According to this law , at absolute zero temperature the entropy of a perfectly crystalline substance is taken as zero ..?
The third law of thermodynamics states that ” The entropy of a perfect crystal at absolute zero temperature is exactly equal to zero.”At absolute zero that is zero Kelvin, the system is said to possess minimum energy. This statement of the third law of thermodynamics holds true only when the perfect crystal has minimum energy state. If the system has minimum disturbances it means that the temperature has approached zero Kelvin where molecular motion seizes to exist. Standard molar entropy is defined as the entropy per mole of a substance at standard conditions and specified temperature.

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The third law of thermodynamics states that the entropy of a perfect crystal at a temperature of zero Kelvin (absolute zero) is equal to zero.Entropy, denoted by ‘S’, is a measure of the disorder/randomness in a closed system. It is directly related to the number of microstates (a fixed microscopic state that can be occupied by a system) accessible by the system, i.e. the greater the number of microstates the closed system can occupy, the greater its entropy. The microstate in which the energy of the system is at its minimum is called the ground state of the system.At a temperature of zero Kelvin, the following phenomena can be observed in a closed system:\tThe system does not contain any heat.\tAll the atoms and molecules in the system are at their lowest energy points.Therefore, a system at absolute zero has only one accessible microstate – it’s ground state. As per the third law of thermodynamics, the\xa0entropy\xa0of such a system is exactly zero.