[justwilliams]

OK I know the title of this sounds crazy but like I am really confused with how a 0th order reactant is possible, because that would mean that there is no effect on rate due to concentration. But, when a solution with an extremely high concentration reacts, wouldn't that mean since it has a higher concentration, the odds of molecules colliding and therefore the reaction rate, should be higher than that the same solution at an extremely low concentration?

[Borek]

0th order just means collisions are not what drives the rate limiting step.

[Corribus]

Often seen with catalyst-driving reactions, because contact with the catalyst, rather than direct contact with other reactants, dominates the reaction rate.  In other words, collisions between reactants in an overwhelming percentage of cases does not lead to reaction - a catalyst needs to be present. So production of product does not depend on the concentration of reactants. The enzyme/catalyst basically acts as a bottleneck. (In such cases, if the catalyst isn't present, you may see higher order reaction kinetics because the reaction in the absence of catalyst DOES require collisions between reactants. But of course that reaction would be extremely slow, possibly so slow it couldn't be observed.)