# Ideal CSTR mixing assumption

Sorry if this seems like a silly question but it has me confused a lot. So I have just started taking Chemical Reaction Engineering, and was studying about continuous-stirred tank reactor (CSTR).

So in my textbook it's stated that "It is normally operated at steady state and is assumed to be perfectly mixed; consequently, there is no time dependence or position dependence of the temperature, concentration, or reaction rate inside the CSTR. That is, every variable is the same at every point inside the reactor. Because the temperature and concentration are identical everywhere within the reaction vessel, they are the same at the exit point as they are elsewhere in the tank. Thus, the temperature and concentration in the exit stream are modeled as being the same as those inside the reactor"

This has me confused a lot, if concentration isn't changing with time, then does the chemical reaction has to occur instantaneously?

So the reactant is instantaneously and uniformly mixed throughout the reactor upon entry, but what about it's conversion into a product?. Because if that's taking time then the statement "concentration isn't changing with time" doesn't make any sense to me.