L2.1: Solubility

• A solution is a homogeneous mixture and exists as a single phase. The particles in a solution are individual atoms, ions, or small molecules.
• A colloid is a heterogeneous mixture and exists as two or more phases, which may be visibly distinct. The particles in a colloid are typically macromolecules or aggregations of small molecules
• A solute dissolves in a solvent to form a solution. Usually, the solvent is the most abundant component. The solubility (S) of a solute is the maximum amount that dissolves in a fixed quantity of solvent at a given temperature.
• Substances that exhibit similar types of intermolecular force dissolve in each other – often expressed by saying “like dissolves in like.” When a solution forms, solute-solute attractions and solvent-solvent attractions are replaced by solute-solvent attractions. This can only occur if the forces within the solute and solvent are similar to the forces that replace them.
• Solubility can be summed up as “like dissolves like.” Most ionic and polar covalent compounds are soluble in water, a polar solvent.
• Small neutral molecules with O or N atoms that can hydrogen bond to water are water soluble. Example: ethanol can hydrogen bond to water.
• Nonpolar compounds are soluble in nonpolar solvents (i.e., like dissolves like). Octane (C8H18) dissolves in CCl4 because both are nonpolar liquids that exhibit only London dispersion forces.
• • Multi Vitamin

  – Provides many essential vitamins

  – “Expensive urine”

  • Water Soluble

  – Vitamin C

  – Must be replenished regularly

  • Fat Soluble

  – Can overdose

  – Vitamin A

  – Can be ingested periodically, stored in body fat
• A saturated solution contains the maximum amount of dissolved solute at a given temperature in the presence of undissolved solute.

  Undissolved solute is in equilibrium with dissolved solute. Solute (undissolved) ↔ solute (dissolved)
• An unsaturated solution contains less than the equilibrium concentration of dissolved solute. If more solute is added, it will dissolve.
• A supersaturated solution contains more than the equilibrium concentration of solute. It is unstable and any disturbance will cause excess solute to crystallize immediately
• For most ionic and molecular solids, solubility generally increases as temperature increases. By dissolving a solid in a solvent at high temperature and allowing it to cool slowly, a supersaturated solution can be made.
• A supersaturated solution contains more than the predicted maximum amount of solute at a given temperature. In contrast, the solubility of a gas decreases with increasing temperature.
• Henry’s law: The solubility of a gas in a liquid is proportional to the partial pressure of the gas above the liquid. The higher the pressure, the higher the solubility of a gas in a solvent.
• As the pressure is increased, more gas particles collide with the liquid surface. More gas particles dissolve until equilibrium is reestablished.