[EvilFrog (OP)]

what are the reaction that do not go faster even if we increase the concentration? How and why this occur?

[Bored chemist]

Yes, but only under odd circumstances.
Imagine a reaction where something absorbs a photon of light and the excited state reacts with a second molecule.
Also imagine that there's enough of this stuff that any photon of light entering the vessel has a good chance of being absorbed. In those circumstances the reaction rate varies with the light intensity but not (much) with concentration.

OK that's odd- but it's useful. You can use it to measure the intensity of the light.

[Nizzle]

Another one i can think of is catalyzed reactions.
If you add two reagents to a solution with a catalyst you'll get to a point where the catalyst is saturated.
Adding extra reagents will not make the reaction go faster.

[EvilFrog (OP)]

thanks!! []

[Bored chemist]

Another one i can think of is catalyzed reactions.
If you add two reagents to a solution with a catalyst you'll get to a point where the catalyst is saturated.
Adding extra reagents will not make the reaction go faster.

The enzymes in the liver that metabolise alcohol reach this sort of "saturation" for anything other than very moderate drinking so it's not just some laboratory curiosity.