Thousands and tens of thousands of compounds are known to the chemist today. It would be impossible to learn properties and behavior of even a fraction of this number if it had to be done on the basis of individual compounds1. Fortunately, most chemical compounds can be grouped together in a few classes. Then, if we can properly classify a compound .we are at once aware of the general properties of the compound from knowledge of the properties of that class or group of compounds. For example. HCl is classed as an acid. and by becoming familiar with the behavior of acids as a distinct class, we are at once aware of the general properties of the compound2. A great many of the compounds we are to study may be classified as acids, bases, salts, metallic oxides, or nonmetallic oxides. Of these five classes of compounds, the first three — acids, bases, and salts — are by far the most important3„
When an acid, base, or salt is dissolved in water the resulting solution is a conductor of the electric current and is termed an electrolyte. If no conduction of current occurs, the compound is known as a nonelectrolyte.
Classification of Common Compounds
By looking at the chemical formulas we may classify many common compounds in the following way.
1. Acids, in the conventional sense, may be recognized by noting that the H is written first in the formula and that the rest of the compound is generally nonmetallic. Ex., HCl, H2SO4, HClO.
2. Conventional bases have OH radicals written last in the formula. The first part of the formula is usually a metal. Ex., NaOH, Ca(OH)2, Fe(OH)3,
3. A salt consists of a metal, written first, combined with a non-metal or radical written last in a formula. Ex., NaCl, Fe2(SO4)3, Ca(ClO)2.
4. Oxides are compounds containing oxygen and only one other element.
If the element other than oxygen is a nonmetal, the oxide is classed as a nonmetal oxide or an acidic anhydride. The latter name comes about because water added to nonmetal oxides under certain conditions produces acids. Likewise, if water is removed from an acid containing oxygen, the acid anhydride (without water) results.
The other class of oxides, metallic oxides or basic anhydrides, consist of oxygen combined with a metal. When water is added under proper conditions to basic anhydrides, bases result and vice versa. Acids
All acids in the conventional sense contain hydrogen, which may be replaced by metals. The negative portion of the acid molecule is composed of a nonmetal or a radical (negative valence group). These negative valence groups (except oxide and hydroxide)are often referred to as acid radicals. All acids are covalent compounds in which the atoms are held together by a sharing of electrons. When an acid is dissolved in water, ions are formed as a result of the transfer of a hydrogen ion(proton) from the acid molecule to the water molecule---for example,