Sometimes atoms give their electrons up altogether instead of sharing them in a chemical bond. This process is known as disassociation. Water, for instance, dissociates by the following formula:

The hydrogen atom gives up a negatively charged electron, gaining a positive charge, and the OH compound gains a negatively charged electron, taking on a negative charge. The H⁺ is known as a hydrogen ion and OH^– ion is known as a hydroxide ion.

The disassociation of water produces equal amounts of hydrogen and hydroxide ions. However, the disassociation of some compounds produces solutions with high proportions of either hydrogen or hydroxide ions. Solutions high in hydrogen ions are known as acids, while solutions high in hydroxide ions are known as bases. Both types of solution are extremely reactive—likely to form bonds—because they contain so many charged particles.

The technical definition of an acid is that it is a hydrogen ion donor, or a proton donor, as hydrogen ions are consist of only a single proton. Acids put H⁺ ions into solution. The definition of a base is a little more complicated: they are H⁺ ion or proton acceptors, which means that they remove H⁺ ions from solution. Some bases can directly produce OH^– ions that will take H⁺ out of solution. NaOH is an example of this type of base:

A second type of base can directly take H⁺ out of an H₂O solution. Ammonia (NH₃) is a common example of this sort of base:

From time to time, the SAT II Biology has been known to ask whether ammonia is a base.

The pH scale, which ranges from 0 to 14, measures the degree to which a solution is acidic or basic. If the proportion of hydrogen ions in a solution is the same as the proportion of hydroxide ions or equivalent, the solution has a pH of 7, which is neutral. The most acidic solutions (those with a high proportion of H⁺) have pHs approaching 0, while the most basic solutions (those with a high proportion of OH^– or equivalent) have pHs closer to 14.

Water has a pH of 7 because it has equal proportions of H⁺ and OH^– ions. In contrast, when a compound called hydrogen fluoride (HF) disassociates, it forms only hydroxide ions. HF is therefore quite acidic and has a pH well below 7. Some acids are more acidic than others because they put more H⁺ ions into solution. Stomach fluid, for example, is more acidic than saliva.

When sodium hydroxide (NaOH) disassociates, it forms only hydroxide ions, making it a base and giving it a pH above 7. Like acids, bases can be strong or weak depending on how many hydroxide ions they put in solution or how many hydrogen ions they take out of solution.

Some substances resist changes in pH even when acids or bases are added to them. These substances are known as buffers. The cell contains many buffers because wide swings in pH can negatively impact the chemical reactions of cell processes.