Authors: Rachel Casiday, Richard Krathen, and Regina Frey
Key Concepts: Molecular basis for solubility; polarity; thermodynamics of dissolution; solubility (quantitative) and solubility product.
In order to use the nutrients that we take in when we eat, we must first break the food down into its nutritive components, dissolve these components, and then carry them to the place where they will be used in the body. If the nutrients are not used immediately, they will either be stored for later use, or excreted in the urine. Understanding the solubility of nutrients in the different substances of the body is important for understanding how they can be used or processed in the body. Scientists have developed several ways to discuss the important concepts of solubility. For salts (ionic solids) that dissociate into ions in water, such as the compounds containing the dietary minerals, a solubility product Ksp is typically given. Vitamins are typically described as soluble in water or in lipids, depending on the functional groups on the molecule. A vitamin's solubility in water or in lipids determines where it can be used, and whether it will be stored in fat cells or excreted from the body if it is not needed for immediate use.
In this tutorial, students learn about the thermodynamics and structural properties of a molecule that make it soluble or insoluble in a given solvent (e.g., water or fat). A table of vitamin structures allows students to examine the two-dimensional (ChemDraw) and three-dimensional (CPK) structures of seven vitamins. Students can also download and view the three-dimensional structures interactively using an interactive molecular-viewing package such as RASMOL. Olestra, the new fake fat in some snack foods, is discussed because it interferes with the body's absorption of fat-soluble vitamins. The solubility of minerals is explained quantitatively, using the concepts of equilibria and the solubility product (Ksp). The role of calcium in the body, equilibria involving blood-calcium concentration, and the solubility of calcium supplements, are discussed in detail.
At Washington University in St. Louis, this tutorial accompanies a second-semester experiment in which students determine the solubility product for Ca(OH)2 and determine the effect of OH- concentration on the solubility of this compound (common-ion effect). The tutorial gives a relevant example (minerals in your body) of where this quantitative solubility product is used. It also develops the students' understanding of the common phrase "like dissolves like".
