The equation BaO + H2O creates the compound Ba(OH)2. BaO is the chemical formula for barium oxide, H2O is the chemical formula for water, and Ba(OH)2 is the chemical formula for barium hydroxide. The entire equation is written as BaO + H2O = Ba(OH)2.
The chemical symbol for barium is Ba, the chemical symbol for hydrogen is H and the chemical symbol for O is oxygen. When the equation is written, the number of atoms on the left side must equal the number of atoms on the right side. The number after the parenthesis indicates two atoms of each element inside the parenthesis.