Writing Equations Chemical Equations Math equations show equalities. Chemical equations show processes (one chemical changes to another chemical). A Chemical Equation Must: Identify all the substances involved. Show the composition of the substances (use formulas). A Chemical Equation Must: Account for all the atoms. (There must be the same number of atoms of every element on both sides of the equation.) Parts of an Equation Reactants Products Coefficients Arrow Example reactants Ca(HCO3)2 + Ca(OH)2 2 H2O + 2 CaCO3 products The ?2? is a coefficient to tell how many molecules of each compound are present. What tells you how many atoms of an element are in a molecule? Symbol Subscript Coefficient Catalyst Question 7 Special Symbols in Equations g?: A gas was produced.