How to recognise the conditions under which changes in pressure would effect system in equilibrium.

Explanation

A. When pressure changes occurs on a chemical reaction in equilibrium, there will be changes in the number of gaseous molecules, if there is difference in the total number of gaseous molecules of reactants and products.
If pressure is increased the reaction proceeds in the direction where the number of gaseous molecules are less. Since the volume of a gas is directly proportional to volume with decrease in number of molecules volume also decreases. As the sum of the masses of the substances in the vessel is changed density increases. So by measuring the density of the equilibrium mixture the effect of change in pressure on the equilibrium can be recognised.