Having a control in an experiment allows you to see what happens when no variables are changed. If you do not have a control, you do not have anything to compare your results with after changing variables of the experiment.
stimulus
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The chemical or chemicals formed in a chemical reaction are called the product of the chemical reaction.
Because it is as old as stone itself.
Lyndon B. Johnson
Symbols are used in lab procedures and in other kinds of instructions to alert you to important information or to the potential for hazards if procedures are not carefully followed.
Biology
A reactant is a substance that participates in and changes during a reaction.