لخّصلي

We read earlier in this chapter that matter may be classified according to its physical state as solid, liquid, or gas. But there is another way to classify matter. Scientists classify matter on the basis of chemical composition as elements, compounds, or mixtures. Elements are the basic substances that cannot be decomposed or broken down into simpler substances by ordinary chemical means. What are these basics substances? Some of the earlier Greek philosophers, including Aristotle, believed that the four basic elements that all things are made of are earth, fire, air, and water. But these ancient Greeks were philosophers, not scientists, and as such they speculated but did not experiment. During the Middle Ages, the alchemists secretly experimented with metals. Finally, in the 18th century, the early chemists disproved the theory of the four elements when they determined that water was composed of hydrogen and oxygen, and air composed of nitrogen and oxygen. After that, many other substances were determined to be elements, and they were categorized as metals and nonmetals. Metals include copper, silver, gold, iron, and lead. Nonmetals include solids, such as carbon and sulfur, and gases, such as oxygen, hydrogen, nitrogen, and helium, The list of elements grew and scientists looked for a pattern in order to understand their behavior. in 1869, the Russian chemist Dmitri Mendeleev organized all the known elements, according to their weights and properties, into a table called the periodic table of elements, To date, 92 natural elements have been identified. Several others, such as plutonium, have been produced artificially. Many of these are extremely rare. One of the interesting facts of science is that it almost all matter including the seas, people, plants and earth - is created from about 20 common elements. Most common substances are not elements, but compounds of elements. A compound is a substance that is made up of two or more elements that cannot be separated by mechanical means. For example, water can only be separated into hydrogen and oxygen by a process known as electrolysis. Salt, sugar, alcohol, benzene, and sulfuric acid are other examples of compounds. Each compound has more than one type of atom within each molecule, and a compound has characteristics thatare distinct from the characteristics of its constituents. For example, when the two atoms of hydrogen combine with one atom of oxygen, an entirely new substance is formed - H20 or water! Salt is compound of sodium which is an active metal, and chlorine, which is a poisonous gas. Yet salt has none of the characteristics of either of these two elements, fortunately. Compounds have a constant or invariable composition. For example, any sample of water contains the same proportion of hydrogen and oxygen. This is called the law of definition composition. Substances which are not homogeneous, or uniform in composition, are called mixtures. Air is essentially a mixture of nitrogen and oxygen. Soil, cement, wood, rocks, and food products are categorized as mixtures. The constituents of a mixture may exist in different proportions. For example, a cake may be prepared with different proportions of flour, butter, eggs, and sugar. Also, each ingredient in a mixture retains its identity. If there is sugar in the cake, the cake will taste sweet.