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Physical Chemistry 54

there are no dynamic changes in the crystal either. Under these conditions, the total
number of possible ways of arranging the material equals one (w=1), and so the entropy
(defined as k Blnw) is equal to zero.
Although absolute zero cannot be reached and perfect crystalline solids cannot be
made, it is still possible to apply the third law. In practice, the absolute entropy of
materials drops to infinitesimally small values at low temperature, and for most purposes
equals zero at the low temperatures which can be routinely achieved in the laboratory.
Because it is possible to measure entropy changes from a reference point using heat
capacity measurements, entropy (unlike the enthalpy and internal energy) has a
measurable absolute value for any system.