3: Solubility

Dissolving a gas in a liquid is usually accompanied by release of heat. When temperature increases, the equilibrium tends to favour escape of gas from the liquid, so its solubility decreases. This is a general trend, not an absolute rule for every unusual system. Gas solubility still depends strongly on pressure and composition.

Solubility depends on the balance between solute–solute, solvent–solvent, and solute–solvent interactions. Stronger favourable attraction between the gas and solvent stabilises dissolved molecules and generally increases solubility at the same pressure and temperature. Molecular size alone is not a universal criterion, and colour has no defining role.

Dissolved salt interacts with water and lowers the fraction of water available for biological and chemical processes; this is described as reduced water activity. The example illustrates that adding a solute changes solvent-related properties. It is not merely a colour change, nor does salt instantly release every gas.

For the same mass of solid, breaking it into smaller particles increases the total surface area exposed to the solvent. More contact allows solvent molecules to collide with and remove solute particles more frequently, increasing the rate of dissolution. It usually does not change the equilibrium solubility at the same temperature.

Stirring continually brings unsaturated solvent into contact with the solute and removes concentration gradients, so dissolution occurs more rapidly. However, at a fixed temperature the equilibrium solubility—the maximum amount that remains dissolved—is determined by the substance and solvent. Stirring does not alter that equilibrium value.

A hot solvent can dissolve a relatively large amount of a solid whose solubility rises strongly with temperature. On slow cooling, the solubility falls and the excess solute separates as crystals. Slow cooling gives ordered crystals and can exclude many impurities. Stirring alone or changing pressure does not provide the required temperature-dependent solubility difference.

A suitable recrystallisation solvent dissolves the desired solid well when hot but only sparingly when cold. The impure sample is dissolved in a minimum amount of hot solvent, then cooled so the desired compound crystallises while soluble impurities remain in the mother liquor. A solvent that dissolves nothing or dissolves everything equally is unsuitable.

Increasing the partial pressure of a gas above a liquid increases the frequency of gas molecules striking the liquid surface. More molecules enter the liquid until a new dynamic equilibrium is established with a larger dissolved concentration. The gas phase does not disappear completely, the liquid does not automatically freeze, and equilibrium still involves continuous molecular motion.

“Low solubility” describes the amount dissolved under a specified reference condition; it does not mean that pressure has no effect. Henry’s law predicts that increasing gas partial pressure increases the dissolved mole fraction. Thus even a poorly soluble gas can dissolve in greater amount at high pressure, although it may still be less soluble than another gas.

Cooling can lower the equilibrium solubility, yet crystallisation may not begin immediately if no suitable crystal nucleus is present. The solution then remains in a metastable supersaturated state. Scratching the vessel, adding a seed crystal, or disturbing the solution can provide nucleation sites and initiate crystal formation. No mass disappears and solubility does not become infinite.

Solubility depends on how favourably the solute interacts with the solvent, as well as on the forces within each pure substance. Changing the solvent can change polarity, hydrogen bonding, dielectric effects, and other stabilising interactions. The solute itself does not change its atomic number, and temperature or pressure need not automatically change.

If the solution cools during dissolution, the process is absorbing heat from its surroundings and is therefore endothermic in the observed direction. Supplying heat by increasing temperature generally favours the dissolving process and increases solubility. The word “likely” is appropriate because real systems can show additional effects, but the equilibrium trend is the stated one.

Warming of the solution indicates that dissolution releases heat, so the process is exothermic in the observed direction. Adding more heat generally favours the reverse direction and may reduce the equilibrium solubility. The decrease is not necessarily complete or universal in magnitude; neither colour nor gas-like pressure dependence controls ordinary solid dissolution here.

Pressure strongly affects gases because gas volume is highly compressible and gas–liquid equilibrium responds to partial pressure. Solids and liquids are nearly incompressible, so pressure usually produces only a small change in their solubility. Therefore the gas rule cannot be extended to every solute, and no universal doubling occurs.

From p = K_H x, rearrangement gives x = p/K_H. At the same pressure, a smaller positive K_H produces a larger dissolved mole fraction x. A larger x indicates greater solubility under the comparison conditions. A small Henry constant does not mean zero solubility or zero pressure; it indicates stronger dissolution in this convention.

The solubility of oxygen and most other gases decreases as water temperature rises. Hot industrial discharge can therefore reduce dissolved oxygen available for fish, invertebrates, and microorganisms. Aquatic organisms may experience stress or suffocation, especially when biological oxygen demand is also high. Heating does not create infinite oxygen or remove the organisms’ respiratory need.

Cooling crystallisation works only when solubility falls substantially as temperature decreases. If the hot and cold solubilities are nearly equal, cooling removes very little dissolved solute from the solution, so the crystal yield is small. Another purification method may be needed. The curve’s temperature dependence, not colour or pressure alone, determines effectiveness.