What happens in an aqueous solution as the concentration of H3O+ decreases? ...

Each water molecule can form four hydrogen bonds with other water molecules due to its polar nature.

Therefore, a hydrogen atom taking part in a hydrogen bond between two water molecules can shift from one water molecule to the other. When this occurs, it leaves its electron behind. Therefore, the hydrogen ion will be a proton written as H+. The molecule that has lost its proton is called a hydroxide ion with the symbol OH-....

Each water molecule can form four hydrogen bonds with other water molecules due to its polar nature.

Therefore, a hydrogen atom taking part in a hydrogen bond between two water molecules can shift from one water molecule to the other. When this occurs, it leaves its electron behind. Therefore, the hydrogen ion will be a proton written as H+. The molecule that has lost its proton is called a hydroxide ion with the symbol OH-. The molecule that has accepted the extra proton is now called H30+ and is known as a hydronium ion. It is sometimes written as simply H+.

The process described above is dissociation of water molecules. However, these molecules can re-form water exists in a state of equilibrium with a neutral pH of 7.

Anytime a substance increases the H+ ion concentration in solution, it is called an acid. A base reduces the H+ ion concentration of a solution. Water is technically both an acid and a base. A shift in the number of H+ ions where they outnumbered the OH- ions would result in a lowered pH and the solution would be acidic. A shift in the number of H+ ions where they were less than the OH- ions would result in an increase in pH.

In a water- based solution at 25 degrees C, the product of H+ ions and OH- ions = 10 -14. Basically, in a neutral solution, there is an equal number of H+ and OH- ions.

The pH of a solution = -log (H+) therefore, in neutral water, the H ion concentration is 10-7 and if one uses the formula above, pH= - (-7)= 7. 

The pH scale is written from 1 to 14. Neutral water has a pH of 7 and values below 7 are increasingly stronger acids. Values above 7 and continuing to 14 are increasingly stronger bases.

In a solution, if the number of H30+ ions (also written as H+ ions) decreases, the pH of the solution will increase. There will be a shift toward more OH- negative ions as the H3O + ions decrease.