Solubility is the maximum quantity of a solute that can uniformly dissolve into a specific amount of a solvent under defined conditions of temperature and pressure. Determining this measurement is foundational in various scientific disciplines, including pharmaceutical development, where it governs drug absorption, and environmental science, where it dictates the movement of pollutants. The process involves both an initial chemical prediction based on molecular structure and a subsequent physical quantification through laboratory measurement.
Predicting Solubility Based on Chemical Nature
Initial determination of solubility relies on the fundamental chemical principle, “like dissolves like.” This rule suggests that substances with similar molecular characteristics will readily mix and dissolve each other. The underlying mechanism is the nature and strength of the intermolecular forces (IMFs) present in both the solute and the solvent.
Polar solvents, such as water, possess molecules with an uneven distribution of electric charge, allowing them to form strong dipole-dipole interactions or hydrogen bonds. These solvents are effective at dissolving other polar solutes, like table salt or sugar, which form comparably strong attractive forces with the solvent molecules. For dissolution to occur, the new attractive forces formed between the solute and solvent must be comparable in strength to the forces holding the pure substances together.
Nonpolar solvents, such as hexane, consist of molecules with a symmetrical charge distribution and primarily exhibit weaker London dispersion forces. These solvents effectively dissolve nonpolar solutes, such as fats or waxes. When a polar solute is introduced into a nonpolar solvent, the strong attractive forces within the polar substance cannot be overcome by the weak forces of the nonpolar solvent, leading to little or no mixing. By assessing the relative polarity of the two materials, a strong initial prediction about forming a solution can be made.
Quantifying Solubility Through Saturation
While chemical nature provides a prediction, determining a precise solubility value requires finding the point of saturation experimentally. A solution is saturated when it holds the maximum amount of dissolved solute possible at a given temperature; any additional solute remains undissolved in a solid state. This experimental process is often conducted using the shake-flask method.
The procedure begins by adding a measured excess of the solute to a known volume of the solvent in a sealed container. The mixture is then shaken and allowed to reach thermodynamic equilibrium, which can take anywhere from 12 hours to several days. Throughout this period, the temperature is strictly maintained to ensure the measured value is accurate for that specific condition.
Once equilibrium is established and a saturated solution is confirmed by the presence of undissolved solid, a precise sample of the liquid solution is carefully separated. A common method for analysis is gravimetric analysis. In this technique, the measured volume of the saturated solution is heated until all the solvent has completely evaporated, leaving only the mass of the dissolved solute behind.
By knowing the initial volume of the solvent and the mass of the solute recovered, the solubility can be calculated and expressed as a concentration. The result is typically reported in standard units, such as grams of solute per 100 milliliters of solvent or as molar solubility (moles per liter). This numerical value represents the precise limit of dissolution for that specific solute-solvent pair under the exact conditions of the experiment.
How External Conditions Change Solubility
The numerical solubility value determined through saturation is dependent on the specific external conditions under which the measurement was taken. Two primary external factors, temperature and pressure, can significantly alter a substance’s ability to dissolve. The effect of temperature on solubility differs markedly between solids and gases.
For the majority of solid solutes dissolved in a liquid, an increase in temperature generally leads to an increase in solubility. The added thermal energy helps to break the forces holding the solid structure together, allowing more solute molecules to disperse into the solvent. Conversely, the solubility of all gases in a liquid consistently decreases as the temperature rises. This occurs because higher temperatures increase the kinetic energy of the dissolved gas molecules, making it easier for them to escape the liquid and return to the gas phase.
Pressure is the second major factor, although its influence is almost exclusively limited to gaseous solutes. The solubility of a gas is directly proportional to the partial pressure of that gas above the liquid, a relationship described by Henry’s Law. When the pressure of the gas above the liquid is increased, more gas molecules are forced into the solution, thereby increasing its solubility. Pressure has a negligible effect on the solubility of solid and liquid solutes.