Scientifically speaking, a molecule of water consists of one Oxygen atom (O) and two Hydrogen atoms (H), which are bound together by both negative electrical charges and positive electrical charges to from H2O. Instead of engaging in a prolonged Chemistry 101 lecture, just let it be understood that a subsequent exchange of electrical charges will occur, causing one Hydrogen atom to break away from the water molecule (H2O) to form a positively charged Hydrogen ion and a negatively charged Hydroxide ion. Within a large body of water, such as a swimming pool, the majority of the fill will exist as scientifically defined water molecules (H2O), but some Hydrogen ions and some Hydroxide ions will exist. It is the proportion of these combined Hydrogen ions and Hydroxide ions to the remainder of the water molecules that determines whether water is Acidic (having a pH level below 7.0) or Alkaline (having a pH level above 7.0), and thus, out of balance.

It is the hope of the authors of poolmanual.com that this excerpt from Chemistry 101 made little or no sense. You do not have to understand the Chemistry of Water in order to understand Water Chemistry. This tab was only included to express that water is volatile, and in a swimming pool, must be tested and adjusted accordingly.