the difference in plant growth is only due to the fertilizer and not other factors such as sunlight or water.”
Let’s explore how to approach and answer each type of question with an
example below!
Example 1: From the example below, we are asked to design a control set-up.
To do this, follow these two key steps:
Identify the changed variable.
Describe a similar set-up where all other variables are kept constant, but the changed variable is removed.
For this question:
Aim: To study the effect of carbon dioxide given out by fish on the growth of water plants.
Changed Variable (Independent Variable): Number of fish, which determines the amount of carbon dioxide released.
Constant Variables:
Number of water plants
Location of fish bowls (next to the window)
Volume of water in the fish bowl
Designing the Control Set-up:
The control set-up should remove the changed variable (the fish) while keeping all other conditions the same.
Therefore, the control set-up will be an identical fish bowl with the same volume of water and number of water plants, placed in the same location, but without any fish inside.
Example 2: Now let’s look at the same example, but this time with a different type of question.
Question: What is the purpose of Set-up C?
Observation: Set-up C has the same volume of water, the same number of plants, and is placed in the same location as Set-ups A and B. The only difference is that it contains no fish.
Analysis: Since there are no fishes, the changed variable (number of fish, which affects the amount of carbon dioxide released) is removed.
Conclusion: With the changed variable removed, Set-up C serves as a control set-up. Its purpose is to ensure that the measured variable (growth of water plants) is affected only by the carbon dioxide given out by the fish, and not by any other factors.
Important Tip: Many students make the mistake of writing vague answers like “to ensure a fair test.” This will not earn full marks. Instead, always explain clearly how the control set-up allows for comparison and proves that the changes in the measured variable are caused only by the changed variable.
Step-by-Step Strategy to Answer Control Set-up Questions
When you see these questions in your school examination, follow this structured approach:
Read the aim of the experiment carefully.
It tells you the relationship between the independent and dependent variables.
Identify the changed variable.
What is different between the set-ups?
Example: Amount of light.
Identify the measured variable.
What is being recorded or observed?
Example: Number of oxygen bubbles released.
Check the constants.
Ensure all other factors are the same (water volume, plant type, etc.).
Design or explain the control set-up.
Remove the changed variable while keeping everything else constant.
Example: A plant placed in darkness (no light).
Write your answer clearly.
Use full sentences and connect the control to the measured outcome.
Common Mistakes to Avoid
Even when students know the theory, small mistakes can cost marks. Here are some to watch out for:
Being too vague. Simply writing “To ensure a fair test” is not enough. Always explain what the control proves.
Forgetting constants. If your control set-up changes more than one variable, it’s invalid.
Confusing changed and measured variables. Remember: you change the independent variable, you measure the dependent variable.
Not answering the question asked. If the question asks for the purpose, don’t just describe the set-up. Tailor your answer.
At first glance, experimental design questions on control set-ups can seem intimidating. But once you understand the logic behind controlled experiments, they become straightforward. By identifying the independent and dependent variables, carefully designing the control group, and writing clear explanations, you’ll be ready to answer the questions with confidence.
So, the next time your exam paper asks, “What is the control set-up?” or “State the purpose of Set-up C,” you won’t panic. Instead, you’ll approach it step by step and tackle it like a pro.
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