In chemistry, pH is a measure of the activity of the (solvated) hydrogen ion. p[H H], which measures the hydrogen ion concentration, is closely related to, and is often written as, pH. Pure water has a pH very close to 7 at 25 °C. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement.