queensland museum learning
Science Skills
Topic
Controlled experiments
Scientists bring a scientific eye to any situation
they observe, and you can too. Even when we see
the same things they do, we may not have their
knowledge and experience to make the same
observations. Although we do not see things as
they do, we can, and often do, see relationships
we want to test.
When scientists carry out investigations, they first
propose an idea to be tested called a hypothesis. This
statement is a tentative idea about a relationship they
see, and they test this hypothesis in a controlled
experiment.
Isolating variables
Scientists identify the factors that could vary (called
variables) before they conduct their investigation. In
their investigation they control all these variables, except
one, which they deliberately change (called the
manipulated variable). In this way they can observe
and measure the effect of the manipulated variable on
the experimental variable.
Other factors to think about
>> Sources of error
Variables that may affect the results and have not
been controlled in the experimental design are
called error.
In the investigation shown, the larvae were taken
from containers from Mt Glorious, near Brisbane.
What, then, is a source of error in this investigation?
>> Interpreting results
What sort of results would be needed to support
this hypothesis? What results would not support
this hypothesis? How would we present these
results?
To see this effect scientists use a control. It is that part
of the investigation where the manipulated variable is
allowed to have no effect on the experimental variable,
see below.
Setting up a controlled experiment
A scientist has observed that mosquito larvae develop
only in brackish pools near mangrove areas, where
trapped salt water has been diluted with rain water.
She thinks salt water kills larvae. To test this idea, she
writes a research question then carries out a controlled
experiment.
Hypothesis: ‘The salt in salt water affects the survival
of soft-bodied animals. So, the survival rate of mosquito
larvae will be less in salt water than in pond water.’
Manipulated variable: Salt concentration of the water
Other variables, which must be controlled: Water,
volume of water, numbers of larvae, type of larvae,
temperature
Control: Wrigglers developing in pond water, to be
compared with wrigglers developing in salty pond water.
queensland museum | po box 3300 | south brisbane bc | queensland 4101 | australia
A controlled experiment: Larvae swim in pond water and
in salty pond water (bottom). The salty water has the salt
concentration of sea water. What variables have been
controlled here? What variable is being measured? This
photo was taken at the start of the experiment.
© Queensland Museum 2007