It is an experimental fact that most of the process including chemical reactions, when carried out in a closed vessel, do not go to completion. Under these conditions, a process starts by itself or by initiation, continues for some time at diminishing rate and ultimately appears to stop. The reactants may still be present, but they do not appear to change into products any more. Do check out Chemistry Questions of other chapters too.
A reaction is said to be reversible if the composition of reaction mixture on the approach of equilibrium at a given temperature is the same irrespective of the initial state of the system, i.e. irrespective of the fact whether we start with reactants or the products. For additional information related to the subject you can check the Chemistry Formula section. If any students need to take the online test to check their concepts or undertstanding then they can visit Quiz for Ionic and Chemical Equilibrium.