The strength of an acid solution is measured by the concentration of H+ ion rather
H3+O ion present in its aqueous solution .
In 98% pure acid the amount of polar H2O being less the acid molecules are less dissociated into H+ ions hence acts as less strong acidic. Its dissociation into ions enhances with dilution. So in this context dilute acid is considered stronger acid than concentrated one.
But concentrated H2SO4 (98% or more) is hihglyhhygroscopic in nature. When it comes in contact with any substance it tries to get diluted by absorbing water molecules from it if available even decomposing it and this dilution process is highly exothermic. So enormous amount of heat is produced and as a result decomposition reaction is found to occur with vigour and may cause burning.. This phenomenon represents the concentrated acid as stronger reactive material than diluted one. For this reason we see that our skin is burnt by the heat generated due to hydration of acid caused by water molecules collected by highly hygroscopic acid from the body in contact with it.
