what the hell... let me explain this.
you see there is such a thing called the equilibrium constant, k. this constant defines where the equilibrium should be at. ie favouring which side. k = concentration of product/concentration of reactant.
but in this case the concentration of the reactant is ultimately constant since it is a solid. ie the density of NaCl(s) does not vary, the concentration of any pure solid or liquid can be regarded as a constant. we can only talk about concentration in terms of ions in a solution or gases in a confined space. therefore the only affector of this dynamic equilibrium can only be the products, ie the ions.
if you want a mathematical explaination of why le chatelier's principle works on chemical reactions consult an advanced chem book or something... lol.
so just remember that solids have a constant concentration, because they are not affected by pressure, temperature, or how much space there is or water there is in a solution and so forth.
so a simple answer would be when adding more solid you are not changing the concentration, so le chateliers principle tells us that we have no shift in equilibrium since only a concentration change can effect the equilibrium position. or another way to put it is that the adding of solid equally affects the foward and reverse reaction of the system, like a catalyst would do.
edit: hsc is fucked... if you're pretty smart person then you'll be confused of why such and such things happen instead of just being told they just do and learn to accept the facts lol
another thing you'll come across is the effect of inert gases... thats a funny one... they just tell you inert gase does not effect the equilibrium without any sought of explaination. when they can just provide a simple explaination in terms of dalton's law of partial pressures which is simple logics if you understand it... but that defeats the philosophy of the hsc: "minimal understanding maximum assimilating"
