Le Chatelier's Principle If a system at equilibrium is subjected to a change of any one of the factors such as concentration, pressure or temperature, the system adjusts itself in such a way as to nullify the effect of that change. Change of pressure : If a system consists of gases, then the concentration of all the components can be alterd by changing the pressure. To increase the pressure on the system,the volume has to be decreased proportionally. The total number of mols per unit volume will now be more and the equilibrium will shift in the direction in which there is a decrease in number of moles i,e. towards the direction in which there can be decrease in pressure. Effect of pressure on melting point : There are two types of solids: (a) Solids whose volume decreases on melting e.g., ice,diamond carborundum magnesium nitride and quratz. Solids (higher volume) hArr Liquid (lower volume) The process of melting is facilitated at high pressure, thus, melting point is lowerd. (b) Solids whose volume increase on melting e.g.,Fe,Cu,Ag,Au,etc. Solid (lower volume) hArr Liquid (higher volume) In this case the process of melting become difficult at high pressure, thus melting point becomes high. (c) Solubility of substances : When solid substances are dissolved in water, either heat is evolved (exothermic) or heat is absorbed (endothermic). KCI+aqhArrKCI(aq)-heat In such cases, solubility increase with increase in temperature. Consider the case of KOH, when this is dissolved,heat is evolved. KOH+aqhArrKOH(aq)+heat In such cases, solubility decrease with increase in temperature. (d) Solubility of gases in liquids : When a gas dissolves in liquid,there is decrease in volume. Thus increase of pressure will favour the dissolution of gas in liquid. A gas 'X' when dissolved in water,heat is evolved. Then solubility of 'X' will increase:
