Hydrogen and hydroxide concentration

[nobahar]

This may not even be a valid question, as I may have misunderstood; but here goes...
At 25C, the K_W of water is 1 x 10^-14 If you increase the amount of hydogen ions present, why does the concentration of hydroxyl ions present alter in such a way that the K_W is maintained (i.e. the product of the concentrations of hydrogen and hydroxyl ions)?
If you introduce protons, then they may associate with water to form hydroxonium ions, or they may associate with hydroxyl ions; in the first there will be an incraese in the hydrogen ion concentration, in the second there will be more water formed, which decreases the hydroxyl concentration but does not alter the hydrogen ion concentration. If it was a mixture of the two (that is, if the protons introduced interact with water both water and hroxyl ions), I still can't see how the two concentrations would be balanced so that the K_W is maintained.
No textbooks seem to give a reason, which suggests I've misunderstood. Can someone help me with this?

 

[Borek]

why does the concentration of hydroxyl ions present alter in such a way that the K_W is maintained

Because that's exactly the way it works?

All chemical equilibria have the same property - at equilbrium reaction quotient equals equilbrium constant. It can be explained in thermodynamical terms (using chemical potentials), it can be explained in kinetic terms (using speeds of forward and backward reactions) - but in fact both explanations just follow the observed reality.

Reality was first, explanations are later

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[nobahar]

Am I right in saying that this ties in with Le Chatelier's principle, then?

 

[Borek]

Yes, just remember that Le Chatelier's principle also describes reality as it was observed.

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[nobahar]

Thankyou Borek.