I did an experiment on the nitrate levels in three water samples of the local river. I used some tap water also. I need to know: was the dependent variable the quality of the water and the independent variable the levels of nitrates? Or should the tap water be the dependent variable? Or should it be the control?|||In an experiment we must change only one factor at a time while keeping all other conditions the same. Scientists call the changing factors in an experiment variables.
An experiment usually has three kinds of variables: independent, dependent, and controlled.
The independent variable is the one that you alter throughout your experiment.
The dependent variable is the variable that you measure to see how it responds to the change made to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable.
The controlled variables are the ones that you try to keep constant throughout your experiment so that they don't affect your experiment.
Most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."
According to these explanations, your independent variable is the different samples of water. Because each case you only change the samples of water. You apply the same method and procedure and what you get : the nitrate level. the nitrate level is your dependent variable.
If you also determine the nitrate level of tap water, it is also an independent variable. But, if you know the nitrate level of tap water and determine it experimentally to be sure the accuracy of your experiment, it becomes a control experiment, not variable. For example if you use the same amount of water in each experiment the amount of water is the controlled variable.
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