What happens to entropy when pressure decreases?

Answer 1

When pressure decreases, entropy increases. Increases in entropy correspond to pressure decreases and other irreversible changes in a system. Entropy determines that thermal energy always flows spontaneously from regions of higher temperature to regions of lower temperature, in the form of heat.

Answer 2

When pressure decreases, the volume generally increases. Since entropy is related to the number of possible microscopic configurations of a system, an increase in volume typically leads to an increase in the number of possible configurations. Therefore, entropy often increases when pressure decreases.