Sam Goldstein, age 14, of Santa Cruz, Calif., for his question:

WHAT ARE PETROCHEMICALS?

Petrochemicals are chemicals made from crude oil and natural gas. They are among the most important materials used in industry today.

Petrochemicals are used in making plastics, detergents, synthetic fibers, paint, synthetic rubber, medicines and fertilizers.

Crude oil and natural gas consist chiefly of compounds of the elements hydrogen and carbon, arranged in different ways. These compounds are called hydrocarbons. One of these hydrocarbons may be separated from the rest, purified and then sold as a petrochemical.

In most cases, the chemical compounds in the oil or gas are first broken apart then put together in another way to make chemicals that were not originally present in the oil or gas. Often, other chemicals are added. Chemicals produced this way are the most important petrochemicals.

Chemicals that are the same as petrochemicals can also be made from other raw materials. For example, they can be made from coal or farm products. Many chemicals even come partly from one and partly from another of these raw materials.

Usually, when most of the supply of some chemical is believed to come from oil or gas, people call it a petrochemical.

The large scale use of oil and gas as raw materials for making chemicals started in the 1920s. The use of petrochemicals spread rapidly. This was partly due to an increased demand for chemicals of all kinds and because the supply of older chemicals could not meet the demand.

Also, some of the newly invented chemicals enabled manufacturers to produce products that could not be made before. This resulted in a demand for these materials to make the things that people wanted and needed.

Large numbers of research scientists and engineers invented and perfected efficient ways to make petrochemicals. Today, experts estimate that at least one fourth of all the chemicals used in industry, by weight, are made from petroleum and natural gas.

One important petrochemical is ammonia, which is produced from natural gas and finds its chief use in commercial fertilizers. Methanol, ethanol and propanol are used as solvents for paint, as automobile antifreezes, in the manufacture of plastics and to make still other chemicals.

Carbon black is an important reinforcing material for automobile tires while manufacturers use ethylene glycol and glycerin in dynamite, paint and cellophane.

Butadiene and styrene serve as important raw materials for synthetic rubber. Dodecyl benzene is the chief raw material for synthetic detergents used for washing clothes and dishes.

Textile companies make synthetic fibers from cyclohexane and paraxylene.