Dissolving is one of the most common physical processes, occurring when one substance appears to vanish into another, such as sugar disappearing in coffee. This phenomenon is a physical change, not a chemical reaction, where the original substances retain their identity but become uniformly mixed at a molecular level. The process results in a homogeneous mixture, meaning the composition is identical throughout and the particles are too small to settle out or be seen.
Defining the Components of Dissolution
The process of dissolution involves three distinct components. The substance that is being dispersed is called the solute, which is typically the component present in the lesser amount. For instance, when making saltwater, the grains of salt act as the solute.
The liquid or gas that facilitates the dissolving by taking the solute particles into itself is known as the solvent, and it is usually the component present in the greatest quantity. In the example of saltwater, the water serves as the solvent. When the solute and the solvent are successfully combined, the resulting homogeneous mixture is referred to as a solution.
A solution is characterized by its uniform appearance and stability, where the individual particles of the solute are dispersed so thoroughly that they will not separate or settle over time. Many types of solutions exist, such as a gas dissolved in a liquid, like carbon dioxide in soda, or a solid dissolved in another solid, such as zinc in copper to form brass.
The Molecular Mechanism of Solvation
The mechanism of dissolving depends on the intermolecular forces between the particles of the solute and the solvent. A principle governing this process is summarized by the phrase “like dissolves like,” which refers to the compatibility of molecular polarity. Polar solvents, such as water, have an uneven distribution of electric charge and are effective at dissolving other polar solutes or ionic compounds like table salt.
Conversely, nonpolar solvents, such as oil or paint thinner, lack these charge differences and are effective at dissolving nonpolar solutes, like fats or waxes. Polar and nonpolar substances do not mix because their differing molecular attractions prevent them from fully intermingling. The process where solvent molecules surround the solute particles is termed solvation.
When water is the specific solvent, this process is given the specialized name of hydration. For an ionic compound like sodium chloride, the partially negative oxygen end of the water molecule is attracted to the positively charged sodium ions. Simultaneously, the partially positive hydrogen ends are attracted to the negatively charged chloride ions. These electrostatic attractions are strong enough to overcome the internal forces holding the salt crystal together.
The process requires an energy trade-off, where energy is absorbed to break the solute-solute bonds and the solvent-solvent attractions. This energy is then released when the new solute-solvent bonds are formed, resulting in the solvated particles being pulled away into the bulk of the solution. If the energy released by the new attractions is comparable to or greater than the energy required to break the initial bonds, the substance will dissolve.
Factors Controlling Dissolving Speed and Limits
The speed at which a solute dissolves, known as the rate of dissolution, is influenced by several physical factors. Increasing the temperature of the solvent increases the rate for solid solutes because the solvent molecules gain kinetic energy, causing them to move faster and collide with the solute more frequently. Agitation, such as stirring or shaking, speeds up the process by continuously bringing fresh solvent molecules into contact with the undissolved solute surface.
Surface area is also a factor; a finely ground powder will dissolve much faster than a large, solid block of the same mass. This occurs because the smaller particles expose a greater total surface area to the solvent, allowing for more points of molecular interaction. These factors affect the speed but do not change the total amount of solute that can ultimately dissolve.
The maximum amount of solute that can dissolve in a specific amount of solvent at a given temperature is defined as its solubility. A solution that has dissolved less than this maximum amount is referred to as unsaturated, meaning more solute could still be added and dissolved. Once the maximum limit is reached, the solution is considered saturated, and any additional solute added will remain as solid at the bottom.
A solution can be prepared to hold more solute than is possible at that temperature, creating a supersaturated solution, though this state is unstable. At the saturation point, a state of dynamic equilibrium exists, where the rate at which the solute dissolves is balanced by the rate at which the dissolved particles recrystallize back into solid form.