John Carlson, age 12, of Williamsport, PA., for his question:

How do mixtures differ from compounds?

In everyday language we can talk about a mixture of bacteria or baking ingredients, and a compound fracture occurs when a broken bone pokes through the flesh and skin. But in chemistry, the words mixture and compound are terms with exact meanings, and no wide awake science student mixes up his mixtures! It’s  his compounds.

In the science of chemistry, the terms mixture and compound take us down to the sub microscopic world of atoms. They are closely linked with the word element, which is another chemical term concerning atoms. An element is made from atoms of the same kind. Gold and pure oxygen are elements. Apart from a few elements, all the countless substances in the world are either compounds or mixtures.

A compound is made from two or more elements in exact proportions. The basic units, of an element is the single atom. The basic unit, of a compound is the molecule. A molecule is a wad of assorted atoms. A molecule of table salt is made wane atom of sodium and one of chlorine. A molecule of water is made of two atoms of hydrogen and one atom of oxygen.

Table salt is a compound made from equal proportions of sodium and chlorine. Water is a compound in which the ratio of ingredients is two to one. This is because of the basic m01coules from which these two compounds are made. A molecule of ordinary sugar, which is sucrose, contains 12 atoms of carbon, 24 atoms of hydrogen and 11 atoms of oxygen. Some molecules contain hundreds of atoms. In plastic and certain other complex compounds, the basic units are linked together in long chains of identical molecules.

The basic ingredients in a chemical mixture may be elements or compounds. There may be atoms of this and molecules of that  but the basic ingredients are not bound together by chemical ties as they are in the molecules. And there is no set ratio between the ingredients. Granite is a stony mixture of silica and feldspar with perhaps a trace of mica. Air is a mixture of gases in which the various ingredients have no chemical ties with each other.

In the whole world, there are about 100 elements, each made from atoms of the same kind. There are 525,000 different compounds made from molecules which are made from assorted atoms. There is no limit to the number of mixtures, such as the stony minerals, the blends of gases and the solutions of watery liquids.