An aqueous solution is a mixture where a solute has been uniformly dissolved in water, which serves as the solvent. The word “aqueous” comes from the Latin word for water, aqua, and this classification is ubiquitous in chemistry and biological processes. In chemical equations, the presence of an aqueous substance is indicated by the state symbol \((aq)\) written as a subscript next to the chemical formula, such as \(\text{NaCl}(aq)\).
Water as the Chemical Solvent
Water’s ability to dissolve a wide variety of substances is due to its molecular structure, which gives it a property called polarity. A water molecule (\(\text{H}_2\text{O}\)) has a bent shape, with the oxygen atom positioned at the center and the two hydrogen atoms attached at an angle of about \(104.5\) degrees. The oxygen atom is more electronegative than the hydrogen atoms, meaning it pulls the shared electrons closer to itself.
This unequal sharing of electrons creates a partial negative charge near the oxygen atom and partial positive charges near the two hydrogen atoms. A molecule with this kind of charge separation is called a dipole. This polarity allows water to follow the general principle of “like dissolves like,” meaning it readily interacts with and dissolves other polar molecules or ionic compounds.
When an ionic compound is placed in water, the negatively charged oxygen end of the water molecule is attracted to the positive ion, while the positively charged hydrogen ends are attracted to the negative ion. These attractions pull the ions apart from the crystal lattice, surrounding them with a shell of water molecules called a hydration shell. Polar covalent molecules, like sugar, are dissolved when water molecules form hydrogen bonds with the solute, separating and dispersing them throughout the solution.
Electrical Conductivity and Dissociation
When a solute dissolves in water, its behavior determines the solution’s ability to conduct electricity. Solutes that produce mobile, charged particles called ions are known as electrolytes. These free-moving ions act as charge carriers, allowing an electric current to flow through the aqueous solution.
For ionic compounds, the process of dissolving involves dissociation, where the crystal structure breaks down and the ions separate completely. Strong electrolytes, such as sodium chloride or strong acids like hydrochloric acid (\(\text{HCl}\)), dissociate or ionize almost \(100\%\) in water, producing a high concentration of ions. This high ion concentration results in a solution with high electrical conductivity.
Conversely, weak electrolytes, such as acetic acid or ammonia, only partially dissociate or ionize. They exist mostly as intact molecules, with only a small fraction forming ions, leading to a much lower concentration of charge carriers. This partial dissociation means that weak electrolyte solutions exhibit low electrical conductivity compared to strong electrolytes. Non-electrolytes, like sugar or ethanol, dissolve without forming any ions, resulting in solutions with essentially no electrical conductivity.
Measuring Concentration in Aqueous Solutions
For any chemical reaction involving an aqueous solution, knowing the exact amount of dissolved solute is necessary for precise calculations. The quantitative measure of the amount of solute present in a given amount of solution is referred to as concentration. This measurement is crucial for accurately preparing solutions and predicting the outcomes of chemical reactions.
The most common unit of concentration used in chemistry laboratories is Molarity, symbolized by the capital letter \(M\). Molarity is defined as the number of moles of solute divided by the total volume of the solution in liters (mol/L). This unit is favored because it directly relates the volume of the solution, which is easy to measure, to the number of particles (moles) of the solute, which dictates the reaction chemistry.
While Molarity is standard for laboratory work, other concentration measures are used for specific applications. For instance, environmental analysis and water quality reports often use parts per million (ppm) or parts per billion (ppb) to express the very small amounts of contaminants or trace elements found in water. Percentage concentration, such as percent mass or percent volume, is also frequently used in industrial and commercial contexts for preparing bulk solutions.