Whenever a chemical equation is written, the elements or compounds which lie to the left side of the arrow are the reactants and those which lie to the right side are products. For the given reaction, reacts are potassium hydroxide 'KOH' and nitric acid 'HNO3' as they are present on the left side of the arrow. They react chemically to produce water 'H2O'and potassium nitrate 'KNO3'.