Pure water is neutral because the total electric charges of the atoms and ions that compose it are equal and opposite, so they cancel each other out. Water that isn't pure, such as drinking water, may contain other elements that can affect its acidity.
The acidity of a substance is measured on the pH scale. Acidic materials have a pH less than 7, while alkaline materials have a pH greater than 7. Pure water, at room temperature, has a pH of almost exactly 7, which means it is neutral.
A water molecule consists of a hydrogen ion bonded to a hydroxide ion. The hydrogen ion is positively charged, while the hydroxide has a negative charge. This gives water an overall electric charge of zero.
Drinking water, however, may contain a number of different minerals that have been dissolved within it. If water contains large amounts of dissolved metal ions, including lead and iron, then it becomes acidic. If water is very acidic, it can be dangerous to drink.
If drinking water contains elements or molecules which are alkaline, then the pH level may increase. Water with high pH levels is called hard water and has a noticeably different taste to pure water.