Solvents and solubility

How to determine the solvent and the principle of selectivity

To designate a mixed-phase component as a solvent, two criteria are used:

1. Aggregate state: the solvent determines the phase. An aqueous salt solution is a liquid because water is liquid.
2. Quantitative factor: when two liquids are mixed, the solvent is designated as the one whose amount of substance (in moles) is greater. For example, in a mixture of 56% ethanol and 44% water, water will be the solvent since the number of moles of water in such a portion is twice as large.

Solubility prediction is based on the rule "like dissolves like". Water excels at dissolving oxygen-containing molecules and binary salts. Organic liquids (benzene, hexane, dichloroethane, alcohols) dissolve organics. For instance, a grease stain cannot be washed away with water, but is easily dissolved by gasoline.

Stages of solution saturation

The ability of a substance to pass into solution has a limit. Depending on concentration, three states of the system are distinguished:

• Unsaturated solution: the added substance continues to actively dissolve.
• Saturated solution: equilibrium is achieved between the precipitate and the dissolved solute. At a given temperature, the substance no longer passes into the liquid phase. Adding further portions will only increase the mass of the precipitate at the bottom, but the concentration of the solution will not change.
• Supersaturated solution: a metastable (unstable) system containing more dissolved substance than the saturation limit allows. Usually obtained by careful cooling. In the absence of crystallization centers, no precipitate forms, but the slightest disruption (rubbing with a rod, dropping a seed crystal) triggers instant crystallization. A striking example is sodium acetate, which turns into "hot ice" upon crystallization.

Thermodynamics and temperature dependence

Dissolution itself is a combination of two opposing energetic processes. The destruction of the crystal lattice requires energy expenditure (temperature drops — an endothermic physical process). The interaction of particles with the solvent, such as hydrate formation, releases energy (temperature rises — an exothermic chemical process).

Depending on which process prevails, the solution may heat up (as with calcium chloride) or cool down significantly (as with ammonium nitrate). This leads to the thermodynamic rule:

• If dissolution absorbs heat, then heating will increase solubility.
• If dissolution releases heat, then heating will decrease solubility.

For gases, the rule is unequivocal: upon heating a liquid, the solubility of any gases always decreases. Boiling can completely remove dissolved gases from water.

Features of gas and liquid solubility

In addition to solids, gases and other liquids can also dissolve:

• Liquids in liquids: can mix infinitely (water and alcohol), have limited solubility with phase separation (water and ether), or not mix at all (water and mercury).
• Gases in liquids: unlike solids, gas solubility is directly dependent on pressure. When a bottle of champagne is opened, the pressure drops sharply, the solution becomes supersaturated, and carbon dioxide is vigorously released. In gas mixtures, the solubility of each component depends on its partial pressure (Henry–Dalton law).

What else to study on this topic

• Calculation of mass fraction in a saturated solution via coefficient $K_s$
• Mathematical apparatus of the Henry–Dalton law
• Stoichiometry of preparing solutions from crystalline hydrates
• Recrystallization as a method of purifying solids from impurities
• Analysis of graphs showing the temperature dependence of salt solubility

Other topics in "Chapter 4. Solutions"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.