Questions on Experiments for PSLE

“What is the aim of the experiment” is a common question in the PSLE. Questions involving experiments take up roughly 9% of the marks. It requires analysis and interpretation of results, which are process skills that will be tested, yet many textbooks and guidebooks fail to address these type of questions. Here I’ll give a quick rundown of the technique to answer these questions.

What is an experiment?

In the PSLE, every experiment is a scientific procedure to test a hypothesis. It is part of the scientific method (which I won’t go into detail here).

Every experiment must have at least 2 set-ups. There must be one changed variable (known scientifically as the independent variable) which is the variable that is being tested. You can identify what it is in a few ways:

  • sometimes it’s stated in the question. eg. “Peter was trying to find out how sunlight affects the growth of the plants”. In this case, the changed variable is the presence of sunlight.
  • sometimes you infer from the set-ups. What is changed between the setups?

There may be more than one measured variable, but in PSLE, it’s usually only one. The measured variable is something measured to find out the effect of changing the variable. Sometimes the effect is something that you can measure directly, but in others, the effect is measured by something else. ¬†For example, the effect could be “growth of plants”. You can pick the height of the plant, mass of the plant, number of leaves, as the measured variable. Another example is the “rate of photosynthesis”, which is usually measured by counting the number of oxygen bubbles in a set period.

It is important to distinguish between cause and effect. The changed variable is the cause of the effect (which is measured with the measured variable).

With this knowledge, answering questions on experiments are much easier.

“What is the aim of the experiment?”

To find out how the [changed variable] affects the [effect / measured variable]

For example, an experiment may be carried out where a torch is placed at several distances from a submerged water plant, and the number of bubbles given off in 1 minute are counted for each distance. The changed variable is the “intensity of light” while the effect is the “rate of photosynthesis”, so the aim of the experiment is “to find out how the intensity of light affects the rate of photosynthesis”.


What is a fair test?

A fair test must have only one changed variable and one measured variable – all other variables need to be kept constant. As long as this is true, it’s a fair test.

“Why should [some variable] be kept constant?”

This is to ensure that all differences in the [measured variable] are due only to the [changed variable] and not to other variables such as [some variable].

“Why this is not a fair test?”

He [didn’t control a variable]. So he could not compare and conclude that the [measured variable] is only due to the [changed variable].



Each experiment must have different results for the different set-ups, otherwise nothing can be concluded. Some questions will tell you that the results are the same for the experiment (maybe because the measurement was too long, eg. if we observe a piece of toasted bread and non-toasted bread after 30 days, and see both pieces have mould on it), and what we can do to the experiment so that we can make a conclusion. The answer would be to make the measured variable more specific so that comparisons can be made.

Another common question is asking what must you observe to make a conclusion. Eg. when observing whether warmth will affect the time for mould to grow, and you have two set ups, one in the cupboard and one in the fridge, you need to compare the two set-ups. Instead of “She must observe that the mould grows in the cupboard.” the answer is:

She must observe that the mould grows faster in the cupboard than the fridge.

“What is the relationship between the height of the drop and the depth of the dent”

As the [changed variable] increases, the [measured variable] increases / decreases

You’ll need to identify which is changed and which is measured and phrase the answer accordingly. Use keywords from the question or data (whether in table form or graph)

“What can she conclude from the experiment?”

Answer should include HOW the changed variable affects the measured variable. eg. instead of “The intensity of light affects the rate of photosynthesis”, the answer is:

The greater the intensity of the light, the greater the rate of photosynthesis

What is a Control?

A control is a set-up used to show that the experiment is valid. It’s a setup where no variables are tested. This provides a baseline for which to compare the results. An example would be an experiment to test the effect of detergent on germination of seeds. We could test seeds with detergent of different concentrations but it would be more ideal to have a control setup with no detergent, to make sure that the detergent is responsible for differences in the germination of seeds. Without the control, if the seeds don’t germinate at all, we won’t know if there is something wrong with the setup, such as the seeds being dead.

“What is the purpose of set up A?” (the control)

“What is the purpose of adding a control?”

Set up A is a control. It allows for comparison of results and to show that the [measured variable] is caused by the [changed variable]”

“Why did she repeat the experiment?”

To improve on the reliability of the results.