There are several specification points in the iGCSE syllabus that mention controlled experiments.  For example in the recent work studied there are two:

• describe how to carry out simple controlled experiments to illustrate how enzyme activity can be affected by changes in temperature
• describe simple controlled experiments to investigate photosynthesis showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll

So what exactly do they mean by a controlled experiment?

In an investigation you need to know what is meant by the independent variable and the dependent variable.  To put it in the simplest terms, the independent variable is the thing you are altering; the dependent variable is the thing you are measuring.

In any experiment there will also be a range of other variables that might affect the dependent variable.  For example in the first bullet point above, enzyme-catalysed reactions are not only affected by changes in temperature.  They can be affected by pH, by the concentration of enzyme and by the concentration of substrate.  A controlled experiment is one in which these other variables – now called control variables – are kept constant to ensure it is a fair test.  So if you were devising an experiment to investigate the effect of temperature on an enzyme reaction, make sure the pH is kept constant by using pH buffers and that the enzyme concentration and substrate concentration are exactly the same in every experimental set up.

It’s as simple as that…….