EXPERIMENTAL  CONTROLS...

 set of experimental conditions where everything is identical, but one variable...
         an experimental control is identical to an experimental treatment
         except for one critical factor being tested.

 all experimental treatments (dependent variables) must be handled exactly alike...
         same time interval, same pH, same temp, age, etc...
         except for independent variable.

 in essence there are 2 parallel experiments:
         one is the EXPERIMENTAL    and    one is the CONTROL