When a substance dissolves, it undergoes a physical change that results in a perfectly mixed substance called a solution. This process involves two components: the substance that is being dispersed, known as the solute, and the substance that does the dispersing, called the solvent. Dissolving is simply the phenomenon where the solute particles separate and spread evenly throughout the solvent, forming a homogeneous mixture that appears as a single phase. The capacity of a solvent to mix with a solute depends entirely on the forces acting between the particles of both substances.
The Molecular Mechanism of Dissolving
Dissolving requires energy to first break existing attractions within both the solute and the solvent. Before a solute can spread, the solvent particles must be pushed apart to create small gaps or “pockets” for the new particles to occupy. Simultaneously, the attractions holding the solute particles together must be overcome, often breaking apart a solid crystal lattice into individual molecules or ions. Both of these initial steps require an input of energy to successfully separate the molecules from their neighbors.
Once separated, the individual particles of the solute and solvent must then be attracted to one another to form a stable solution. For a solute to successfully dissolve, the energy released from the formation of these new solute-solvent attractions must be sufficient to offset the energy required for the initial separation steps. Solvent molecules surround the newly freed solute particles, a process known as solvation, which shields them and prevents them from re-associating with the undissolved solid. If the solvent molecules are not strongly attracted to the solute particles, the substance will not dissolve, as the original solute-solute attractions remain stronger.
Understanding Solubility: The “Like Dissolves Like” Rule
Solute and solvent compatibility is determined by the principle summarized as “like dissolves like.” This rule relates to molecular polarity, which describes how electrical charge is distributed within a particle. A polar molecule, like water, has a slightly positive and a slightly negative end due to the unequal sharing of electrons. Nonpolar molecules, such as those in oils, have a more even distribution of charge.
The “like dissolves like” rule states that polar solvents will readily dissolve polar solutes, and nonpolar solvents will dissolve nonpolar solutes. For example, a polar solvent like water can dissolve an ionic solute like salt because the charged ends of the water molecules are strongly attracted to the positive and negative ions of the salt. Conversely, nonpolar paint thinner is effective at dissolving nonpolar grease because both substances lack significant charge differences, allowing them to mix freely. When a polar and a nonpolar substance are mixed, such as oil and water, the strong attractions between the polar molecules exclude the nonpolar molecules, causing them to separate into distinct layers.
Factors That Influence Dissolving Speed
While the nature of the solute and solvent determines if dissolving will occur, several actions influence how quickly it happens. Increasing the temperature of the solvent speeds up the process because solvent molecules gain more kinetic energy and move faster. This increased motion leads to more frequent and forceful collisions with the solute, helping to break it apart more rapidly.
Mechanical agitation, such as stirring, significantly increases the rate of dissolving. Stirring constantly moves the newly dissolved solute particles away from the surface of the undissolved solid. This ensures that fresh solvent, which has not yet reached its local capacity, is always in contact with the solute surface, maintaining a high concentration gradient. The final factor is increasing the surface area of the solute, such as by crushing it into a powder. Since dissolving is a surface phenomenon, a smaller particle size exposes more individual solute particles to the solvent simultaneously, accelerating the rate of dissolution.
When Dissolving Stops: Saturated Solutions
Every solvent has a limit to the amount of solute it can hold at a specific temperature. When a solvent has dissolved the maximum possible amount of solute, the resulting mixture is called a saturated solution. At this point, any additional solute added will fall to the bottom of the container and remain undissolved.
This apparent halt in the process is actually a state of dynamic equilibrium. Within a saturated solution, solute particles continue to dissolve into the solvent at the same rate that dissolved particles are crystallizing back out. The concentration of the dissolved solute remains constant because the opposing processes of dissolving and crystallization are perfectly balanced. Solutions that contain less than this maximum amount are termed unsaturated, while unstable solutions holding more solute than the saturation limit are called supersaturated.