What is a balanced equation?

• A. An equation that has equal numbers of each type of atom on both sides.
• B. An equation that shows the change of state.
• C. An equation that only uses words.
• D. An equation that does not balance atoms.

Answer: (A)

The key idea is conservation of mass: the same number of atoms of every element must appear on both sides of a chemical equation. That’s why the correct choice states equal numbers of each type of atom on both sides. To see this in action, balance the reaction by adjusting the coefficients, not the subscripts; for example, 2 H2 + O2 → 2 H2O shows two hydrogen molecules and one oxygen molecule on the left, and two water molecules on the right, giving the same counts of hydrogen and oxygen atoms on both sides. A change of state refers to phase changes, not atom counts. A word equation uses only words, not chemical formulas. An unbalanced equation would violate mass conservation because it would not have the same number of atoms of each element on both sides.