In chemistry, a measure of the acidity or basicity of an aqueous solution, or of the molar Concentration of hydrogen ions in solution. Solutions less than pH 7 are called acidic, and those greater than pH 7 are basic or alkaline. Pure water has a pH very close to 7. Acidic and basic are two extremes that describe chemicals, just like hot and cold describe extreme temperatures. Mixing acids and bases can cancel out their extreme effects; much like mixing hot and cold water. A substance that is neither acidic nor basic is neutral. The pH scale ranges from 0 to 14. Each whole pH value is ten times stronger than the preceding value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10 times 10) more acidic than a pH of 6..