Sodium chloride (NaCl), or table salt, exhibits a significant difference in electrical conductivity depending on its state. In its common solid state, table salt is a poor conductor of electricity. However, when the substance is heated until it liquefies, it suddenly becomes an excellent conductor of an electric current. This transition highlights the requirements for charge movement and requires looking closely at the structure of the compound in both its solid and molten forms.
The Fixed Structure of Solid Sodium Chloride
Sodium chloride is an ionic compound, consisting of positively charged sodium ions (\(\text{Na}^+\)) and negatively charged chloride ions (\(\text{Cl}^-\)), held together by a strong electrostatic attraction (an ionic bond). In the solid state, these ions arrange themselves into a highly ordered, repeating three-dimensional structure called a crystal lattice.
The ions are locked into fixed positions, only able to vibrate slightly around their designated points within the structure. For any material to conduct electricity, it must contain charged particles that are free to move throughout the substance.
Since the constituent ions are immobilized in their fixed positions, there are no mobile charge carriers available to transport an electric current. Unlike metals, which conduct electricity via the movement of delocalized electrons, solid ionic compounds lack these free-moving electrons. The fixed nature of the ions prevents electrical conduction.
The Transition to a Molten, Conductive State
The conversion of solid sodium chloride into its molten state requires a significant input of thermal energy due to the strength of the ionic bonds. Sodium chloride melts at approximately 801°C, the temperature necessary to disrupt the powerful electrostatic forces holding the lattice together.
Once the melting point is reached, the rigid, fixed arrangement of the ions is overcome, and the crystal lattice completely collapses. The \(\text{Na}^+\) and \(\text{Cl}^-\) ions become dissociated from their fixed positions and are now free to move randomly throughout the entire volume of the liquid.
The resulting liquid is a pool of dissociated, mobile ions, which are the required charged particles for electrical conduction. This freedom of movement allows molten sodium chloride to function as an electrolyte.
How Mobile Ions Carry the Electric Current
The presence of mobile ions in the molten state directly enables the flow of electricity when an external electric potential is applied. When two electrodes—a cathode (negative) and an anode (positive)—are placed into the molten salt, the random motion of the ions becomes directional.
The positively charged sodium ions (cations) are attracted to and move toward the negatively charged electrode, the cathode. Conversely, the negatively charged chloride ions (anions) are drawn to and migrate toward the positively charged electrode, the anode. This organized movement of oppositely charged ions in opposite directions constitutes the electric current within the molten salt.
This mechanism of charge flow, involving the physical transport of ions, is fundamentally different from the way current flows through a metal wire. In metallic conduction, electrons move, but the atoms remain in place. In molten sodium chloride, the entire charged particle—the ion—must physically move to carry the current, making it an example of ionic conduction.