What is the equilibrium constant?

Imagine a bathtub that someone forgot to plug, but they left the tap open. If the water from the tap is running fast enough, after some time the speed at which water drains and the speed at which water fills the bathtub will equalibrate, and the water level inside the bathtub will remain constant. Although the system is constantly changing ('new' water flowing in and 'old' water draining), you can't notice it as the apparent level of water remains the same. It is the same with chemical reactions. Imagine a reversible chemical reaction: A <-> B The speed at which A gets converted to B would be v(forward)=k(forward)[A] At the same time, we lose some product because it gets converted back to A. The speed at which B gets converted to B would be v(reverse)=k(reverse)[B] In equalibrium, the two rates are the same. Therefore we can say: v(forward)=v(reverse) k(forward)[A]=k(reverse)[B] k(forward)/k(reverse)=[B]/[A], which is the equilibrium constant, K. It is useful for many chemical calculations, and a general definition is: for a reaction a A + b B -> c C + d D, the equilibrium constant K=([C]^c*[D]^d)/([A]^a*[B]^b).