Q.At absolute zero temperature the entropy of a pure crystal is zero.

Entropy tracks directly with temperature -- water vapour above $100^\circ$C has high entropy (molecules roam freely); cooling to liquid, then to ice, steadily lowers entropy as molecular freedom is progressively lost; cooling the solid crystal further slows even lattice vibration toward near-zero disorder. At absolute zero (0 K, $-273^\circ$C), theoretically ALL molecular motion stops -- but absolute zero is a limit a real object can only approach, never actually reach. …