What the requirement for a reversible reaction to be at equilibrium ?

A reversible reaction is a chemical change in which the products can be converted back to the original reactants under suitable conditions. In a reversible reaction, changing the reaction conditions e.g. concentration, pressure or temperature will change the net direction the reaction goes i.e. more to the right (forward) or more to left (backward).It also means a reversible reaction does not go to completion in either direction and all components, original reactants or ensuing products, ALL co-exist in the reaction mixture (see notes on chemical equilibrium).This means the reaction can go in either direction i.e. A + B ==> C + D or C + D ==> A + BA reversible reaction is shown by the sign , a half-arrow to the right (direction of forward reaction), and a half-arrow to the left (direction of backward reaction).It is really important you understand that the terms right & left AND forward & backward are used in the context of how the equation is presented.Most reactions are not reversible (irreversible) and have the usual complete arrow only pointing to the right.

Because a model would make the investigation easier to understand

Woah that’s a lot to read xd i’ll try though. i’ll get back t you if. i know the andwer