When water interacts with other substances, the outcome varies. Some substances, like table salt, dissolve completely and change the water’s electrical nature. Other substances, such as sugar, also dissolve but do not produce the same electrical changes. This difference is tied directly to the chemical structure of the dissolved substance. Understanding this process centers on whether a substance remains whole or breaks apart when mixed with water.
Solutes and Solvents: Defining the Key Players
A solution is a uniform mixture where the solute is dispersed evenly throughout the solvent. When water is the solvent, the resulting mixture is called an aqueous solution. The solute’s behavior is classified based on whether it produces free-moving, charged particles.
Electrolytes are solutes that generate ions—electrically charged atoms or molecules—when dissolved in water. This release of ions allows the water to conduct an electric current. Nonelectrolytes, in contrast, dissolve but do not form any ions in the process.
The mechanism of an electrolyte breaking apart is called dissociation, the separation of a compound into its constituent ions. Nonelectrolytes do not undergo dissociation when placed in water. They enter the solution as whole, neutral molecules, which is why the resulting mixture does not conduct electricity.
The Chemistry of Dissociation
To understand dissociation, examine how an electrolyte, such as sodium chloride (NaCl), behaves in water. Sodium chloride is held together by ionic bonds, where electrons are transferred between the sodium and chlorine atoms, creating positive sodium ions (\(Na^+\)) and negative chloride ions (\(Cl^-\)). In its solid form, these ions are locked into a rigid crystal lattice structure.
Water molecules are polar, having a slightly negative end (oxygen) and a slightly positive end (hydrogen). When the salt crystal is introduced, these polar water molecules are strongly attracted to the charged ions on the crystal’s surface. The negative ends surround the positive sodium ions, while the positive ends surround the negative chloride ions.
This attraction, known as ion-dipole attraction, is strong enough to overcome the ionic bonds holding the crystal together. The water molecules pull the individual ions away from the solid structure, surrounding and stabilizing them in the solution, a process called hydration or solvation. The resulting solution contains mobile, charged ions, which is necessary for electrical conductivity.
How Nonelectrolytes Dissolve Without Dissociating
Nonelectrolytes, such as table sugar or ethanol, differ from electrolytes because their atoms are held together by covalent bonds. In a covalent bond, atoms share electrons rather than transferring them, resulting in a neutral molecule without pre-formed ions. Because the attractive forces are within the molecule itself, the molecule remains intact when dissolved.
When a nonelectrolyte like a sugar molecule is placed in water, the polar water molecules are still attracted to the molecule’s polar regions. The water molecules surround the entire sugar molecule, pulling it away from the solid crystal structure. This process is known as molecular dispersion or simple solvation.
Water molecules disrupt the weaker intermolecular forces holding the solid sugar molecules together. However, they lack the strength to break the stronger covalent bonds within each individual sugar molecule. The molecule is dispersed throughout the solvent as a whole, neutral unit, and does not split into charged components. This lack of dissociation means that, while a sugar solution may look similar to a salt solution, it remains non-conductive.