Introduction to Acid
Acid, a chemical compound that is capable of releasing hydrogen ions (electrically charged hydrogen atoms) for a chemical reaction. All acids contain hydrogen, but not all hydrogen-containing compounds are acids. Most acids also contain oxygen.
Long, chainlike compounds called nucleic acids are found in all living cells and are essential to life. Amino acids, also found in living organisms, are the building blocks of proteins. Other acids, such as sulfuric acid and nitric acid, are of tremendous importance industrially. Sulfuric acid, familiar as the acid in automobile storage batteries, is the largest single product of the chemical industry.
Properties of Acids
At ordinary temperatures, most pure acids are solids. Many, however, are liquids, and a few are gases. Some acids, such as prussic acid (hydrogen cyanide), are deadly poison.
When acids are dissolved in water in sufficiently high concentration they typically have the following properties:
1. They have a sour taste. (The term acid is derived from acidus, the Latin word for sour.)
2. They are corrosive.
3. They will turn blue litmus paper red.
4. They will dissolve many metals (for example, iron, tin, and zinc) and at the same time release hydrogen gas.
5. They will conduct an electric current, with the simultaneous liberation of hydrogen gas.
Acids will react with substances called bases (for example, sodium hydroxide) to form salts (such as sodium chloride, or table salt). A base is a substance that is capable of accepting hydrogen from another substance; thus it can be considered the opposite of an acid.
Structure of Acids
The simplest type of acid has a molecule consisting of one atom of hydrogen and one atom of a nonmetallic element. A molecule of hydrochloric acid, for example, contains one atom of hydrogen and one atom of chlorine. Hydrochloric acid can be obtained by dissolving hydrogen chloride, a gas, in water. Both fndrochloric acid and hydrogen chloride have the chemical formula HCl, H being the symbol for hydrogen and Cl the symbol for chlorine.
Sodium chloride, one of the many chemical compounds classified as a salt, can be formed by combining metallic sodium (Na) with hydrochloric acid. The sodium replaces the hydrogen, as indicated by the chemical equation for this reaction:
2 Na + 2 HCl>2 NaCl + H2
Sulfur, nitrogen, carbon, phosphorus, fluorine, and arsenic are among the other common elements that combine with hydrogen, or with hydrogen and oxygen, to form acids. Most of the common acids can be prepared by the reaction of oxides of nonmetals with water. In contrast to hydrochloric acid and other simple acids, some acids contain dozens of atoms in each of their molecules. The largest nucleic-acid molecules each contain millions of atoms.
Organic and Inorganic Acids
Acids, like all chemical compounds, are classified as either organic or inorganic. Organic acids occur in, or can be produced from, animal and vegetable matter. In addition to hydrogen, organic acids always contain carbon and at least one other element.
Nucleic acids and amino acids are organic. Other common organic acids are acetic acid, found in vinegar and cider; citric acid, found in citrus fruits, gooseberries, and currants; tannic acid (tannin), found in oak galls; formic acid, which occurs in insects and plants; lactic acid, a constituent of sour milk; oleic acid, found in animal fats and vegetable oils; and oxalic acid, found in rhubarb, spinach, and other edible plants.
Organic acids are used as food additives, in medicines, in dyeing and bleaching, in the tanning of leather, and in the manufacture of various chemicals.
Inorganic acids, which contain no carbon, are sometimes called mineral acids. In order of tonnage produced in the United States, the most important inorganic acids are sulfuric, phosphoric, and nitric. Inorganic acids are used in the manufacture of fertilizers, plastics, explosives, synthetic textiles, paints, dyes, solvents, and many other substances.
Strength of Acids
The ability of an acid to release hydrogen ions is specified by its strength. A strong acid (for example, sulfuric or hydrochloric acid) dissociates (breaks up) completely, or nearly completely, into positive and negative ions in a dilute (nonconcentrated) water solution. Only a small percentage of a weak acid (for example, acetic acid), on the other hand, forms ions in solution, the major portion of the acid remaining in the form of molecules. As a general rule, inorganic acids are stronger than organic acids. The formation of positive and negative ions is the reason that an acid in a water solution will conduct electricity.
The strength of an acid is denoted by its pH. The pH value of a dilute solution is given approximately by the following expression:
pH = log 1/[H+]
In this expression [H+] is a number whose value is determined by the quantity of hydrogen ions per unit volume. The pH of a solution can range from 0 to 14. Pure water, which is neutral (that is, neither an acid nor a base), has a pH of 7. Substances with pH values of less than 7 are acids, and substances with pH values of more than 7 are bases. Strong acids have pH values near 0, while strong bases have pH values near 14.
Certain indicators, such as methyl orange and bromphenol blue, which will change color at a known pH, can be used to find the pH of a solution. Electric meters that give a direct reading of the pH of a solution are also used by chemists.