Ion entropies vary as
expected on the basis that they are related to the degree to which the ions order the
water molecules around them in the solution. Small, highly charged ions induce local
structure in the surrounding water, and the disorder of the solution is decreased more
than in the case of large, singly charged ions. The absolute, Third-Law standard molar
entropy of the proton in water can be estimated by proposing a model of the structure
it induces, and there is some agreement on the value −21 J K−1 mol−1. The negative
value indicates that the proton induces order in the solvent.