Solute and solvent Knowledge-Based Study

Study notes

1. What Is a Solution?

   In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, the particles of the mixed substances are distributed uniformly at the molecular or ionic level, meaning the physical properties and composition are identical throughout any sample taken from the mixture.

   Every simple binary solution consists of two fundamental components:

   • Solute: The substance that is dissolved.
   • Solvent: The substance that does the dissolving and provides the medium in which the solute particles disperse.

   Typically, the component present in the greater quantity is designated as the solvent, while the component present in the lesser quantity is the solute.

   Identifying Components in Common Systems

   Identifying which substance acts as the solute and which acts as the solvent depends on their initial physical states and relative amounts:

   • Solid Dissolved in a Liquid: When table salt (sodium chloride) dissolves in water, water retains its liquid state while salt disperses throughout it. Therefore, sodium chloride is the solute and water is the solvent.
   • Gas Dissolved in a Liquid: In carbonated water, carbon dioxide gas is dissolved under pressure into liquid water. Here, carbon dioxide is the solute and water is the solvent.
   • Liquid Dissolved in a Liquid: When two liquids form a solution, such as rubbing alcohol (a mixture of isopropanol and water), the component with the larger volume or molar amount is the solvent. For example, in a 70% isopropanol and 30% water mixture by volume, water is the solute and isopropanol is the solvent.
   • Gaseous Solutions: Air is a homogeneous mixture of gases. Nitrogen makes up roughly 78% of dry air by volume, making nitrogen the solvent, while oxygen (about 21%), argon, and trace gases act as solutes.

   The Role of Water as a Universal Solvent

   An aqueous solution is any solution in which water serves as the solvent. Because water is a highly polar molecule with partial positive and partial negative regions, it readily dissolves many ionic compounds and polar covalent substances.

   During dissolution, solvent molecules surround individual solute particles in a process called solvation (or hydration when the solvent is water):

   1. Solute-solute attractive forces must be overcome.
   2. Solvent-solvent intermolecular attractions must be disrupted to accommodate solute particles.
   3. Solute-solvent attractive forces form, releasing energy and stabilizing the dispersed particles throughout the solution.

   If the solute-solvent interactions are sufficiently favorable compared to the individual solute-solute and solvent-solvent interactions, the solute dissolves completely to form a stable homogeneous solution.