An acid is a substance that increases the concentration of H3O ions in solution. Look at the reaction below of phenol (C6H5OH) with water: Upper C subscript 6 upper H subscript 5 upper o upper H (a q) plus upper H subscript 2 upper O (l) right arrow upper C subscript 6 upper H subscript 5 upper O superscript minus (a q) plus upper H subscript 3 upper O superscript plus (a q). Which reactant acts as an acid by contributing a H ion to create H3O in the chemical equation