Pure water isn’t a good conductor of electricity because it lacks ions necessary for carrying an electric current. In a solution, electrical conductivity depends on the presence of charged particles, either ions or electrons, that can move freely and carry charge. Pure water consists mostly of water molecules (H2O) and contains very few ions.
When an ionic compound, such as table salt (sodium chloride, NaCl), dissolves in water, it dissociates into its constituent ions, sodium (Na+) and chloride (Cl-) ions. These ions are responsible for conducting electricity in the solution because they can move freely in the water, carrying charge.
However, pure water undergoes autoionization to a very small extent, where a small fraction of water molecules dissociate into ions:
H2O (l) ⇌ H+ (aq) + OH- (aq)
This process generates very low concentrations of hydrogen ions (H+) and hydroxide ions (OH-) in the water. These ions are responsible for its slight conductivity. But the concentration of these ions in pure water is extremely low, about 10^-7 moles per liter each, due to the self-ionization constant of water (Kw = [H+][OH-] ≈ 10^-14 at 25°C).
Because of this low concentration of ions, pure water’s electrical conductivity is very low compared to that of an ionic solution like saltwater. The movement of ions in solution is what allows for the flow of electricity, and since pure water contains very few ions, it doesn’t conduct electricity well.
Furthermore, water molecules themselves are not good conductors of electricity. While water is a polar molecule with a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom, the movement of these polar molecules in response to an electric field is minimal compared to the movement of ions in an ionic solution. This limited movement of water molecules contributes to the overall low conductivity of pure water.
Temperature also affects the conductivity of water. Generally, as temperature increases, the kinetic energy of water molecules increases, leading to more ionization and therefore higher conductivity. However, even at elevated temperatures, pure water still has much lower conductivity compared to ionic solutions due to its low ion concentration.
It’s important to note that the purity of the water also affects its conductivity. Pure water, in the sense of being free from impurities, has very low conductivity. However, if impurities such as dissolved salts or acids are present, they can significantly increase the conductivity of the water by providing additional ions.
In summary, pure water isn’t a good conductor of electricity because it lacks sufficient ions to carry an electric current. While it undergoes autoionization to a small extent, generating low concentrations of hydrogen and hydroxide ions, the overall ion concentration is too low for effective conductivity. Additionally, water molecules themselves do not conduct electricity well, further contributing to the low conductivity of pure water.