CHAPTER XI.

OTHER COMMON ELEMENTS AND COMPOUNDS.

The

104. Carbon.-Carbon is an abundant element that occurs in the solid condition in three distinct forms-charcoal, graphite, and diamond. Charcoal is a soft, amorphous, dull black solid obtained by heating wood and other organic substances out of contact with air. Graphite is a soft bright crystalline variety of carbon often called "black-lead." Diamond is an extremely hard transparent variety of carbon, its crystals often being octahedral in shape. specific gravity of charcoal is 1'5, of graphite 2'5, and of diamond 3'3. Though these three varieties of carbon are physically so unlike each other, they are chemically the same substance, for on burning them in oxygen the same chemical compound is produced, viz. carbon dioxide. When an element occurs in forms that are physically dissimilar, but chemically identical, the forms are said to be allotropic modifications of the element.

FIG. 100.-Model of octahedron.

Carbon occurs in nature, not only free, but also in combination with other elements to form numerous compounds. It occurs combined with oxygen to form carbon dioxide; it is present, associated with oxygen and lime, in the minerals called carbonates; and it is a constituent of nearly all animal and vegetable matter. Wood, bone, sugar, bread, and coal all contain carbon united with hydrogen and oxygen.

Experiment 83.-Place some chips in a hard glass tube, one end of which is then drawn out as shown (Fig. 101). Heat the tube strongly, and notice that the wood chars or blackens and a volatile oil is driven out, which will burn at the mouth of the tube. A large amount of liquid condenses on

the cool parts of the tube. found to contain free carbon.

On breaking the tube the blackened mass is
The liquid is mainly water formed from the

hydrogen and oxygen that have been expelled. Carbon can be set free and obtained in a similar way from bread, starch, bone, and sugar. The process of heating out of contact with air thus illustrated is called destructive distillation.

Charcoal is very porous, and floats on cold water. On being placed in boiling water, the air is expelled from the pores, and the charcoal sinks. Charcoal has also a great absorbent power for many gases, the absorbed gases passing into

its pores and becoming oxidized by the oxygen of the air in the pores. It is therefore used as a deodorizer and disinfectant. Decomposing animal matter gives off noxious gases, but when sprinkled with charcoal the offensive smell ceases. The following experiment further illustrates the absorbing power of charcoal :

Experiment 84.-Fill a large test-tube with ammonia-gas and invert it over a basin of mercury (Fig. 102). Pass into the test-tube a small lump of dry, freshly prepared charcoal. The charcoal soon absorbs the ammonia, and the pressure of the atmosphere on the surface of the mercury in the basin causes the liquid to rise and fill the tube.

105. Carbon Dioxide. When a piece of charcoal is burnt in oxygen, the carbon and the oxygen unite to form a compound called carbon dioxide. The same substance is also formed when any carbon compound is burnt in oxygen or in the air. Most combustibles that are used are carbon compounds, the carbon being united in most cases with the element hydrogen.

FIG. 102.

Experiment 85.-Hold a dry jar over a candle-flame or a burning taper for about a minute. The jar gathers some of the invisible products of combustion. Its sides become bedimmed with moisture formed by the union of the hydrogen in the candle with the oxygen of the air. The jar also contains some carbon dioxide, formed by the union of the carbon in the candle with the oxygen of the air. This can be shown by pouring lime

MEWATER

FIG. 103.

(From Newth's "Elementary Practical Chemistry.”)

water into the jar and shaking it up.

The lime-water is turned milky by the carbon dioxide, owing to the lime uniting with the carbon dioxide to form particles of insoluble calcium carbonate.

In the same way treat the coal-gas flame and the flame of a spirit-lamp, and in both cases it will be seen that carbon dioxide is produced when coalgas or spirit is burnt.

To obtain a quantity of carbon dioxide for experiments, we generally act upon one of the chemical compounds called "carbonates." Limestone or chalk is a chemical compound called calcium carbonate, and contains three elements—a metal calcium, the solid carbon, and the gas oxygen. Its chemical formula is represented thus, CaCO3. Carbon dioxide can be obtained from any form of calcium carbonate (limestone or marble), either by the action of heat or by the action of an acid. Other carbonates, as magnesium carbonate (magnesite), can be treated in the same way.

Experiment 86.-Place some magnesia alba (magnesium carbonate powder) in a hard glass tube. Fit it with cork and delivery tube, as in

Fig. 104. Support the tube on a stand and strongly heat it. The carbon dioxide driven off may be collected in a small trough in the usual way. The magnesium carbonate is split up by the heat into oxide of magnesium which remains in the tube, and carbon dioxide which is driven off. Powdered calcium carbonate may be treated in this way, but it requires a much greater heat to drive out the gas. The above reaction is as follows :

Experiment 87.-Put into a flask some pieces of calcium carbonatemarble, limestone, or common chalk. (The sticks of "chalk" used for the blackboard are not really chalk, but a preparation of calcium sulphate.) Fit up the flask with thistle and delivery tube as shown in Fig. 105. Pour down the thistle funnel dilute hydrochloric acid until the acid just covers the end of the thistle tube. A violent effervescence or bubbling, due to the liberation of a gas, soon begins. The gas may either be collected at the pneumatic trough or, as it is 11⁄2 times heavier than air, it may be collected by simply allowing it to fall down to the bottom of the jar, and so gradually to lift out the lighter air. This method of collecting a gas is known as the method of " downward displacement." In order to ascertain when the jar is full, we make use of the fact that the gas does not support combustion. Insert slowly, therefore, a lighted taper into the jar, and the place where it is extinguished will indicate the quantity of carbon dioxide the jar contains.

+ 2 HCl

=

The following equation represents the chemical change :CaCO3 CaCl2 + H2O + CO2 Calcium carbonate. Hydrochloric acid. Calcium chloride. Water. Carbon dioxide. 106. Experiments to illustrate the Properties of Carbon Dioxide. It will have already been noticed that carbon dioxide is a colourless gas that is neither combustible nor a supporter of combustion. A small animal placed in a jar of the gas is soon suffocated for want of free oxygen.

Properties of Carbon Dioxide.

135

After

(a) Pass the gas from the delivery tube into a beaker of water. a short time test the water with a blue litmus paper. It will be found to be acid. The gas has dissolved in the water to form a feeble acid, carbonic acid. The name "carbonic acid" is sometimes applied to the gas itself. Boil the water. The gas is driven out of the solution, and the water is no longer acid to litmus.

(b) Pass the gas into lime-water for some time. Note that insoluble calcium carbonate is first formed (see equation), but after a time is redissolved, and the water becomes clear again.

This action of carbon dioxide on lime-water is characteristic of the gas. (c) The heaviness of the gas is shown by the way it is collected. It may also be illustrated by pouring the gas from one jar to another, like one pours water. The fact that the gas has changed jars may be proved by lime-water or by testing with a candle.

(d) Shake up a little clear lime-water in a large bottle of fresh air. Notice the slight cloudiness produced, proving that air contains a little of the gas. By means of a glass tube breathe into the bottle and again shake. The milkiness produced shows that air expired from the lungs contains much carbon dioxide. In fact, experiment (b) can be shown by blowing down a glass tube into lime-water.

It is of great importance to notice that calcium carbonate is insoluble in pure water. If, however, we take some limewater in a glass beaker, and cause carbonic acid gas (carbon dioxide) to bubble through it for a considerable time, we have seen that, although it becomes milky at first, from the particles of chalk (calcium carbonate) produced, yet, on continuing to bubble in more carbonic acid, it becomes clear. We thus learn that, although, as stated above, calcium carbonate is quite insoluble in pure water, it dissolves in water which contains carbon dioxide in solution. On boiling the clear solution thus obtained, the carbon dioxide is driven off, and the calcium carbonate is again precipitated. Rain-water dissolves a little carbonic acid as it falls through the air, and it obtains more as it percolates through ground containing decaying animal and vegetable matter. In this way it becomes able to dissolve calcium carbonate, and in those parts of the country where chalk and limestone rocks abound, parts of the rocks are slowly dissolved away, and the water of the springs and streams contains small quantities of calcium carbonate in solution. Such water is called hard water, and when used for washing purposes it only "lathers" with difficulty. Water which contains