* Acids donate H+ ions: Acids are substances that release hydrogen ions (H+) into solution.

* Water auto-ionizes: Water molecules can react with each other to form hydronium ions (H3O+) and hydroxide ions (OH-):

* 2H2O(l) ⇌ H3O+(aq) + OH-(aq)

* Le Chatelier's Principle: Adding acid (H+) shifts the equilibrium of this reaction to the left, favoring the formation of water molecules and consuming hydroxide ions (OH-).

In summary:

* Increasing the hydrogen ion concentration (adding an acid) reduces the hydroxide ion concentration.

* This is a direct consequence of the auto-ionization of water and Le Chatelier's principle.

Note: The relationship between hydrogen ion concentration ([H+]) and hydroxide ion concentration ([OH-]) is defined by the ionic product of water (Kw):

* Kw = [H+][OH-] = 1.0 x 10^-14 at 25°C

* As [H+] increases, [OH-] must decrease to maintain a constant Kw value.