Students
are learning to balance equations. Today they learned that reactants
are what you start with and are on the left side of the equation.
Products are on the right side of the arrow and are what is made by
process of a chemical change.

Because of the Law of
Conservation of Mass, the number of atoms have to be equal on both
sides. To balance an equation, the coefficients are changed.
Coefficients are the big numbers in front that tell you how many
molecules there are. The subscripts (the little lower numbers) are not
allowed to be changed because those are there to make neutrally bonded
molecules (what we learned in the last unit.

By changing the coefficients and counting the number of atoms on both sides of the arrow, balancing can be achieved.