What happens to an acid or a base when it is diluted with water?

Adding water to an acid or base will change its pH. Water is mostly water molecules so adding water to an acid or base reduces the concentration of ions in the solution.  When an acidic solution is diluted with water the concentration of H+ ions decreases and the pH of the solution increases towards 7.  To make the pH change by 1, a tenfold dilution is required (eg adding 9 cm3 of water to 1 cm3 acid). The acid is becoming less acidic. Similarly, when an alkali is diluted with water the concentration of OH- ions decreases. This  causes the pH of the alkali to fall towards 7, making the solution less alkaline as more water is added.

Explanation:

water and alcohols have similar properties because water molecules contain hydroxyl groups that can form hydrogen bonds with other water molecules and with alcohol molecules, and likewise alcohol molecules can form hydrogen bonds with other alcohol molecules as well as with water.