Electrical conductivity measures a material’s ability to pass an electric current. In solids like metals, current is carried by the flow of negatively charged electrons. However, the mechanism differs in a liquid or solution. In a solution, the electrical current relies on the movement of charged atoms or molecules known as ions, not electrons.
The Essential Role of Mobile Ions
A solution’s ability to conduct electricity depends entirely on the presence and mobility of charged particles. Pure water, for example, is a very poor electrical conductor because it contains an extremely small number of naturally occurring ions. When a substance that can dissociate is dissolved in water, the resulting solution becomes conductive because the compound breaks apart into mobile, charged ions.
This process, called dissociation, involves the solvent (usually water) separating the solute into cations (positively charged ions) and anions (negatively charged ions). When an electric field is applied, these free ions move: cations migrate toward the negatively charged electrode (cathode), and anions move toward the positively charged electrode (anode). This synchronized movement constitutes the electric current flowing through the solution.
Ionic mobility, the speed at which ions move, directly influences the solution’s conductivity. Most ions must navigate around the water molecules, but the hydrogen ion (\(\text{H}^+\)) and hydroxide ion (\(\text{OH}^-\)) possess a unique, highly efficient transfer mechanism. These ions rapidly transfer their charge to adjacent water molecules, allowing current to pass much quicker than if the entire ion had to physically move. This special mechanism is why solutions containing strong acids or bases often exhibit exceptionally high conductivity compared to other solutions with similar ion counts.
How Different Solutes Affect Conductivity
Solutes are classified based on the degree to which they form ions, which directly correlates with conductivity. This classification defines whether a substance is a strong electrolyte, a weak electrolyte, or a non-electrolyte.
Strong Electrolytes
Strong electrolytes, such as table salt (sodium chloride), strong acids (\(\text{HCl}\)), and strong bases (\(\text{NaOH}\)), dissociate almost completely when dissolved in water. Because they produce a maximum number of free ions, solutions made with strong electrolytes exhibit very high conductivity.
Weak Electrolytes
Weak electrolytes, including weak acids (acetic acid) and weak bases (ammonia, \(\text{NH}_3\)), only partially break into ions when dissolved. Only a small percentage dissociates, meaning the solution contains a mixture of ions and whole, un-dissociated molecules. This partial ionization results in fewer charge carriers being available, giving these solutions a moderate to low level of electrical conductivity.
Non-Electrolytes
In contrast, non-electrolytes are substances that dissolve in water but do not produce any ions at all. Common examples include molecular compounds like sugar (glucose) and alcohol (ethanol). When these substances dissolve, the molecules remain intact, and because there are no mobile charged particles to carry the current, the resulting solution is virtually non-conductive.
External Variables That Change Conductivity
A prepared electrolyte solution’s conductivity is not fixed and can be modulated by external factors.
Concentration
Electrolyte concentration is a primary variable, as increasing the dissolved solute leads to a greater number of available ions. A higher concentration of charge carriers allows a larger current to flow, thus increasing the solution’s conductivity. However, this effect is not limitless; at very high concentrations, ions begin to interact with each other more strongly, sometimes forming pairs that reduce the number of truly free and mobile charge carriers.
Temperature
Temperature is the other significant external factor that influences a solution’s conductivity. When the temperature of the solution increases, the ions gain kinetic energy and move more quickly through the solvent. This increased ionic mobility facilitates the quicker transfer of charge between the electrodes, leading to a higher overall conductivity. For most aqueous electrolyte solutions, conductivity typically increases by about 1–3% for every one-degree Celsius rise in temperature.