An aqueous solution is a homogeneous mixture where a substance is dissolved in water. The term is derived from the Latin word aqua, meaning water, which identifies the defining component of this solution. This type of solution is common in laboratories and nature. It forms the medium for nearly all life processes, such as the blood in the body and the sap in plants. Understanding aqueous solutions is necessary for chemical and biological studies.
Defining the Aqueous Solution
Every solution consists of two primary components: a solute and a solvent. The solvent is the substance present in the largest amount, acting as the dissolving medium for the other components. Conversely, the solute is the substance that is dissolved within the solvent, and it is typically present in a smaller proportion.
An aqueous solution is defined by water always being the solvent. For example, when table salt (sodium chloride) is stirred into water, the salt acts as the solute and the water is the solvent. The oceans are also massive aqueous solutions, containing various salts and gases dissolved in the water. Since the mixture is homogeneous, the dissolved particles are uniformly distributed and will not settle out over time.
The Essential Role of Water’s Polarity
Water’s ability to dissolve many substances stems from its molecular structure and polarity. A single water molecule is composed of two hydrogen atoms bonded to one oxygen atom, creating a bent shape. Since the oxygen atom is more electronegative than the hydrogen atoms, it pulls the shared electrons closer to itself.
This unequal sharing of electrons results in a partial negative charge on the oxygen side of the molecule and partial positive charges on the hydrogen sides. A molecule with this separation of charge is called a dipole, and this polarity is what makes water such an effective dissolving agent. Water molecules surround the particles of a solute, with the positive hydrogen ends attracting negative ions or negative parts of a polar molecule.
The negative oxygen end attracts positive ions or positive regions of the solute particle. This action, known as solvation, pulls apart the solute’s structure, causing it to break down into individual ions or molecules that are then surrounded by a “hydration shell” of water. This process allows water to dissolve a wide range of ionic compounds and other polar substances.
How Aqueous Solutions Are Represented in Formulas
A notation is used to communicate the state of a substance in a chemical equation. To indicate that a substance is present as an aqueous solution, chemists append the state symbol (aq) to its chemical formula. For instance, sodium chloride dissolved in water is written as NaCl(aq).
This notation clarifies that the substance is dissolved in water, contrasting with other state symbols like (s) for a solid, (l) for a liquid, or (g) for a gas. The use of (aq) is important in reactions, as it signifies that the dissolved particles are free to move and interact with other substances in the solution.