pH is a measure of how acidic a substance is. Acidity is caused by hydrogen ions in a liquid, with more hydrogen ions making the liquid more acidic. However, the concentration of hydrogen ions can range from just a few ions per litre to many billions of ions per litre of liquid, so the pH scale has been designed to make the numbers a bit more manageable. pH numbers range from 1 to 14, with 1 being very acidic, and 14 being very alkaline (the opposite of acidic) with neutral solutions (such as distilled water) having a pH of 7.