Where Does Salt Come From?
Salt is one of the things with which we come in contact with daily perhaps more than any other. With the exception of water, probably no one thing is used more by all civilized people than salt.
You have already learned in our talk on elements the difference between a mere mixture of substances and a chemical compound. You remember that when some substances are only mixed together, they do not lose their identity. In a compound the substances are always combined in fixed proportions and the properties of the compound are often very different from those of the things that make it. Common salt is made of two substances, that are not at all like salt, and are very different from each other. One, sodium, is a soft, bluish metal, and the other is chlorine, a yellowish-green gas. The chemical name for salt is sodium chloride which is derived from the two names sodium and chlorine.
Sodium and chlorine are both what we have learned to call elements. An element being a substance which cannot be separated into substances of different kinds. There are now known about seventy such elements. All the substances around us are composed of these elements alone, or chemically united in different compounds, or simply mixed together. Most of them, however, are mixtures, not of separate elements, but of compounds. The soil under our feet is a mixture of compounds. Water is also a compound. Pure compounds very rarely occur naturally. Salt is sometimes found almost pure; but generally is mixed with so many other things that we have to take them out to get absolutely pure salt. For practical every-day use it is unnecessary to purify the salt.
Salt is found in large quantities in the sea water, in which it is dissolved with some other substances. It is also found in salt beds, formed by the drying up of old lakes that have no outlets; salt wells, that yield strong brine; and salt mines, in which it is found in hard, solid, transparent crystals, called rock salt. Rock salt is the purest form in which salt is found and, to prepare it for market, it is merely necessary to grind it or cut into blocks. The greatest deposit of salt in the world is probably that at Wielizka in Poland, where there is a bed 500 miles long, 20 miles wide, and 1,200 feet thick. Some of the mines there are so extensive that it is said some of the miners spend all their lives in them, never coming to the surface of the earth.
A trip through these mines is interesting. In one of them can be seen a church made entirely of salt. The salt supply of the United States is obtained chiefly from the salt wells of Michigan and New York, the Great Salt Lake in Utah, and the rock-salt mines of Louisiana and Kansas.
In the arts and manufactures, the most important uses of salt are in glazing earthenware, in extracting metals from their ores, in preserving meats and hides, in fertilizing arid soil, and also, as we shall presently see, in the manufacture of soda. Of equal importance, perhaps, is its use in food. Most people think it not only lends a pleasant flavor, but is itself an important article of diet. It is certain, that all people who can obtain it use salt in their food, and where it is scarce, it is considered one of the greatest of luxuries.
Soda is of interest to us, not so much on account of its use in our households, as because it plays on extremely important part in two industries that contribute greatly to our comfort, viz., the manufacture of glass and soap.
Soda is not found naturally in great abundance, as salt is, but is generally made from other substances. Formerly it was made almost entirely from the ashes of certain plants. One, known as the Salsoda soda-plant, was formerly cultivated in Spain for the soda contained in it, and the ashes, or Barilla, as they were called, were soaked in water to dissolve out the soda. Now, however, the world’s soda supply is produced from common salt by two processes, known from the names of their inventors as the Leblanc and Solvay processes.
WHERE WE GET SODA
In the Leblanc process the first step is to treat the salt, or sodium chloride, with sulphuric acid. As a result of this, a compound of sodium, sulphur, and oxygen, called sodium sulphate is formed, together with another acid containing hydrogen and chlorine, and called hydrochloric acid. This acid is driven off by boiling, and the sodium sulphate is left.
The next step in the process is to convert the sodium sulphate, or “salt cake,” into soda, or, to give it its chemical name, sodium carbonate. This change is brought about by mixing the salt cake with limestone and coal and heating the mixture. Just what changes go on when this is done, are not known, but the chief ones are probably the following: the coal, which consists for the most part of an element called carbon, takes the oxygen out of the sodium sulphate, and unites with it to form carbonic acid gas, leaving a compound of sodium and sulphur called sodium sulphide; this acts on the limestone, which is composed of a metal, calcium, in combination with carbon and oxygen, and causes the sulphur in the sodium sulphide to combine with the calcium, forming calcium sulphide, while the sodium combines with the carbon and oxygen and forms the desired compound, sodium carbonate. After the heating, the resulting mass which contains calcium sulphide, sodium carbonate, and some unburned coal, and is known as “black ash,” is broken up and treated with water. This dissolves the sodium carbonate, leaving the rest undissolved, and when part of the water is evaporated crystals containing sodium carbonate and water are formed. By heating these the water may be driven off, and the sodium carbonate left behind as a white powder.
The Solvay, or ammonia soda, process consists in forcing carbonic acid gas through strong brine, to which a considerable quantity of ammonia has been added. When this is done, crystals are formed in the brine, which are composed of a compound of hydrogen, sodium, carbon, and oxygen, and are called sodium bicarbonate. This substance, which is the soda we sometimes use in baking bread, is decomposed by heating, into water and sodium carbonate, the soda used for washing.
The Leblanc process was formerly used almost altogether for making soda; but in recent years the Solvay process has come into extensive use, and it is said that now more than half the soda of the world is made in this way.