Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture composed of a solvent, the major component, and a solute, the minor component. The physical state of a solution—solid, liquid, or gas—is typically the same as that of the solvent. Solute concentrations are often described with qualitative terms such as dilute (of relatively low concentration) and concentrated (of relatively high concentration).
In a solution, the solute particles (molecules, atoms, and/or ions) are closely surrounded by solvent species and interact through attractive forces. This dissolution process is called solvation. When water is the solvent, the process is known as hydration. For the solvation, the solute–solvent interactions should be stronger than solute–solute and solvent–solvent interactions. Precipitation is the opposite of solvation and occurs due to strong solute–solute interactions.
Solubility is the measure of the maximum amount of solute that can be dissolved in a given quantity of solvent. Temperature, pressure, and molecular polarity are some of the important factors that affect solubility. Solubility equilibrium is established when the dissolution and precipitation of a solute species occur at equal rates.
Like Dissolves Like
To predict if a solute will be soluble in a given solvent, the rule of thumb is “like dissolves like.” Polar or ionic solutes dissolve in polar solvents due to resulting ion–dipole or dipole–dipole interactions with the solvent molecules. Such interaction will not be possible with a nonpolar solvent. Nonpolar solutes dissolve in nonpolar solvents through intermolecular dispersion forces.
Hydrophilic, Hydrophobic, and Amphiphilic Compounds
Water is a polar solvent. Solutes that are soluble in water are called ‘hydrophilic’ or ‘water-loving’. For example, when solid KCl is added to water, the positive (hydrogen) end of the polar water molecules is attracted to the negative chloride ions, and the negative (oxygen) ends of water are attracted to the positive potassium ions. The water molecules surround individual K+ and Cl− ions, reducing the strong forces that bind the ions together and letting them move off into solution as solvated ions.
A solute that is insoluble in water is termed as ‘hydrophobic’ or ‘water-fearing’. Such solutes, like oil, are unable to form hydrogen bonds with the surrounding water molecules due to the stronger solute–solute interactions. As a result, the solute particles cluster together and remain undissolved.
Compounds that have both polar and nonpolar groups are called ‘amphipathic’ or ‘amphiphilic’. For example, soaps, which are the salts of fatty acids. They have a hydrophobic tail of nonpolar hydrocarbons and a polar hydrophilic head. The cleaning action of soaps and detergents can be explained in terms of the structures of the molecules involved. The hydrocarbon (nonpolar) end of a soap or detergent molecule dissolves in or is attracted to nonpolar substances, such as oil, grease, or dirt particles. The ionic end is attracted by water (polar). As a result, the soap or detergent molecules become oriented at the interface between the dirt particles and the water, so they act as a kind of bridge between two different types of matter, nonpolar and polar. As a consequence, dirt particles become suspended as colloidal particles and are readily washed away.
This text is adapted from OpenStax Chemistry 2e, Section 11.1: The Dissolution Process, Section 11.3: Solubility, and Section 11.5: Colloids.