Chapter 2 - Water

Abstract

Water represents approximately 65% of the total body mass of an adult human. Water has unique physical properties (melting and boiling temperatures, and heat of vaporization) that depend on its molecular structure. The elements in H2O form a 104.5degrees angle, which gives the molecule its polar nature. The negative charge in water is displaced toward the oxygen atom, while the positive charges are displaced toward the hydrogen atoms. This allows the formation of hydrogen bonds between different water molecules. The polarity of water molecules influences their interaction with other ionic and polar nonionic substances. In contrast, nonpolar compounds are hydrophobic and do not dissolve in water. Water is an electrolyte; pure H2O at 25°C ionizes into H+ and OH−, at a concentration of 0.0000001M or 10−7M for each ion. The product [H+]·[OH−] is a constant called water ion product (Kw); its value at 25°C is 10−7·10−7=10−14. When [H+]=[OH−], the solution is neutral. Compounds that yield H+ to the solution function as acids, while those that accept H+ from the medium function as bases or alkalis. According to their degree of ionization, acids and bases can be strong or weak. The pH of a solution corresponds to the logarithm of the reciprocal of the [H+] or the negative logarithm of [H+]. For pure water at 25°C, the pH is 7. Buffers minimize changes in pH in a solution. Buffers are generally formed by a weak electrolyte (acid) and its salt (of a strong base). The pH of a buffer can be calculated by knowing the acid dissociation constant (Ka) and the initial concentrations of the acid and the salt. The relationship between these values is expressed in the Henderson-Hasselbalch equation.