be almost no energy difference between the arrangements . . . AB AB AB . . . and . . .
BA AB BA . . . , so W > 1 even at T = 0. If S > 0 at T = 0 we say that the substance has
a residual entropy. Ice has a residual entropy of 3.4 J K−1 mol−1
. It stems from the
arrangement of the hydrogen bonds between neighbouring water molecules: a given
O atom has two short O–H bonds and two long O···H bonds to its neighbours, but
there is a degree of randomness in which two bonds are short and which two are long.