Carbon dioxide is a colorless, odorless compound naturally present in the atmosphere. Pure carbon dioxide, whether as a gas, liquid, or solid dry ice, does not conduct electricity. However, the situation changes when this molecule dissolves into water, a common occurrence in nature and beverages.
What Makes a Substance Conductive
Electrical conductivity depends on the presence of mobile, charged particles that are free to move and carry an electric current. There are two primary types of charge carriers in materials.
The first type involves free electrons, found in metals like copper or silver. In these materials, valence electrons are delocalized, forming a “sea of electrons” that can flow easily when an electrical field is applied.
The second type consists of mobile ions, which are atoms or molecules that have gained or lost electrons, acquiring a positive or negative charge. These ions are the charge carriers in molten salts or in electrolytes dissolved in a solvent like water. If a substance lacks both free electrons and mobile ions, it is considered an electrical insulator.
Electrical Behavior of Pure Carbon Dioxide
Pure carbon dioxide is a non-metal composed of one carbon atom double-bonded to two oxygen atoms, forming a linear molecule. This bonding structure involves the sharing of electrons, known as a covalent bond. The electrons are tightly held within the molecule, and there are no free or delocalized electrons available to carry a current.
The linear geometry of the O=C=O molecule means that while the individual bonds are slightly polar, the opposing forces cancel each other out. This results in a non-polar molecule with no net electrical dipole moment. This lack of freely moving charged particles means that pure gaseous, liquid, or solid carbon dioxide (dry ice) is an electrical insulator.
When CO2 Dissolves in Water
The lack of conductivity in pure carbon dioxide changes once it encounters water, such as in carbonated drinks or rainwater. When carbon dioxide gas dissolves in water, the molecules react with the water to form carbonic acid (\(\text{H}_2\text{CO}_3\)). The equilibrium reaction is: \(\text{CO}_2 (\text{aq}) + \text{H}_2\text{O} (\text{l}) \rightleftharpoons \text{H}_2\text{CO}_3 (\text{aq})\).
Carbonic acid is classified as a weak acid because it only partially dissociates in the solution. This dissociation releases a small number of mobile ions, primarily hydrogen ions (\(\text{H}^+\)) and bicarbonate ions (\(\text{HCO}_3^{-}\)). The reaction is \(\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^{-}\), which is the source of the solution’s slight acidity and conductivity.
Because only a small fraction of the carbonic acid molecules ionize, the resulting solution is considered a weak electrolyte. The presence of these mobile ions allows the solution to conduct electricity, although the conductivity remains relatively low compared to solutions of strong electrolytes. The degree of this weak conductivity is directly proportional to the amount of dissolved carbon dioxide.
Real World Importance of CO2’s Conductivity
The non-conductive nature of pure carbon dioxide has significant practical applications, particularly in fire safety. \(\text{CO}_2\) fire suppression systems are used to extinguish Class C fires, which involve energized electrical equipment. Using a non-conductive agent in these scenarios prevents short circuits and electrical shock hazards.
The gas blankets the fire, displacing the oxygen required for combustion without causing damage to sensitive electronics or leaving a residue. Conversely, the slight conductivity of water containing dissolved \(\text{CO}_2\) impacts industries dealing with water quality. For instance, the conductivity of carbonated water must be accounted for in industrial processes and measurements where mobile ions can alter sensor readings.