All the others are weak acids. A strong acid is one that dissociates completely when it dissolves in water. Dissociate means that the substance splits apart into ions. There are only a few strong acids in water, they are: HBr, HCl, HI, HNO3, HClO4 and H2SO4 (for the first H only) All other acids are weak acids. Chemists use the term strong or weak with regard to acids and bases to tell how much the substance dissociates in water.