Question

What happens to matter when in a thermodynamic equilibrium?

Answer 1

First of all I will address your last concern, which translates into: equilibrium doesn't necessarily mean that nothing is moving. As an example, particle in a solution at equilibrium can move from one side to the other as long as almost the same number of particle move the opposite direction (this usually happens, say, because of thermal agitation/Brownian motion...). What doesn't change with time is then the average concentration of particles in the solution. Now using again this analogy, suppose you have an isolated system made of, say, sea water and distilled water. When you first mix them you can still distinguish them, as one has a higher concentration of salts. After a while, internal gradients, that arise because the system is out of equilibrium, will drive the salts to homogenise and the end result is water with a homogeneous concentration of salts, after a suitably long relaxation time. Same happens with temperature: if you replace sea and distilled water with hot and cold water, the same mechanism will homogenize your system. Temperature gradients will mix water around until the temperature is the same everywhere. Then gradients disappears and dynamical equilibrium settles in (water is not completely static, but molecules flow around because of thermal agitation).

Answer 2

Explanation:

First of all I will address your last concern, which translates into: equilibrium doesn't necessarily mean that nothing is moving. As an example, particle in a solution at equilibrium can move from one side to the other as long as almost the same number of particle move the opposite direction (this usually happens, say, because of thermal agitation/Brownian motion...). What doesn't change with time is then the average concentration of particles in the solution. Now using again this analogy, suppose you have an isolated system made of, say, sea water and distilled water. When you first mix them you can still distinguish them, as one has a higher concentration of salts. After a while, internal gradients, that arise because the system is out of equilibrium, will drive the salts to homogenise and the end result is water with a homogeneous concentration of salts, after a suitably long relaxation time. Same happens with temperature: if you replace sea and distilled water with hot and cold water, the same mechanism will homogenize your system. Temperature gradients will mix water around until the temperature is the same everywhere. Then gradients disappears and dynamical equilibrium settles in (water is not completely static, but molecules flow around because of thermal agitation).