When a solid is added to a liquid, the familiar process of dissolution occurs. This action, such as stirring sugar into tea, is a physical change where the solid’s structure breaks down without forming a new chemical substance. Understanding this phenomenon requires looking closely at the particles involved and the forces that govern their interactions.
Defining the Components of Dissolution
The process of dissolution involves three components. The substance being dissolved is the solute (the solid material). The liquid that does the dissolving is called the solvent, which is typically present in the larger amount. When the solute particles are uniformly dispersed throughout the solvent at a molecular level, the resulting homogeneous mixture is called the solution. For example, when salt is mixed into water, the salt is the solute, the water is the solvent, and the resulting saltwater is the solution.
The Molecular Mechanism of Dissolving
Dissolution begins when solvent molecules collide with the outer surface of the solid solute. The solvent must overcome the attractive forces holding the solid particles together, such as lattice energy in an ionic crystal or intermolecular forces in a molecular solid. As solvent molecules bombard the solid, they pry individual solute particles—atoms, ions, or molecules—away from the main crystal structure.
Once separated, the solute particles become surrounded by solvent molecules in a process called solvation (or hydration when the solvent is water). The solvent molecules orient themselves around the solute particle, creating a shield that prevents the particle from reattaching to the remaining solid. These solvated particles then diffuse throughout the liquid, resulting in a stable, uniform mixture that defines the solution.
Factors Influencing Dissolution Rate
The speed at which a solid dissolves is known as the dissolution rate, and it is influenced by several physical factors. Increasing the temperature of the solvent speeds up the rate of dissolving for most solids. Higher temperatures give solvent molecules greater kinetic energy, causing them to move faster and collide with the solid surface more frequently and with greater force, effectively breaking apart the solid’s internal forces.
Agitation, such as stirring or shaking the mixture, also increases the rate. Stirring continuously brings fresh, unsaturated solvent into contact with the solid solute, moving away layers of solvent that have already dissolved some solute. Without stirring, a concentrated solution layer can build up at the solid’s surface, slowing the rate of particle release.
Breaking the solid into smaller pieces increases its surface area and accelerates the rate of dissolution. Granulated sugar dissolves faster than a sugar cube because the smaller particles expose more of the solute’s surface to the solvent. This allows more solvent molecules to attack the solid simultaneously.
Limits to Dissolving: Saturation
Dissolution has a limit, as there is a maximum amount of solid a given liquid can dissolve at a specific temperature. This maximum amount is termed solubility, and once reached, the solution is considered saturated. At saturation, a process called dynamic equilibrium is established.
Even when no more solid appears to dissolve, the processes of dissolution and crystallization do not stop. Solute particles continue to break away from the solid into the solution. However, at the exact same rate, an equal number of dissolved particles leave the solution and reattach to the undissolved solid. This constant, balanced exchange means the net concentration of the dissolved solid remains unchanged, creating a steady state.