(COC) Experimental Design Lesson

Experimental Design

image of door: Why is the door squeaking? Is it the window? Is it the hinges? Is it the door knob?One step in the scientific method is actually testing your predictions/hypothesis to see if the outcome is what you think it should be. In order to form conclusions, a controlled experiment must be conducted.  A controlled experiment is one that only tests one thing at a time.  What you are testing is called a variable. If you test more than one variable at a time, you will not know what actually caused the outcome.  For instance, I have a problem with a squeaky door. If I punch out the window, grease the hinges, and remove the doorknob and the door no longer squeaks; I am happy but do I know what actually worked to cause the door to stop squeaking? No! Instead, I should test one thing at a time to determine what is causing the door to squeak.  In this lesson, you will look at the components necessary to develop a controlled experiment.

In order to develop a controlled experiment, it is important to have both an experimental group and a control group. The experimental group is the group in which the experiment is being conducted.  This is the group that is exposed to the independent variable and the results are being observed and recorded. The control group on the other hand is not exposed to the independent variable. This group is separated from the experiment so that the scientist can compare the results to see if the independent variable did in fact have an effect on the experiment.  

The variables in an experiment are the factors being tested.  There are several kinds of variables.

  • Independent Variable - the variables that the experimenter changes to test their dependent variable.  It stands alone and is not changed by the other variables the experimenter is trying to measure.  It is important to have only one independent variable for each experiment. For example:  If an experimenter is measuring if fertilizer affects a plant's height growth, then the amount of fertilizer is the independent variable. The fertilizer is what the experimenter is adding to change the outcome of the experiment.
  • Dependent Variable - is 'dependent' on the independent variable. As the experimenter changes the independent variable, the change in the dependent variable is observed and recorded. For example: If an experimenter is measuring if fertilizer affects a plant's height growth, then the height of the plant is the dependent variable.  The height of the plant is what is being observed and measured.
  • Control Variables - the factors that remain the same and constant in the experiment. For example: If an experimenter is measuring if fertilizer affects a plant's height growth, then the control variables would be the amount of sunlight, amount of water, type of plant, size of the pot, etc.

Example:

A chemist made an observation that more sugar could dissolve in her sweet tea when hot than when cold.  In order to test her hypothesis, the chemist conducted an experiment in which she followed the procedures below:

  • She measured 100 mL of tea into 5 beakers.  She heated each beaker to a different temperature.
  • She added sugar in 50-gram increments, stirring to ensure all dissolved before adding more. Once the sugar was dissolved and settled to the bottom, no additional sugar was added.
  • After she collected the data, she made the following graph:  

Sugar Solubility Graph of grams in sugar dissolved in water and temperature with the curve going up

  • As she analyzed the data, she was able to see a pattern.
  • She formed the conclusion that more sugar could dissolve at higher temperatures.

 

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