Discussion

Solution Concentrations.    Solutions are homogeneous mixtures that contain a solute and solvent.   The amount of the solute that dissolves in the solvent is dependent on many factors including temperature, pressure and structure of the substances.  In general, the solute is more likely to be soluble or miscible if the polarities are similar, “like dissolves like”; however, the solubility rules for ionic compounds often help us understand which solutes are more soluble in water solutions.   The solubility of a compound in the solvent is defined as the

Solubility =   grams of solute per 100 mL of solvent

The density and composition of a solution are also related to the amount of the solute present.  These terms, however, do not quantitatively define the exact amount of solute present.  For this, several numerical values are used to define the concentration of the solution.  Generally, these values define the amount of solute present in the amount of solvent present in mass, moles or volume.  The definitions of each concentration provide the basis for calculations with specific units.  

Paper chromatography.     Chromatography can be thought of as a dynamic extraction where the solute continually has an option of two phases. This is accomplished by having one phase move by a stationary phase. Chromatography was discovered by Michael Tswett (a Russian botanist) early in the 20th century. Tswett allowed a mixture of pigments extracted from plants to percolate down through a column of calcium carbonate. Solvent was added from the top as needed to cause continuous movement of the pigments down the column. Pigments that were more strongly attracted to the stationary phase (calcium carbonate) and had less affinity for the solvent (moving phase) moved down the column more slowly than pigments that had greater affinity for the solvent and had weaker attraction for the stationary phase. Tswett observed that the pigments had separated into several differently colored bands as a result of the fact that they moved down the column at different rates. The term chromatography was coined to describe the phenomenon.