Common table salt, or sodium chloride (NaCl), is usually associated with flavor rather than electrical flow. Most people associate conductivity with metals like copper, where electricity is carried by the rapid movement of free electrons. Salt is a highly conditional conductor, and the determining factor lies in its atomic structure and physical state. Understanding how salt works as a conductor requires shifting focus from the flow of electrons to the movement of charged atoms.
The Necessary Condition: Ions and Electrolytes
The fundamental difference between salt and metal conductivity lies in the type of charged particle responsible for carrying the current. Sodium chloride is an ionic compound, meaning it is built from atoms that have either gained or lost electrons, resulting in positively and negatively charged particles called ions. These charged particles are the sodium cation (\(\text{Na}^{+}\)) and the chloride anion (\(\text{Cl}^{-}\)). In this system, the electrical charge is carried by the physical movement of these entire charged atoms.
For any substance to conduct electricity, it must contain charged particles that are free to move under the influence of an electric field. A substance that conducts electricity when dissolved or when molten is classified as an electrolyte. When an external voltage is applied to an electrolyte, the positive sodium ions are drawn toward the negative electrode, and the negative chloride ions move toward the positive electrode. This directed movement of oppositely charged ions constitutes an electric current.
Conductivity Based on Physical State
The ability of salt to function as an electrolyte is dependent on its physical state. In its familiar solid form, table salt is organized into a rigid, crystalline lattice structure. The positive and negative ions are held firmly in fixed positions by strong electrostatic forces. Because the ions are locked into this tight structure, they are not mobile and cannot move to carry a charge, making solid salt an electrical insulator.
The situation changes completely when salt is introduced to water or heated to its melting point of about 801 degrees Celsius. When dissolved in water, the polar water molecules surround and pull the \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) ions apart, freeing them from the crystal lattice. Once free in the aqueous solution, these mobile ions can move toward the oppositely charged electrodes, allowing the solution to conduct electricity. Similarly, melting the salt also breaks the lattice, creating a molten state with highly mobile ions capable of charge transfer.
Ionic Conduction in Practical Applications
The principle of ionic conduction is applied in numerous real-world systems. Seawater, for instance, is a strong natural electrolyte because it contains a high concentration of dissolved salts, primarily sodium chloride. This high concentration of mobile ions is why saltwater is a much better conductor of electricity than pure water, which contains very few ions.
Industrial processes like electrolysis rely on the conductivity of molten or aqueous salts to drive chemical reactions. For example, the production of chlorine gas and sodium metal involves running an electric current through a concentrated salt solution. This process highlights that ionic conduction involves mass transfer, where the ions themselves physically move and react at the electrodes, a fundamental difference from the purely electronic flow in household wiring. The technology of modern batteries, including advanced lithium-ion and solid-state batteries, also utilizes electrolytes, which are often specialized salts engineered for high ion mobility. These systems depend on the movement of ions between the battery’s electrodes to store and release electrical energy.