nonmetals) polar substances, such as alcohols and sugars, also are soluble in water because of the dipole-dipole (or hydrogen-bonding) interactions. However, covalently bonded nonpolar substances, such as fats and oils, aren’t soluble in water. Check out Chemistry For Dummies (written by this book’s coauthor, John T. Moore, and published by Wiley) for a discussion of chemical bonding.
Figure 2-2 shows the structure of a typical amphipathic molecule. The molecule appears on the left of Figure 2-2, with its hydrophilic and hydrophobic regions shown. The right side of Figure 2-2 is a symbolic way of representing the molecule. The round head is the hydrophilic portion, and the long tail is the hydrophobic portion.
FIGURE 2-2: Structure of a typical amphipathic (both water-loving and water-hating) molecule.
Certain amphipathic molecules, such as soap molecules, can form micelles, or very tiny droplets that surround insoluble materials. This characteristic is the basis of the cleaning power of soaps and detergents. The hydrophobic portion of the molecule (a long hydrocarbon chain) dissolves in a nonpolar substance, such as normally insoluble grease and oil, leaving the hydrophilic portion (commonly an ionic end) out in the water. Soap or detergent breaks up the grease or oil and keeps it in solution so it can go down the drain.
A micelle behaves as a large polar molecule (see Figure 2-3). The structure of a micelle is closely related to the structure of cell membranes.
FIGURE 2-3: Structure of a micelle, composed of amphipathic molecules, with their hydrophilic heads pointing out.
Hydrogen Ion Concentration: Acids and Bases
In aqueous solutions — especially in biological systems — the concentration of hydrogen ions
Even minor changes in hydrogen ion concentration can have dire consequences to a living organism. For example, the human body can function properly only if its blood’s hydrogen ion concentration falls within a very small range. Hydrogen ion concentrations higher or lower than this range can cause death.
Because living organisms are so dependent on pH, we review the concepts of acids, bases, and pH in the following sections.
Achieving equilibrium
When the concentrations of hydrogen ions
The equilibrium of hydrogen ions is present in all aqueous solutions. Water may or may not be the major hydrogen ion source (usually, it isn’t). Water is a contributor to the hydrogen ion concentration because it undergoes autoionization, as shown by the following equation:
The double arrow
The value of the constant Kw, like all Ks, is only constant if the temperature is constant. In the human body, where
In pure water, at 25°C,
Understanding the pH scale
Expressing hydrogen ion concentrations in an exponential form, such as
For instance, in the case of a solution with a hydrogen ion concentration of
Table 2-1 gives similar calculations for some hydrogen ion concentrations.
