Experimental Design questions IGCSE Biology

I have been posting comments about the questions that appear year after year on iGCSE Biology papers.  Questions like the one below are found in every past paper we have.  I call these the “Design an experiment to” questions for obvious reasons…..

“Rivers are sometimes polluted by warm water from power station outflows.  This is known as thermal pollution and can affect the growth of plants.  Design an experiment to investigate the effect of water temperature on the growth of plants.  6 marks. November 2010”

As you all know, the mark scheme for this kind of design an experiment question is based around the acronym CORMS.

C – how do you change the independent variable?

The independent variable is the thing you are going to change to see its effect.  In this experiment it is the temperature of the water.  So how are we going to change it?  Well it might appear obvious but you need aquatic plants living in water baths at a range of temperatures, say 10,20,30,40,50,60 degrees.  Try to make your independent variable continuous if it is possible – the range of temperatures above is much better than just one set of plants in hot water, another in cold water.

O – what organisms (or other biological material) will you use?

To get this mark you will need to say something about the plants you will use in your investigation.  For the experiment to produce reliable results, there are many features of the plants that will need to be kept the same in each water bath.  Same species, same age of plants, same starting size, same surface area of leaves etc.  There are other factors too about the plants that need to be controlled.  Can you think of any others?

R – reliability

In order to produce reliable results you will need to set up multiple repeats of each experiment so anomalous readings disappear as you average your results.  How would you do this?  Well in the example above, I would set up 5 identical water baths at each temperature.  We are investigating six different temperatures so we will need 30 water baths.  Don’t worry about this.  For research as vital as this fascinating experiment, no expense should be spared……

M – how are you going to measure the dependent variable?

There are often two possible marks for this and you will see M1 and M2 on the mark schemes.  The key idea is often the same however (there’s a shock)  The first mark is for identifying what you will measure about the plants to measure growth.  There are lots of alternatives depending on what kind of plant you are using.  I am picturing a small floating algae growing in my water baths so I would measure the mass of the plants.  (Dry mass would be better but this would lead to destructive sampling – plants won’t grow further if you dehydrate them completely in an oven before weighing them……)  You could measure the height of the stem of a plant, or the total surface area of water covered.  It doesn’t really matter which thing you choose as long as it is a sensible measure of growth.  What will M2 be awarded for?  Well it is essential you leave all 30 waterbaths for exactly the same length of time between measurements.  How frequently will you measure the growth of your plants?  Every hour would be too often, so perhaps every day would be sensible.  So a statement that says “use a mass balance to measure the total mass of the plants in each water bath every day for a period of 10 days” will be certain to get both M marks…

S – what factors do you need to standardise to make the experiment a fair test?

You will have mentioned some of these “fair test” factors in the mark point O above.  Now it is time to show that you understand what factors other than the temperature of the water will effect the growth of your plants.  Growth of plants is done by photosynthesis so I would be aiming to show you understand the other factors that will effect rates of photosynthesis:  i.e. light intensity. light wavelength and carbon dioxide concentration.  All three should be kept constant and I would say how:  same lamp at the same distance from the water baths, carbon dioxide in water controlled by dissolving same mass of sodium hydrogencarbonate in the water.  There are often two S marks but by stating all three important control variables this should guarantee we get both.

Now I have written this post without looking at the mark scheme.  “Promise….  Honestly Sir I wouldn’t cheat myself like that…..”  But here it is and look we would have got full marks.  Full Marks = A* #result




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