Sodium chloride, commonly recognized as table salt, is an ionic compound composed of positively charged sodium ions (\(\text{Na}^{+}\)) and negatively charged chloride ions (\(\text{Cl}^{-}\)). Whether this substance conducts electricity depends entirely on its physical state. In its solid, crystalline form, sodium chloride is an electrical insulator and does not conduct a current. However, when heated sufficiently to transform it into a liquid, or “molten” state, the compound becomes an excellent electrical conductor. This change happens because melting frees the charged particles, allowing them to move and carry an electric current.
Why Solid Sodium Chloride Does Not Conduct
In its solid state, sodium chloride exists as a highly ordered structure known as a giant ionic lattice. This crystalline arrangement is held together by strong electrostatic forces of attraction between the \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) ions, which are packed in a fixed, repeating pattern.
Although sodium chloride is made up of charged particles, these ions are not mobile; they are tightly bound within the rigid lattice. For any material to conduct electricity, it must possess mobile charge carriers, such as electrons or ions. Since the ions are immobile and there are no free-moving electrons, solid sodium chloride is an electrical insulator.
How Molten Sodium Chloride Becomes a Conductor
The transition to an electrical conductor happens when sodium chloride is heated past its melting point, approximately \(801^\circ\text{C}\). When the compound reaches this temperature, it changes from a rigid solid into a molten state. The high thermal energy breaks down the strong electrostatic forces holding the crystalline lattice together.
Once the lattice structure is destroyed, the fixed \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) ions become mobile and are free to move randomly throughout the liquid volume. When an electrical voltage is applied, these mobile charged ions act as charge carriers, a process called ionic conductivity. The positive sodium ions migrate toward the negative electrode (cathode), and the negative chloride ions move toward the positive electrode (anode), completing the electrical circuit. This mechanism differs from metallic conductivity, where electricity is carried by the movement of electrons. A substance that conducts electricity due to the movement of ions is classified as an electrolyte.
Industrial Uses of Molten Salt Electrolysis
The ability of molten sodium chloride to conduct electricity is applied in a major industrial process known as molten salt electrolysis. This technique uses electricity to drive a non-spontaneous chemical reaction, decomposing the compound into its elemental components. The most common setup for this is the Down’s cell, which is designed to handle the high temperatures required.
When a direct current is passed through the molten salt, the \(\text{Na}^{+}\) ions are reduced at the cathode, forming liquid sodium metal (\(\text{Na}\)). Simultaneously, the \(\text{Cl}^{-}\) ions are oxidized at the anode, producing chlorine gas (\(\text{Cl}_2\)). This process is the primary industrial method for obtaining pure sodium metal, a highly reactive substance used in chemical synthesis.
The process requires significant energy due to the high operating temperature needed to keep the salt molten. To make the industrial process more economically viable, other salts, such as calcium chloride, are often added. This addition lowers the mixture’s overall melting point, reducing the energy cost necessary to maintain the liquid state. The resulting products, sodium metal and chlorine gas, are collected separately and have widespread applications.