Dissolution is a fundamental physical process, ranging from making coffee to the functioning of biological systems. Dissolving is the spontaneous process where one substance uniformly disperses into another at a molecular level, forming a homogenous mixture. This mixing occurs because the attractive forces between the different substances are strong enough to overcome the forces holding the original substances together. Understanding this molecular interaction reveals how everyday actions, like adding salt to soup, are driven by complex chemical physics.
Defining Solute Solvent and Solution
To discuss dissolution, we must define the three main components involved. The substance being dissolved is the solute, typically present in the lesser quantity of the mixture. The solvent is the substance doing the dissolving, usually present in the greater amount. For example, when table sugar is mixed into water, the sugar is the solute and the water is the solvent.
The final product is the solution, a homogenous mixture where the solute particles are distributed evenly throughout the solvent. Solutions are characterized by their uniformity; any sample taken will have the same concentration and properties. Once fully dissolved, the individual solute particles cannot be easily separated from the solvent through simple physical means like filtration.
The Molecular Mechanism of Dissolving
The actual process of dissolving, known as solvation, involves a three-step energy exchange at the molecular level. Energy must first be put in to break the forces holding the solute particles together and to separate the solvent particles slightly. This initial energy input is an endothermic process, requiring heat from the surroundings. The third step is the formation of new attractive forces between the solute and solvent particles, which is an exothermic process that releases energy.
For ionic compounds, like sodium chloride (table salt), the mechanism involves ion-dipole interactions with a polar solvent like water. Water molecules have a slightly negative oxygen end and slightly positive hydrogen ends. These water molecules collide with the salt crystal; the negative oxygen ends attract the positive sodium ions, and the positive hydrogen ends attract the negative chloride ions. These strong attractions pull the individual ions away from the crystal lattice, surrounding them with a shell of water molecules called a hydration shell.
For molecular compounds, such as table sugar, the process is different because the molecules do not break into ions. Instead, polar water molecules form new intermolecular bonds, such as hydrogen bonds or dipole-dipole attractions, with the sugar molecules. These new attractions must be strong enough to separate the individual sugar molecules from one another. The solute particles are then fully surrounded by solvent molecules and dispersed throughout the solution.
Why Some Things Dissolve and Others Do Not
The ability of a solute to dissolve in a solvent is governed by the principle of “like dissolves like,” which describes the compatibility of the intermolecular forces of the two substances. Substances are categorized by their polarity, which describes the distribution of electric charge across the molecule. Polar molecules, such as water, have an uneven charge distribution resulting in distinct positive and negative poles. Non-polar molecules, such as oils, have an even charge distribution.
The “like dissolves like” rule states that polar solvents dissolve polar and ionic solutes, while non-polar solvents dissolve non-polar solutes. This occurs because the new attractive forces formed between “like” molecules are strong enough to overcome the initial forces holding the solute and solvent apart. Water, a highly polar solvent, can dissolve salt and sugar, but it cannot dissolve oil because the non-polar oil molecules are not strongly attracted to the polar water molecules.
When a polar and a non-polar liquid are mixed, they remain separate and are called immiscible, as seen with oil and water. The strong attractions between the water molecules cause them to exclude the non-polar oil and remain clustered together. Solubility is a competition between the solute-solute, solvent-solvent, and solute-solvent attractive forces, with only similar forces yielding a true solution.
Factors That Influence Dissolving Speed
While the nature of the solute and solvent determines if something will dissolve, the speed is influenced by external, kinetic factors. Increasing the temperature of the solvent significantly speeds up the dissolution rate. Higher temperature translates to higher kinetic energy for the solvent molecules, causing them to move faster and collide with the solute more frequently and with greater force.
Stirring or agitation of the solution is an effective method. When a solute begins to dissolve, a layer of saturated solvent forms immediately around the undissolved substance, slowing the process. Stirring moves this saturated layer away and continuously brings fresh, unsaturated solvent molecules into contact with the solute, maintaining a high rate of dissolution.
The third factor is the surface area of the solute; a crushed substance dissolves faster than a solid block of the same mass. Dissolution is a surface phenomenon that depends on the solvent molecules colliding with the outer layer of the solute. Grinding the solute into smaller pieces greatly increases the total exposed surface area, allowing a larger number of solvent molecules to interact simultaneously.