The microstate describes a system completely and exactly in terms of the positions and velocities (or other degrees of freedom) of all of its constituents. Hence for a bowl of water, the microstate will tell you exactly where each molecule is and how fast and in what direction it is moving.

Macrostate on the other hand, is a macroscopic property. What it means is that the system is described in terms of its measurable, macroscopic properties only. It greatly simplifies matters because we don’t have to worry about each and every particle, but their properties observed as a complete system.

A single macrostate can thus correspond to a multiple of microstates, provided each of those appear to be identical when measured macroscopically.

For the bowl of water, if any two molecules collided and exchanged momentum it would change the microstate of the system. However, macroscopically it won’t make any noticeable difference (Volume, Temperature etc.). Hence the macrostate remains the same but microstate is changed.

 

Macrostates vs Microstates
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