Ionic compounds do conduct electricity when dissolved in water. This ability stems from their unique chemical structure and how they interact with water molecules. Understanding this process involves examining what ionic compounds are, water’s specific properties, and the mechanism by which charged particles move through a solution.
What are Ionic Compounds?
Ionic compounds are substances formed from the chemical combination of a metal and a non-metal. This bond, known as an ionic bond, involves the transfer of electrons from the metal atom to the non-metal atom. For example, in table salt (sodium chloride, NaCl), a sodium atom donates an electron to a chlorine atom.
This electron transfer results in the formation of charged particles called ions. The atom that loses electrons becomes a positively charged ion, known as a cation, while the atom that gains electrons becomes a negatively charged ion, called an anion. In their solid state, these oppositely charged ions are held together in a rigid, repeating three-dimensional structure called a crystal lattice. Due to this fixed arrangement, ions in a solid ionic compound cannot move freely, making them poor conductors of electricity.
The Unique Role of Water
Water (H₂O) enables ionic compounds to conduct electricity because it is a polar molecule. This polarity arises from the uneven sharing of electrons between the oxygen and hydrogen atoms within a water molecule. The oxygen atom attracts electrons more strongly, giving it a slight negative charge, while the hydrogen atoms develop slight positive charges.
When an ionic compound is placed in water, the charged ends of the water molecules are attracted to the oppositely charged ions in the crystal lattice. The slightly negative oxygen ends of water molecules surround the positive cations, and the slightly positive hydrogen ends surround the negative anions. This interaction, known as ion-dipole interaction, weakens the strong electrostatic forces holding the ions together in the solid lattice. The water molecules then pull individual ions away from the crystal, a process called dissociation. Each separated ion becomes surrounded by a shell of water molecules, known as a hydration shell, which stabilizes it and allows free movement.
How Ions Carry Electricity
Once ionic compounds dissociate in water, the free-moving ions carry an electrical current. When an electric field is applied to the solution, these charged ions move directionally. The positively charged cations are attracted towards the negative electrode, while the negatively charged anions migrate towards the positive electrode.
This movement of charged particles through the solution constitutes the flow of electricity. Unlike in metals where electrons carry the current, in ionic solutions, the ions themselves act as charge carriers. The higher the concentration of these free ions in the solution, the greater its ability to conduct electricity. Ions physically move through the liquid, transporting matter.
Why Pure Water Doesn’t Conduct Well
Pure water, without dissolved substances, is a poor conductor of electricity. This is because water molecules are electrically neutral and do not break apart into ions. While a tiny fraction of water molecules can autoionize to form hydrogen (H⁺) and hydroxide (OH⁻) ions, their concentration is very low.
Therefore, pure water lacks sufficient free, mobile charged particles for effective current. The ability of tap water or other natural water sources to conduct electricity is due to the presence of dissolved ionic impurities, such as salts and minerals, not the water molecules themselves. These dissolved impurities provide the necessary ions that enable electrical conduction.