Acids are deeply involved in countless biological, geological and technological processes. Bacteria produce lactic acid that preserves food, soil acids release nutrients from rock-based fertilizers and acids in batteries lead to reactions that generate electrical energy. Hydrochloric acid, often abbreviated as HCl, is a common example of a strong acid, and specific pH values can be achieved through mixtures of hydrochloric acid and water.
The pH scale, which typically ranges from 0 to 14, measures the concentration of hydrogen ions in a substance. Acids have pH values less than 7, bases have pH values greater than 7 and pH 7.0 is the neutral point. The pH scale is negative and logarithmic, which means that a factor-of-ten increase in hydrogen ion concentration corresponds to a decrease of one unit on the pH scale. When an acidic substance is added to water, the overall pH of the solution decreases.
Broken Molecules, Free Ions
When an acid is added to water, the molecules of the acid separate into individual ions. This process is known as disassociation. A molecule of hydrochloric acid, for example, is composed of a hydrogen atom and a chlorine atom. When these molecules dissolve into water, they separate into a positively charged hydrogen ion and a negatively charged chlorine ion. This leads to an increased concentration of hydrogen ions and thus to a lower pH. Hydrochloric acid is classified as a "strong" acid, which means that virtually all of the molecules disassociate. Many other acids -- such as acetic acid, commonly referred to as vinegar -- are classified as "weak" acids. When weak acids are added to water, only some of the molecules disassociate.
An Extreme Acid
Pure hydrochloric acid has a theoretical pH of zero -- in other words, it is extremely acidic. In practical situations, however, hydrochloric acid exists only as a diluted substance. Consequently, the effective pH of hydrochloric acid is dependent upon the degree of dilution. Because the pH of hydrochloric acid is so low, large pH changes occur even when tiny amounts are added to a neutral solution such as water. One example of diluted hydrochloric acid is human stomach acid, which has a pH value around 3.
The degree of pH change that occurs when strong acids such as hydrochloric acid are added to water corresponds directly to the dilution factor, because all of the acidic molecules release one hydrogen ion. Since the pH scale follows a logarithmic relationship, a factor-of-ten dilution corresponds to a pH change of one unit. In other words, if 1 milliliter of hydrochloric acid is added to 10 milliliters of pH-neutral water, the concentration of hydrogen ions decreases by one factor of ten, and thus the pH of the final solution will be one unit higher than the pH of the original hydrochloric acid. If 1 milliliter of hydrochloric acid is added to 100 milliliters of water, the concentration of hydrogen ions decreases by two factors of ten and the pH increases by two units.