The main lesson on Temperature and pH gives a brief description of pH, but does not talk much about chemistry. This page is meant for those who wish to learn more about the chemistry associated with pH, including how the daily and nightly activities of aquatic plants affect pH.
The pH of a solution is basically a measure of the concentration of hydrogen ions in moles per liter of solution. (A mole is a chemical mass unit, defined to be 6.022 x 1023 molecules, atoms, or some other unit.). The term pH is an abbreviation, with p standing for the German word for power (or potential), potenz, and H standing for Hydrogen. So pH literally means "the power of hydrogen."
You are probably familiar with the chemical formula for water, H2O. This formula tells us that each molecule of water is made up of 2 hydrogen ions (H+) and one oxygen ion (O-2). Remember in chemistry that ions are either positively or negatively charged. Ions with a positive charge are called "cations," while ions with a negative charge are referred to as "anions." Ions with the same charge (positive to positive or negative to negative) repel each other, but ions of opposite charge attract each other. So if you put a bunch of hydrogen and oxygen ions together, the positive charge on the hydrogen ion will be attracted to the negative charge on the oxygen ion. Each oxygen ion carries two negative charges, so an oxygen ion will attract and bond with two positive hydrogen ions. When they join together, they form a water molecule, H2O. Because all of the charges are matched up, the water molecule has a neutral charge.
If there are exactly enough hydrogen and oxygen ions in a solution, all of the charges will match up and you will get a neutral solution of pure water. So what happens if there are more hydrogen ions leftover when all of the oxygen ions have formed water molecules, or what if there aren't enough hydrogen ions to bind to the negative "ports" on the oxygen ions? This is where the measure of pH comes in.
Lets begin with the scenario where we have an abundance of hydrogen ions. All of the oxygen ions bind with two hydrogen ions forming water molecules, but there are still hydrogen ions left over.
In this situation, the solution is acidic, and pH will be less than 7. The more excess hydrogen ions, the lower the pH.