Solubility

Main Ideas:

• Acidic solutions contain higher concentrations of H+ ions than pure water and have pH values below 7.
• Basic, or alkaline, solutions contain lower concentrations of H+ ions than pure water and have pH values above 7.

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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.

Water tends to disassociate into H+ and OH- ions. In this disassociation, the oxygen retains the electrons and only one of the hydrogens, 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.

An acid causes an increase in the numbers of H+ ions and a base causes an increase in the numbers of OH- ions.

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.