Pure water is neutral on the pH scale because the scale measures the relative amount of free hydrogen and hydroxyl ions caused by the presence of non-water compounds. Pure water is the neutral reference point around which the scale was developed.
A solution with a pH of less than 7 is considered acidic. Acidic solutions contain more free hydrogen ions than hydroxyl ions. Basic solutions contain more hydroxyl ions than hydrogen ions and have a pH above 7. Water found in nature is generally not neutral. Bodies of water contain nutrients and heavy metals that affect the pH. Rain water contains carbon dioxide from the atmosphere, causing an acidic pH of 5.6.