Sodium chloride (\(\text{NaCl}\)), commonly known as table salt, is an ionic compound consisting of a positive sodium ion (\(\text{Na}^+\)) and a negative chloride ion (\(\text{Cl}^-\)). For a substance to conduct electricity, it must contain mobile charged particles, such as free electrons or mobile ions. Whether \(\text{NaCl}\) can conduct electricity depends entirely on its physical state. In the case of sodium chloride, the presence and mobility of its constituent ions determine its ability to transmit an electric current.
Why Solid Salt Does Not Conduct Electricity
Solid sodium chloride is an electrical insulator because its charged particles are locked into a rigid structure. The positive \(\text{Na}^+\) and negative \(\text{Cl}^-\) ions are held together by strong electrostatic forces, forming a highly ordered crystalline lattice. This stable, three-dimensional arrangement prevents the ions from moving freely throughout the compound.
When a voltage is applied, the fixed positions of the ions mean they cannot migrate toward an oppositely charged electrode. Since the movement of charged particles is a prerequisite for electrical flow, no current can pass through solid salt. Furthermore, \(\text{NaCl}\) lacks the delocalized electrons present in metals, which are the primary charge carriers in metallic conductors. The tightly bound electrons cannot move to carry a current, resulting in the solid’s non-conductivity.
Conductivity When NaCl is Molten
If solid sodium chloride is heated to its melting point of approximately \(801^\circ\text{C}\), the ionic bonds holding the crystal lattice are overcome. The thermal energy supplied breaks the rigid structure, causing the solid to transition into a molten state. This allows the \(\text{Na}^+\) and \(\text{Cl}^-\) ions to separate and move randomly.
In this molten state, the ions become highly mobile charge carriers free to travel throughout the liquid. When an electrical potential is introduced, the positive \(\text{Na}^+\) ions migrate toward the negative electrode, and the negative \(\text{Cl}^-\) ions move toward the positive electrode. This directed movement of charged ions constitutes an electric current, making molten \(\text{NaCl}\) an effective electrical conductor. The mechanism of conduction here is ionic, where the ions themselves carry the charge, as opposed to metallic conduction carried by electrons.
Conductivity When NaCl is Dissolved in Water
Sodium chloride most commonly conducts electricity when dissolved in water, forming a solution categorized as an electrolyte. Water is a polar solvent, meaning its molecules have slight positive and negative ends. This polarity is the driving force behind the separation of the ions.
When salt is mixed with water, the polar water molecules surround the \(\text{Na}^+\) and \(\text{Cl}^-\) ions, pulling them away from the crystal lattice in a process called dissociation. The negative oxygen ends of the water molecules attract and surround the positive \(\text{Na}^+\) ions, while the positive hydrogen ends surround the negative \(\text{Cl}^-\) ions.
This process, known as solvation, results in hydrated ions that are completely separated and freely dispersed throughout the solution. These solvated \(\text{Na}^+\) and \(\text{Cl}^-\) ions are highly mobile and act as the necessary charge carriers.
When a voltage is applied to the salt water, the mobile ions are directed toward the electrodes of opposite charge, creating a flow of electricity. The conductivity of the aqueous solution is directly proportional to the concentration of these free ions; the more salt that is dissolved, the greater the number of mobile ions available to carry the current. Salt solutions are described as strong electrolytes due to this ability to conduct electricity via ion movement.