Why Water is an Universal Solvent?
It can be quite correctly argued that life exists on Earth because of the abundant liquid water. Other planets have water, but they either have it as a gas (Venus) or ice (Mars). This relationship is shown in Figure 1.
Recent studies of Mars reveal the presence sometime in the past of running fluid, possibly water. The chemical nature of water is thus one we must examine as it permeates living systems: water is a universal solvent, and can be too much of a good thing for some cells to deal with.
|Figure 1. Water can exist in all three states of matter on Earth, while only in one state on our two nearest neighboring planets.|
Water is polar covalently bonded within the molecule. This unequal sharing of the electrons results in a slightly positive and a slightly negative side of the molecule. Other molecules, such as Ethane, are nonpolar, having neither a positive nor a negative side, as shown in Figure 2.
|Figure 2. The difference between a polar (water) and nonpolar (ethane) molecule is due to the unequal sharing of electrons within the polar molecule. Nonpolar molecules have electrons equally shared within their covalent bonds.|
These link up by the hydrogen bond discussed earlier. Consequently, water has a great inter-connectivity of individual molecules, which is caused by the individually weak hydrogen bonds, shown in Figure 3 that can be quite strong when taken by the billions.
|Figure 3. Formation of a hydrogen bond between the hydrogen side of one water molecule and the oxygen side of another water molecule.|
Water has been referred to as the universal solvent. Living things are composed of atoms and molecules within aqueous solutions (solutions that have materials dissolved in water). Solutions are uniform mixtures of the molecules of two or more substances. The solvent is usually the substance present in the greatest amount (and is usually also a liquid). The substances of lesser amounts are the solutes.
The solubility of many molecules is determined by their molecular structure. You are familiar with the phrase “mixing like oil and water.” The biochemical basis for this phrase is that the organic macromolecules known as lipids (of which fats are an important, although often troublesome, group) have areas that lack polar covalent bonds. The polar covalently bonded water molecules act to exclude nonpolar molecules, causing the fats to clump together. The structure of many molecules can greatly influence their solubility. Sugars, such as glucose, have many hydroxyl (OH) groups, which tend to increase the solubility of the molecule. This aspect of water is illustrated in Figure 4.
|Figure 4. Dissolution of an ionically bonded compound, sodium chloride, by water molecules.|
Water tends to disassociate into H+ and OH– ions. In this disassociation, the oxygen retains the electrons and only one of the hydrogen, becoming a negatively charged ion known as hydroxide. Pure water has the same number (or concentration) of H+ as OH– ions. Acidic solutions have more H+ ions than OH– ions. Basic solutions have the opposite. The pH of several common solutions is shown in Figure 5. An acid causes an increase in the numbers of H+ ions and a base causes an increase in the numbers of OH– ions.
|Figure 5. pH of some common items.|
The pH scale is a logarithmic scale representing the concentration of H+ ions in a solution. Remember that as the H+ concentration increases the OH– concentration decreases and vice versa.
If we have a solution with one in every ten molecules being H+, we refer to the concentration of H+ ions as 1/10. Remember from algebra that we can write a fraction as a negative exponent, thus 1/10 becomes 10-1. Conversely 1/100 becomes 10-2 , 1/1000 becomes 10-3, etc.
Logarithms are exponents to which a number (usually 10) has been raised. For example log 10 (pronounced “the log of 10”) = 1 (since 10 may be written as 101). The log 1/10 (or 10-1) = -1. pH, a measure of the concentration of H+ ions, is the negative log of the H+ ion concentration.
If the pH of water is 7, then the concentration of H+ ions is 10-7, or 1/10,000,000. In the case of strong acids, such as hydrochloric acid (HCl), an acid secreted by the lining of your stomach, [H+] (the concentration of H+ ions, written in a chemical shorthand) is 10-1; therefore the pH is 1.