Year 8 Science · Unit 4 · Lesson 3
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Learning Goals
True or False? Fix the false ones
Circle T or F for each statement. If the statement is false, rewrite it correctly on the line below.
A well-designed experiment can have more than one independent variable at a time to save time.
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The dependent variable is the factor the scientist deliberately changes in an experiment.
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Controlled variables ensure a fair test by preventing other factors from affecting the dependent variable.
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A confounding variable makes an experiment more reliable because it adds extra information.
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Scenario
A student investigates whether fertiliser type affects tomato plant height. They use four pots: one with organic fertiliser, one with chemical fertiliser, one with no fertiliser, and one with double the chemical fertiliser dose. However, each pot is placed in a different location in the greenhouse — near a window, in the centre, near a fan, and in a corner.
(a) Identify the independent variable and the dependent variable in this experiment.
(b) Identify the confounding variable introduced by placing pots in different locations. Explain why this makes the experiment unfair.
(c) How would you redesign the experiment to make it a fair test? Name at least three variables you would control and how.
1. CSIRO researchers test new crop varieties across Australian farms. They control soil moisture, planting depth, and pest management so only the crop variety differs. Why is this strict control important for the reliability of their recommendations?
Wrap Up
In one sentence, what was the main idea of this lesson?