An aqueous solution is defined as any solution where water acts as the solvent, deriving its name from the Latin word aqua (water). This type of solution forms a homogeneous mixture where one or more solutes—which can be a solid, liquid, or gas—are dissolved uniformly throughout the water. Water’s ability to dissolve a wide array of substances makes aqueous solutions the primary medium for countless chemical reactions in both laboratory and natural settings.
Understanding Water as the Universal Solvent
Water’s effectiveness as a solvent stems from its molecular structure, which is bent and highly polar. The oxygen atom strongly attracts electrons, giving it a partial negative charge, while the two hydrogen atoms carry partial positive charges. This uneven distribution creates a dipole moment, meaning the molecule has distinct positive and negative ends, similar to a tiny magnet.
This polarity allows water to interact with other charged particles and polar molecules, following the principle that “like dissolves like.” When an ionic compound, such as table salt (sodium chloride), is placed in water, the negatively charged oxygen ends of water molecules are attracted to the positive sodium ions. Simultaneously, the positively charged hydrogen ends are drawn to the negative chloride ions.
These attractions overcome the internal bonds holding the solute together, pulling the ions apart in a process called dissociation. The separated ions are then surrounded by a shell of water molecules, known as a hydration shell, which keeps them dispersed. This process of surrounding a solute particle to stabilize it is called solvation. Water can dissolve more substances than any other liquid, earning it the nickname “universal solvent,” though it cannot dissolve nonpolar substances like oils.
Dissolving Substances Electrolytes and Nonelectrolytes
A substance’s behavior when dissolved in water determines whether the resulting aqueous solution can conduct an electric current. Substances that dissociate into ions when dissolved are known as electrolytes. These mobile, charged particles allow electricity to pass through the solution. Electrolytes include most soluble salts, strong acids (like hydrochloric acid), and strong bases (like sodium hydroxide), which ionize almost completely in water.
Strong electrolytes, such as sodium chloride, produce a high concentration of ions and are excellent conductors of electricity. Weak electrolytes, like certain weak acids, only partially dissociate, resulting in fewer ions and poor conductivity. Electrical conduction in an aqueous solution is defined by the movement of these charged ions through the water, not the movement of electrons as in a metal wire.
Conversely, nonelectrolytes dissolve in water but remain intact as neutral molecules, meaning they do not break apart into ions. Common examples include table sugar (sucrose), ethanol, and urea. Because these dissolved particles lack an electrical charge, their aqueous solutions cannot conduct electricity. The solute’s molecular structure determines whether it forms ions, distinguishing an electrolyte from a nonelectrolyte.
Aqueous Solutions in Biological Systems and Chemical Reactions
Aqueous solutions are the medium for nearly all biological activity on Earth. The cytoplasm within every cell and extracellular fluids like blood plasma are complex aqueous solutions. These environments allow nutrients, ions, and signaling molecules to be transported throughout the organism and waste products to be carried away.
All metabolic processes, including the breakdown of food and the synthesis of proteins, rely on reactants being dissolved in water. Water’s ability to dissolve charged molecules and facilitate their interaction enables reactions like acid-base neutralizations and oxidation-reduction to occur efficiently. For example, digestion involves hydrolysis reactions, where water breaks down larger molecules.
In chemical notation, the state of a substance dissolved in water is represented by the symbol “(aq)” placed after the chemical formula. This designation is used in chemical equations for reactions like precipitation, where two aqueous solutions form an insoluble solid. The “(aq)” symbol indicates that reactants are dispersed at the molecular or ionic level, a necessary condition for quick reaction.