Acids, Alkalis and Salts

by George Henry Joseph Adlam

It is perhaps somewhat surprising at first to be told that all these forms are composed of the same chemical substance, yet on this point the evidence is definite and unmistakable. All the varieties dissolve readily in dilute hydrochloric acid with effervescence caused by the escape of carbon dioxide gas; moreover, if any of the purer forms, such as pearl, marble, or Iceland spar, are heated to redness for some time, they all lose about 44 per cent. by weight, leaving a residue which is pure lime.

Quicklime. The making of lime from limestone or chalk is called lime burning. The operation is carried out in a structure called a lime kiln, which is usually a barrel-shaped vertical shaft surrounded by substantial brickwork. There are two main methods of procedure, the one continuous and the other intermittent. In the continuous process, the kiln is filled up with limestone and fuel (generally coke) in alternate layers. Combustion is started at the bottom and maintained by a regulated draught. As the charge works down, the addition of limestone and fuel is continued from the top, while the lime is removed from the bottom of the kiln. The lime produced by this method has the ashes of the fuel mixed with it. To avoid this, the more modern type of kiln has four lateral fire grates outside the actual kiln.

For the intermittent method, a kiln is required which has a fireplace at the bottom. Over this a rough arch is built of large pieces of limestone, laid dry, and then the kiln is filled up with pieces of limestone which decrease in size from below upwards. The fire is kindled beneath the arch and urged by a regulated draught. The heating is maintained for three days and nights, after which time the charge is allowed to cool down.

Carbonic Acid Gas in Nature. Although the solvent action of carbonic acid is very small compared with that of strong acids, it is nevertheless great in comparison with that of water. This is shown especially in its action on limestone, an action from which several important consequences arise. Rain, as it falls through the air, dissolves a little carbon dioxide and, although this is only an exceedingly dilute solution of a very weak acid, its cumulative effect, especially in limestone districts, is very great; it hollows out enormous caves and causes the formation of those fantastic creations in stone known as stalactites and stalagmites.

When a drop of water charged with carbonic acid gas falls on limestone, it dissolves a little of that substance, forming calcium bicarbonate, which may be regarded as a compound of calcium carbonate, carbon dioxide, and water. Little by little, the solid rock is hollowed out and a cave, or perhaps an underground watercourse, is formed.

Again, the drop of water charged with calcium bicarbonate may find its way to the roof of a cave. As it hangs from the roof while it gathers strength to fall, a little of the carbon dioxide escapes, and a minute quantity of calcium carbonate is deposited. In this way, a stalactite looking like an icicle in stone gradually grows downwards.

When the drop reaches the floor of the cave, a little time elapses before it sinks into the ground; again a little carbon dioxide escapes, and a small quantity of calcium carbonate is formed. Little is added to little, and in the course of ages the stalagmite grows upward from the floor and ultimately meets the stalactite to form a continuous column of glistening crystallized calcium carbonate.

Hard and Soft Water. Water that is used for domestic or manufacturing purposes is described as either hard or soft. Soft water produces a soap lather almost at once; hard water forms at first a scum or curd which has no detergent properties, and only after a time gives the soap lather which is required. The difference is due to the relative amount of dissolved solid contained in the water.

Only distilled water or rain water collected in the open country is perfectly soft, for this is the only kind of water which on being evaporated to dryness leaves no solid residue. In districts where the underlying strata are composed of hard insoluble rock, such as granite or millstone grit, the water contains very little dissolved matter and is relatively soft. In a limestone or chalk country, water is very hard and in many cases has to be softened either before delivery or before use.

The chief impurities which cause hardness are the chlorides, sulphates, and bicarbonates of magnesium and calcium. The chlorides and sulphates are not affected in any way by boiling, and the hardness which is due to them is said to be “permanent.” The bicarbonates, on the other hand, are decomposed when the water is boiled, and then they cease to cause the water to be hard. This part of the hardness is spoken of as “temporary” hardness.