Ionic product of water (Kw)

[Strawman]

Got a really lame question that probably has a really simple answer:

 

I know that if you multiply the concentration of Hydroxide ions (OH-) and Hydronium ions (H+) in any solution at 25 degrees it will always add up to 10-14. This is because they are 10-7 each. I've been told/read this is because if you add and acid to a solution (something that donates protons) they form Hydronium ions increasing the concentration of H30+ ions.

 

Why does this decrease OH- concentration?

 

HCl + H20 = H30+ +Cl-, no more. H+ increases OH- stays the same. Should be H+= 10-6 and OH- =10-7 = 10-13.

 

I can't get to grips with why one affects the other. If it is because H+ reacts with H20 forming H30+, surely this has no affect on OH-. Is it a concentration thing? or is something else going on that i've not thought of?

 

I need a really simple explanation please.

 

 

[insane_alien]

le chateliers principle. you shift the equilibrium to favour forming water by increasing the ion concentration. of course there are so many hydronium ions that this drastically reduces the number of OH- ions.