Solutions that contain dissolved salts conduct electricity because they release charged particles into solution that are capable of carrying an electric current. Scientists can quantify the extent to which they conduct electricity with conductivity measurements. In general, the conductivity of salt solutions increases as the amount of dissolved salt increases. The exact increase in conductivity, however, is complicated by the relationship between the concentration of the salt and the mobility (i.e., speed) of its charged particles.
To a chemist, the term “salt” refers to more than simple table salt. As a class of compounds, “salt” refers to chemicals comprised of a metal and a nonmetal. The metal assumes a positive charge and is referred to as a “cation,” whereas the nonmetal assumes a negative charge and is an “anion.” Chemists refer to such salts as “ionic” compounds. Electrostatic interactions, which simply refers to the attractive forces between the oppositely charged metal and nonmetal, hold ionic compounds together as solids.
Ionic Compounds in Water
Some ionic compounds dissolve in water. Chemists classify such compounds as “water-soluble.” When they dissolve, they break into their respective ions (or, in technical terms, they “dissociate”). Table salt (chemical name sodium chloride, chemical formula NaCl) breaks into sodium ions (Na?) and chloride ions (Cl?).
Not every ionic compound dissolves in water. Solubility guidelines, such as those available at the website of Clackamas University, provide chemists and students a general understanding of which compounds will or will not dissolve.
In basic terms, concentration simply refers to the amount of substance dissolved in a given amount of water. Scientists have developed numerous units for specifying concentration, such as molarity, normality, mass percent and parts per million. The exact unit of concentration runs secondary, however, to the general principle that higher concentration means a larger quantity of dissolved salt per unit volume.
Many people are surprised to learn that pure water is actually a poor conductor of electricity. The relevant term in the previous statement is “pure.” Virtually any water from a natural water source (river, lake, ocean) will act as a conductor because it will contain dissolved salts.
“Good” conductors allow for the easy, sustained flow of electric current. In general, a good conductor possesses charged particles that are relatively mobile (i.e., free to move). In the case of salts dissolved in water, the ions represent charged particles, and because they are solvated in water, the ions exhibit relatively high mobility.
Conductivity and Concentration
The equations governing conductivity indicate that the conductivity of a solution depends on the number of charge carriers (i.e., the concentrations of the ions), the mobility of the charge carriers and their charge.
Theoretically, conductivity should increase in direct proportion to concentration. This implies that if the concentration of sodium chloride, for example, in a solution were doubled, the conductivity should also double. In practice, this does not hold true. The concentration and mobility of the ions are not independent properties. As the concentration of an ion increases, its mobility decreases. As a consequence, experimentally the conductivity increases linearly with respect to the square root of concentration instead of in direct proportion.