Unit 2
Elements of Chemical Change

2-11. WATER

Water is the most important liquid to all living organisms. It comprises about 57 percent of your body weight. It is the solvent for or is contained in most of the nutrients your body requires for growth or maintenance. It is also the primary vehicle for almost all liquid pharmaceutical preparations. Because of the inherent importance of water in the practice of medicine, it is essential to acquire some knowledge of the properties of water and aqueous (water-based) solutions.

Properties of Water. We are all familiar with some properties of water. We know that generally water is a bland-tasting, colorless liquid. Other specific properties of water are of importance in medicine.

Importance of Properties. The properties above are the specific reasons that water is so valuable to living systems and to pharmaceutical preparations. The wide difference between the freezing point (water as ice) and the boiling point (water as steam or vapor) indicates that water will be a liquid at most of the temperatures encountered under normal conditions. An example should help emphasize the importance of these properties. If we wanted to prepare a liquid drug solution for a patient who could not swallow capsules, we used a liquid vehicle with a freezing point of 25 C (77 F) and a boiling point of 30 C (86 F), we would be giving the patient a worthless product. As the patient left home, the drug solution would boil if it were a normal summer day (temperature = 86 F), and when the patient entered his air-conditioned home, the remaining solution might become a solid which could not be poured from the bottle. We also want our vehicle to be as unreactive as possible so that only the drug is exerting a pharmacological effect.

Structure of Water. The properties of water may best be explained by examining the structure of the water molecule. The water molecule consists of two hydrogen atoms bonded covalently to one oxygen atom. The three atoms are bound together as shown below.

This arrangement leads to an electron-rich atom, oxygen, on one end and two electron-poor atoms, hydrogen, on the other end. This results in a molecule that resembles a bar magnet in that it has a negative pole and a positive pole, as shown below.

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Actually, there are not distinct electrical charges on the molecule, only partial charges, referred to as d+ and d- (the Greek letter delta, d, meaning partial). While these charges are only partial, they are still strong enough for water to be referred to as a polar molecule, meaning that it has a positive and negative end.

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Hydrogen Bond. The polarity of the water molecule gives rise to an unused type of bond between water molecules, the hydrogen bond. This bond is the electrical attraction between the partially negative oxygen atom of one molecule and the partially positive hydrogen atom of another molecule.

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The hydrogen bond is a very weak attraction about 1/10 to 1/20 the strength of the hydrogen-oxygen covalent bond. The hydrogen bond explains why water has such a high boiling point in relation to other compounds of similar molecular weight. For example, methane (CH4, molecular weight = 16) boils at a temperature below 0C, while water (molecular weight = 18) boils at 100C. Methane does not exhibit hydrogen bonding.

Water Purification. We are all familiar with some of the ecological problems facing the world today. Water is subject to mineral and biological contamination. Since we will often be using water in the preparation of our products, we must be concerned with its purity and the methods utilized for its purification. There are two common methods of water purification used at Army medical treatment facilities--distillation and ion exchange.