Ordinary water molecules have 2 Hydrogen atoms bonded to an Oxygen atom. Water is denoted by the chemical symbol H2O. Overall, this molecule is neutrally charged.

However, two water molecules can react with one another to produce a positively charged ion (H3O+) called Hydronium and a negatively charged ion (OH-) called Hydroxide. H3O+ is often simply denoted by H+. These ions are very reactive, that is, they don't like to be alone but will readily bind with many other substances.

Pure water has equal amounts of Hydronium and Hydroxide ions but some substances have more H+ ions than pure water and some substances have less H+ ions. Substances with larger numbers of H+ than pure water are called acidic and substances with less H+ are called alkaline.

Pure water is neutral, neither acidic nor alkaline. For comparison, battery acid has a pH of 1.0 and is extremely acidic. Lye has a pH of 13.0 which is very alkaline.

pH is not a linear scale but is a logarithmic scale so that a substance with a pH of 5.0 is 10 times more acidic than a substance with a pH of 6.0 and a substance with a pH of 10.0 is 10 times more alkaline than a substance with a pH of 9.0. Can you think of another measurement scale that is logarithmic?
ph Diagram

Why Does pH Matter

pH affects many chemical and biological processes in water bodies. The largest variety of aquatic organisms prefer a pH range of 6.5-8.0. pH outside this range lowers the diversity of aquatic organisms by reducing reproduction and stressing these organisms. Low pH also allows toxic metals and compounds to become mobile and available to aquatic organisms and plants.