Dissolution is a fundamental chemical process where one substance disperses evenly into another at a molecular level, forming a homogeneous mixture known as a solution. The substance being dissolved is called the solute, and the substance doing the dissolving is the solvent. For example, when table sugar (solute) dissolves in water (solvent), the resulting sugar solution is homogeneous. The solute particles become completely surrounded by the solvent particles, becoming indistinguishable to the eye.
The Molecular Mechanism of Dissolution
Dissolving occurs through a precise sequence of three energy-related steps. First, energy is required to separate the solute particles, overcoming the attractive forces holding them together. Simultaneously, the solvent particles must also separate slightly to create spaces to accommodate the incoming solute particles, which also requires energy input.
The third step is solvation, the mixing of the separated solute and solvent particles. New attractive forces form between the solute and solvent particles, which releases energy. If water is the solvent, this process is specifically termed hydration. The overall spontaneity of dissolution depends on the net energy balance between the energy absorbed in the first two steps and the energy released in the third.
The ability of a solute to dissolve in a solvent is largely dictated by the principle of “like dissolves like,” which refers to the similarity of their intermolecular forces. Polar solvents, such as water, dissolve polar solutes like salts and sugars. Conversely, nonpolar solvents, like oil, readily dissolve other nonpolar substances, such as fats and waxes.
When the new solute-solvent attractions are stronger than the original attractions, the process releases heat, making the dissolution exothermic. If the energy required to break the original bonds is greater than the energy released during mixing, the process absorbs heat from the surroundings, resulting in an endothermic dissolution. The resulting solution is a stable state where the solute particles are uniformly dispersed.
Factors Controlling the Rate of Dissolution
The rate of dissolution can be controlled by modifying the interaction between the two components. Increasing the solvent’s temperature is an effective way to accelerate the process for most solid solutes. Higher temperatures increase the kinetic energy of solvent molecules, causing them to collide with the solute particles more frequently and forcefully. This intensified molecular activity helps to quickly dislodge the solute particles from their bulk structure.
Another significant factor is the surface area of the solute particles exposed to the solvent. When a solid solute is ground into smaller pieces, such as powdered sugar, the total surface area available for contact with the solvent increases substantially. Because dissolution is a surface phenomenon, the greater surface area allows for many more solvent molecules to interact with the solute simultaneously, speeding up the overall rate.
Agitation, such as stirring, also dramatically increases the speed at which a solute dissolves. When a solute dissolves, a layer of concentrated solution forms around the undissolved solid, which slows further dissolving. Stirring continuously removes this saturated layer, bringing fresh, unsaturated solvent into contact with the remaining solute particles. This action maintains a high concentration gradient, thereby maximizing the rate of particle separation and solvation.
Solubility and the Saturation Point
Solubility defines the maximum amount of a solute that can dissolve in a specific amount of solvent under specific conditions. Solubility is typically measured at a fixed temperature because temperature significantly influences this maximum limit. The intermolecular forces between the solute and solvent ultimately determine the substance’s inherent solubility.
A solution is classified as unsaturated if it contains less than the maximum amount of dissolved solute possible for that temperature. If more solute is added to an unsaturated solution, it will continue to dissolve completely. The solution reaches its saturation point when no more solute can dissolve, and any additional solute will settle to the bottom.
A saturated solution represents a dynamic equilibrium where the rate at which the solid solute dissolves is equal to the rate at which the dissolved solute re-crystallizes. Supersaturation is an unstable condition created by carefully heating a saturated solution, dissolving more solute, and then gently cooling it. A supersaturated solution holds more dissolved solute than is stable at that cooler temperature.
If a tiny crystal or a rough surface is introduced into a supersaturated solution, the excess solute will rapidly precipitate out. This process of crystallization is a quick return to the stable saturated state, forming solid crystals on the surface of the added seed.