Salt water conducts electricity, but sugar water does not, a difference that seems puzzling given that both substances dissolve readily in water. This observation points to a fundamental chemical distinction in how these two familiar compounds interact with water molecules. The ability of a liquid solution to carry an electric current is determined by the nature of the particles released into the water, not just whether a substance dissolves. Understanding this reveals why one solution is conductive and the other is an insulator.
What Is Required for a Liquid to Conduct Electricity
The flow of electricity in a liquid solution requires the presence of mobile, electrically charged particles. In liquids, the current relies on charged atoms or molecules, called ions, to move the charge.
A substance that produces these mobile ions when dissolved in water is known as an electrolyte, and its solution can conduct electricity. Conversely, a non-electrolyte dissolves but does not yield any charged particles, meaning the solution cannot carry a current. Pure water is a very poor conductor, but the addition of an electrolyte dramatically increases its conductivity by supplying the necessary charge carriers.
How Dissolving Salt Creates Mobile Charges
Salt, chemically known as sodium chloride, is held together by a strong attraction between oppositely charged atoms. In its solid state, the positive sodium ions and negative chloride ions are locked into a rigid crystal structure, preventing them from moving freely.
When salt is added to water, the polar water molecules are strongly attracted to these charged ions. The water molecules surround the ions and exert enough force to pull the individual ions out of the crystal lattice. This process is called dissociation, and it separates the salt into its constituent ions, which are then free to move throughout the solution.
Why Dissolving Sugar Does Not Create Mobile Charges
Sugar, specifically sucrose, is a compound where the atoms share electrons to form neutral molecules. These shared electron bonds hold the atoms together in an intact molecular structure. Unlike salt, there are no pre-existing charged ions within the solid sugar crystal.
When sugar dissolves, the water molecules surround the sugar particles, pulling them away from the solid structure. This process is called hydration, but it does not break the bonds within the sugar molecule itself. The entire sugar molecule remains intact as a single, electrically neutral unit. Because the resulting solution contains only these whole, neutral sugar molecules, there are no mobile charged particles to carry the electrical current.
The Fundamental Chemical Difference
The contrasting behaviors of salt and sugar stem entirely from the fundamental type of chemical bonding within each compound. Salt is an ionic compound, meaning it is composed of already-charged particles held together by strong electrostatic forces. Water simply separates these pre-formed ions, making them available for electrical conduction.
Sugar, conversely, is a molecular compound, also called a covalent compound, where atoms are linked by shared electrons, resulting in a neutral overall structure. Water is only strong enough to break the weaker forces holding the individual sugar molecules to each other in the crystal. It cannot break the stronger covalent bonds inside the molecule to create new, charged particles. The presence or absence of these mobile ions created by dissociation is the sole reason for the difference in electrical conductivity.