Rodney Ambrosini, age 13, of Monroe, La., for his question:

WHAT IS CITRIC ACID?

Citric acid is a common organic acid that gives lemons, oranges and other citrus fruits their sour taste. Lemon juice contains six to seven percent of the acid.

The name "citric" comes from the Latin word "citrus," which means citron tree. The citron tree is similar to the lemon and lime trees. Carl Scheele, a Swedish chemist, first isolated citric acid from lemon juice in 1784.

Citric acid is used as a flavoring for soft drinks and pharmaceuticals. Industry uses it in chemicals, alkyd resins and as a mordant or dye fixative.

Citric acid is prepared commercially from fermentation of sugar and by extraction from lemon juice, lime juice and pineapple canning residues.

Pure citric acid forms colorless, odorless crystals that have a pleasant, sour taste. It is very soluble in water. Citric acid combines with metals to form salts called citrates.

Citrin (with the letter "i" as the next to the last letter) is a chemical substance that belongs to a group of chemicals called flavonoids. Scientists do not know exactly what citrin is, or if it is essential to mans health. But they have found that flavonoid substances help to control bleeding from the capillaries in the body. Capillaries are the smallest of all blood vessels.

Citrin, also called vitamin P, affects the capillary walls, making them less likely to hemorrhage. Citrin used in medicine is usually prepared from lemon peel and paprika.

Citron (with the letter "o" as the next to the last letter) is a large, usually sour fruit much like the lemon. It ranks among the largest citrus fruits. The fruit is six to 10 inches long, or longer, and shaped like an egg.

Citrus is the name of a group of trees and shrubs which belong to the rue family. Some citrus fruits are the orange, grapefruit, lemon, mandarin, kumquat, tangerine, bitter orange, citron, shaddock and bergamot.

Citrus trees grow wild in parts of India and Southeastern Asia. The Chinese were the first to cultivate citrus trees, about 1000 B.C. These trees and shrubs have been grown in other parts of the world for their fruit for many years.

Citrus trees are thorny, but usually very attractive. They are evergreen with long, shiny, pointed leaves. The flowers are very fragrant.

Ripe citrus fruits are yellow to orange red in color. All citrus fruits are a type of berry that scientists call a hesperidium.

Citrus fruits are valuable foods. They contain large amounts of vitamins and minerals. Citrus fruits are also usually high in vitamin C.

In the United States, citrus trees grow best in Florida, Texas, California and Arizona.