Stating a hypothesis
A scientific hypothesis is a suggested explanation for an observation, which can be tested by an investigation or experiment. The hypothesis leads to predictions about the investigation. This can be accepted or rejected based on results from the investigation. Features of a sound hypothesis include the following: • It offers an explanation for an observation or a problem. • It is written as a statement and not a question. • It can be tested by experimentation/investigation. • It is based on observations and prior knowledge of the investigation. • It leads to predictions about the investigation.
Example of a hypothesis: The pH has an effect of the growth rate of plant A.
Designing a method for your investigation Research, plan and draw up a step-by-step method for your investigation. If appropriate, compare your chosen method with other methods and give reasons for your preference. Make sure you consider the following: • List all the apparatus and materials needed to perform the investigation. • Consider experimental procedures that would minimise experimental errors, for example, minimising the error of parallax.
• List all the safety precautions that should be taken.
Make sure that you identify all the variables in the experiment. A variable is any characteristic or property able to take any one of a range of values. Investigations test the effect of changing one variable on another (see Table 1). • The variable that the investigator will change is called the independent variable. • The variable that will change as a result of changes made to the independent variable is called the dependent variable.
• Variables that are kept constant throughout the experiment are called controlled variables. • In all investigations the investigator changes only one variable. • Besides the experimental group, also set up a control group. A control is set up under the same test conditions as the experiment, but the independent variable is not changed.
Table 1 Independent and dependent variables in three investigations Aim
Investigate the effect of varying …
Temperature Light intensity Soil pH
on the following … Enzyme activity Rate of photosynthesis Independent variable
Temperature Light intensity
Plant height at age three months pH
Variables Dependent variable Enzyme activity
Rate of photosynthesis Plant height
The scientific method 9
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