Experiment: Solutions and SolubilitiesDecember 20, 2008
SOLUTIONS AND SOLUBILITIES
What is a solution? Solutions are homogeneous mixtures of two or more pure substances. A homogeneous mixture is a physical combination of two or more pure substances whose distribution throughout the mixture is uniform. What this means is that if we were to make a solution and take only a portion of the solution at random called an aliquot, the proportion of each pure substance in the aliquot would be the same as the proportion of that pure substance in the whole solution. We call these proportions to the whole solution the concentration.
Within any solution there are electrical interactions called intermolecular forces between the solute and solvent molecules. In aqueous solutions the polar water molecules will be attracted to other molecules that are also polar.
The water molecules will surround each solute molecule and form a “solvent cage” isolating each solute molecule. We say the solute molecules have been solvated. If the bonds holding the solute molecule together are weak enough, the weakest bond within the solute molecule could be broken by the electrical force of attraction the water molecules have for the solute molecules. If this should take place the solute molecule will undergo what is called ionization forming ions (fragments of the solute molecule) within the solution. In some cases the ionization process can be quite complete while in other cases the ionization process will occur in a very limited manner. Those solutions that do not undergo ionization have the solute molecules solvated within the solution and are called non-electrolytes. Such solutions will not conduct electrical current because of the absence of ions within the solution. Those solutions whose molecules undergo ionization during the solution process are called electrolytes and those solutions will conduct electrical current because of the presence of ions.
Energy Transfers In Solution Formation
When solutions form the solvent molecules form solvent cages around the solute molecules. This solvation process involves a certain amount of thermal energy to be exchanged between the solution system and its immediate surrounding environment. Some solutions absorb energy as they are formed. The solution is said to have an endothermic Heat of Solution.
Solute + Solvent + Thermal Energy —-> Solution
Most aqueous solutions involving liquid or solid solutes will have endothermic Heats of Solution. However, a few are exceptions to this statement.
Other solutions involving gaseous solutes in water will release thermal energy during the solution formation process. These solutions are said to have an exothermic Heat of Solution.
Solute(g) + Solvent —–> Solution + Thermal Energy
Solutions can be unsaturated, saturated, or supersaturated. Unsaturated solutions are those that are below the solubility limits of the solute in that solvent. Saturated solutions are those that are at the solubility limits. Supersaturated solutions are those solutions that are above the solubility limits. Supersaturated solutions are meta stable. Such solutions will have the excess solute crystalize out with any disturbance of the supersaturated solution. Providing a tiny crystal of the solute or scratching the sides of the container which introduce micro chips of glass into the supersaturated solution so the excess solute can crystalize upon its surface, will illicit the dramatic crystallization of the excess solute out of the solution restablishing a saturated solution.