Sodium chloride (NaCl), known commonly as table salt, is a compound that does not conduct an electric current in its solid form. However, when dissolved in water, the resulting solution becomes a highly effective conductor of electricity. The answer lies in understanding the fundamental requirements for electrical conduction and how the physical state of the salt affects its internal structure.
The Foundation: Ions as Charge Carriers
Electrical current is fundamentally the movement of charged particles. Sodium chloride is an ionic compound, formed when a sodium atom transfers an electron to a chlorine atom, creating a positively charged sodium ion (\(\text{Na}^+\)) and a negatively charged chloride ion (\(\text{Cl}^-\)). These charged particles, called ions, are the carriers of electrical charge in salt. The movement of these ions, rather than electrons, facilitates the transfer of electrical current through a salt solution, defining ionic conduction.
Why Solid Sodium Chloride Does Not Conduct
Despite containing charged ions, solid sodium chloride is an electrical insulator. In its solid state, the \(\text{Na}^+\) and \(\text{Cl}^-\) ions are locked into a rigid, highly ordered crystal lattice. This structure is held together by strong electrostatic forces between the oppositely charged ions.
The ions are fixed in their positions and cannot move freely throughout the structure. Since electrical conduction requires the movement of charge carriers, the immobility of the ions prevents the flow of current. Without the ability to migrate, the ions cannot transport charge in response to an external voltage.
The Requirement for Mobility: Melting and Dissolving
The substance gains the ability to conduct electricity only when the rigid crystal lattice is broken down, allowing the ions to move freely. This mobility can be achieved through melting the salt or dissolving it in a solvent like water.
Melting
Melting sodium chloride requires heating it to over \(800^{\circ}\text{C}\). At this high temperature, the thermal energy overcomes the strong ionic bonds. Once molten, the ions are liberated from their fixed positions and become mobile, allowing the liquid salt to conduct electricity.
Dissolving
Dissolving salt in water is a more common process. Water is a highly polar molecule, meaning it has partial positive and negative charges. When salt is introduced, water molecules surround the ions: the negative ends attract the positive \(\text{Na}^+\) ions, and the positive ends attract the negative \(\text{Cl}^-\) ions.
These electrostatic attractions pull the \(\text{Na}^+\) and \(\text{Cl}^-\) ions away from the crystal lattice. Once separated, each ion is surrounded by a hydration shell of water molecules. This shell shields the ion’s charge, preventing it from rejoining other ions and allowing it to move independently through the solution. This dissolution process grants the ions the necessary mobility to become effective charge carriers.
The Mechanism of Ionic Current Flow
Once the ions are free to move in the solution, an external electrical potential drives the current flow. When electrodes are placed into the salt water and connected to a power source, the resulting electric field guides the movement of the charged ions.
The positively charged sodium ions (cations) are attracted to the negatively charged electrode (cathode). Conversely, the negatively charged chloride ions (anions) migrate toward the positively charged electrode (anode). This coordinated, directional movement of positive and negative ions constitutes the electrical current through the solution.
At the electrodes, the ions transfer their charge, completing the circuit. This process is known as electrolysis, where the flow of electric current results in a chemical change at the electrode surfaces. The continuous migration of ions toward the oppositely charged terminals is the mechanism by which dissolved sodium chloride conducts electricity.