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Variables are an important part of science projects and experiments. What is a variable? Basically, a variable is any factor that can be controlled or changed in an experiment. Scientific experiments have three types of variables.
Types of Variables

Independent Variable
The independent variable is the one condition that you change in an experiment.

Example: In an experiment measuring the effect of temperature on solubility, the independent variable is temperature.

Dependent Variable
The dependent variable is the variable that you measure or observe. The dependent variable gets its name because it is the factor that is dependent on the state of the independent variable.

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  Example: In the experiment measuring the effect of temperature on solubility, solubility would be the dependent variable.

  
  

   

  
  


• Controlled Variable
  

  A controlled variable or constant variable is a variable that does not change during an experiment.

  
  

   

  
  

  http://chemistry.about.com/od/sciencefairprojects/a/What-Is-A-Variable-In-Science.htm