Parts of an experiment to identify and label:

Independent Variable, Dependent Variable, Subjects, Control, and the different Constants

A “variable” is something that can change.
A “constant” is something that does NOT change.
The “subjects” are what the test is done to in order to see how they change or react.
A “control” is one subject that is left alone and not tested on, to use as a normal standard for comparison.

1. Independent Variable: What you want to test. It is the variable you are going to manipulate or change, it is what you are testing.

• I want to know if red is a better color for plants to grow higher in.
• I want to see if potting soil is better than my garden soil to make larger flowers.

This is called the Independent variable or the I.V.  It is what is being tested.  Such as the red color, or potting soil.

Use these sentences:

“I want to know if ________, will ….”   Or   “I want to test ______”

What goes into the blank is the IV.

2. Dependent Variable: What change will happen after you tried the new condition (the IV) on the subject?  What will you measure to know if it worked, or did not work? 

You will measure any changes, these measurements are recorded and are called data.

• I want to know if red is a better color for pants to grow higher in.
• I want to see if potting soil is better than my garden soil to make larger flowers.

This is called the Dependent Variable or the  D.V.  The change in the subject is “dependent” on what is being tested. The plant’s height will be dependent on the color of lights. The size of the flowers is dependent on the potting soil used.

Use this to determine the DV:  What is the data you will be taking? What will you measure?

3. Control: How will you know if the IV you tested worked or not?

If you just try red color and the plant grows 30 cm, did it help or not?
A normal plant grows to 40 cm in the same time, so the red light did not help.

If you use a new potting soil and the flowers grew to 15 cm, did the potting soil help or not.
A plant’s flower in normal, regular soil grows to 10 cm, so the new potting so did help.

The control is the same subject that does not get the test done to it. It is used as a standard or comparison against the tested subjects.

Use this to help: Which subject was not tested on, that is the normal, regular one. It will be the same subject (like the plant) but not tested on.  The plant is grown under normal light or in the normal, regular soil.

4. Constants: Constant means to stay the same. When conducting a test only one thing can be tested at a time. If you want to test red lights and put 2 plants under them. Then you don’t water plant number 1, and give plant number 2 water.  Plant number one dies. Was it the red lights or lack of water?

To make a test fair all things must be equal, only the thing you want to test can be different.

Everything must be held constant but the one thing you are testing. For our examples with plants, constants would be water, sunlight, temperature, the same kind of plants, the same age.

Practice what you learned

1. Bailey wants to find out which frozen solid melts the fastest:  water ice, or  juices.  She pours three juices (apple, orange, and cranberry) into the empty cubes of an ice tray, and fills one with pure water and then places the ice tray in the freezer over night.  The next day, she pulls the ice tray out and sets each cube on its own plate.  She then waits and watches for them to melt.  When the last part of the frozen liquid melts, she records the time. She makes sure the cubes are the same size, the plates and air are the same temperatures. 

• What would be the hypothesis that Bailey is using?

Based on Bailey’s observations and reading about melting times, she thinks that frozen water (pure ice) will melt faster than water with something else it in. Put that into a sentence.

Hypothesis:  Pure water ice will melt faster than frozen juice. 

• Independent Variable is what she is testing: I want to find out if frozen juices melt the faster than water. The IV is the frozen juices. 
• Dependent Variable is what measurement will be taken to determine that the results are: How fast they melt.  The DV is melting time.  The minutes to melt. 
• Control is the one that is normal, regular, unchanged. Used as the standard to compare.  The control is the pure water ice which is normal with nothing added.
• Constants are all things that are the same for all groups, what is NOT being tested and you do not want them to interfere.  If one sample was left to melt in a refrigerator and one sample put in a warm oven it would not be a fair test.

Constants: Amount of ice, the temperature of the room, the plate the ice is put on, the humidity and air flow around the ice, the shape of the ice cubes…