62. Elements And Compounds

Elements And Compounds. Every body, every mass of matter, is either an element, a compound, or a mixture. Iron, silver, sulphur, and oxygen are elements; water, wood, lime, and carbonic acid are compounds.

A compound may be decomposed or divided into separate substances. For example, if an electric current is passed through water, the water slowly disappears, and two gases are formed. These gases are entirely unlike, and neither resembles the water from which it was produced. Likewise, lime can be divided into two other substances, calcium and oxygen. Any substance which can thus be decomposed or divided into other substances, is called a compound.

There are substances, however, which have never been decomposed into other substances. By no known process can sulphur be separated into other substances; the same is also true of iron, gold, arsenic, and many other substances. Substances which have never been decomposed are called elements.

63. Combustion is very rapid chemical combination.

The atoms of some of the elements have a very great affinity, or attraction, for those of other elements, and when they combine they rush together with such rapidity and force that heat and light are produced. Oxygen, for example, has a great attraction for nearly all the other elements. An atom of oxygen is ready to combine with almost any substance with which it comes in contact. Oxygen has a particular liking for carbon, and whenever these two elements come in contact at a sufficiently high temperature they combine with great rapidity. The combustion of coal in the furnace of a boiler is of this nature. The temperature of the furnace is raised by kindling the fire, and then the carbon of the coal begins to combine with oxygen taken from the air. The combination is so rapid and violent that a great quantity of heat is given out.

The elements which enter into combustion are oxygen and, usually, either carbon or hydrogen. Coal, wood, and other fuels are composed almost entirely of these three elements. Combustion is, therefore, the rapid chemical combination of oxygen with either carbon or hydrogen, or both.

64. When carbon and oxygen combine they form a gas called carbon dioxide, which is denoted by the symbol CO2; when hydrogen and oxygen combine they form water (H1O). These are called the products of combustion. When, as is ordinarily the case, the oxygen is obtained from the air, the nitrogen of the air passes into the furnace along with the oxygen. It takes no part in the combustion, and passes off up the chimney with the CO2. Hence, nitrogen is also a product of combustion in air.

There is one other case that may occur: the combustion of carbon may not be complete. If insufficient air or oxygen is supplied to the burning carbon, it is possible for the carbon and oxygen to form another gas, carbon monoxide, or CO, instead of carbon dioxide (CO2.)

The combustion of a pound of carbon to form CO, of course, requires only half of the oxygen that would be necessary to form CO2 . This is because in CO gas 1 atom of carbon seizes 1 atom of oxygen instead of 2 atoms. To burn a pound of carbon to CO2 requires 11.6 pounds of air. To burn it to CO would, therefore, require but 5.8 pounds of air.

65. The quantity of air required in practice to properly burn coal at different rates of combustion, per square foot of grate surface per hour, is about as follows:

Table 9

There is little or no difference in the amount of air required, per pound, to burn anthracite or bituminous coal.