Chapter 7 · Chemical Reactions
7.3The Chemical Equation
9 min · two checks
Predict
What do (g), (l), (s), and (aq) tell you that the formulas do not?
The idea
- Read formulas, coefficients, and state symbols.
- Explain why coefficients and subscripts do different jobs.
A chemical equation lists reactants on the left and products on the right. The arrow means “react to produce,” not “equals” in the arithmetic sense, though atoms do balance. Coefficients are the large numbers in front of formulas and they count formula units or molecules. Subscripts belong to the formula and must not be changed to force a balance. State symbols sit in parentheses: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Unbalanced skeleton equations still name the substances. H₂ + O₂ → H₂O is the right cast and the wrong count: two oxygen atoms on the left and one on the right. Balancing, next section, adjusts coefficients only. If heat is required you may see a triangle or “heat” over the arrow. If a catalyst is used, its formula goes over the arrow because it is not consumed as a reactant.
Keep these
- Coefficients count molecules. Subscripts define them.
- (s), (l), (g), (aq) report state.
- Never balance by editing a subscript.