Carbonic acid is a weak acid that is created when carbon dioxide (CO2) is dissolved in water (H2O), resulting in the chemical formula H2CO3. When the acid dissociates, or gives up a hydrogen ion, the resulting molecule is called a bicarbonate ion. Carbonic acid appears frequently in the natural world. It can be found in sodas, champagne, and blood. The acid even appears in rain.
During the making of soda, carbon dioxide is dissolved in water. As stated, this process also creates carbonic acid. This acid, along with phosphoric acid and other acids, provides the tart taste in many sodas. It also provides a slight burning sensation that a person feels when ingesting a fizzy drink. Thus, it is the acid that makes fizzy drinks taste fizzy.
Carbonic acid plays an important role in keeping the body’s pH stable. The normal pH of bodily fluids is around 7.4 and must be kept close to this value in order for the body to function properly. If the pH changes, whether up or down, enzymes can stop functioning, muscles and nerves can start weakening, and metabolic activities becomes impaired. The bicarbonate ion released from acid serves as a buffer that helps resist changes in pH. This means it can act as an acid or a base as the need arises.
Acids are defined as any substance that releases hydrogen ions into solutions. Bases are substances that accept those hydrogen ions. When excess hydrogen ions build up in the body — i.e. the fluids become more acidic — then bicarbonate ions accepts those extra hydrogen ions and keeps the body’s pH at a normal level. In the inverse, if the hydrogen ion levels drop too much — i.e. the fluids become too alkaline — then carbonic acid gives up hydrogen ions in order to keep the blood’s pH normal. This process is also seen during the transport of oxygen and carbon dioxide.
Carbonic acid even appears as a normal occurrence in rain. As rainwater falls through the air, it absorbs carbon dioxide, producing carbonic acid. Thus, when it reaches the ground, it has a pH of about 5.5. This should not be confused with acid rain which is caused when emissions, such as sulfur oxides and nitrogen oxides, from burning fossil fuels rises to the air. As it falls, rain absorbs these components, producing acids which can make the pH in rain fall to as little as two.