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Bring a very little of the solid supported on a clean platinum wire (s. Exp. 2 of this Chap.) into the flame of a Bunsen-lamp: the flame is coloured pale grass-green.

The production of a pale grass-green colour in a flame which before was non-luminous is a test for the presence of a compound of barium.

When barium sulphate is heated for some time in contact with a large quantity of a concentrated solution of sodium carbonate, a part of the barium sulphate is changed to barium carbonate, and sodium sulphate is simultaneously formed; sodium sulphate dissolves easily in water. On these reactions we may found a method for proving that the solid substance obtained as described is a sulphate. Add sodium carbonate to some warm water as long as the solid dissolves; put a little of this solution in a test tube; to the rest of it add some of the solid which is to be tested, and allow the liquid with the solid suspended in it to boil for 5 minutes or so. Meanwhile prove that the solution of sodium carbonate you have prepared is quite free from sulphate, by adding excess of hydrochloric acid and barium nitrate (s. Exp. 8, Chap. VI.). Now return to the boiling solution of sodium carbonate holding the solid in suspension; allow it to settle until you obtain a clear supernatant liquid; pour off some of this (through a filter if necessary), and prove the presence of a sulphate in this liquid.

Barium oxide therefore interacts with sulphuric acid to form barium sulphate; but barium sulphate is a salt; therefore barium oxide is a basic oxide.

In this chapter you have learned something of the meanings of the terms basic oxide and acidic oxide; but you have seen that to understand the terms more fully it is necessary to know what is meant by an acid, and what is meant by a salt.

Incidentally you have learned how to detect a nitrate and a phosphate, and also compounds of potassium, of barium, and of sodium. Finally you have learned how to neutralise an acid by an alkali.

Reference to "ELEMENTARY CHEMISTRY." Chap. VIII. pars. 123 to 126, and 130 to 132.

CHAPTER VIII.

ACIDS AND SALTS.

Exp. 1. To three quantities of dilute sulphuric acid in beakers * add, respectively, zinc, zinc oxide, and zinc carbonate; gas is evolved in the first and third beakers; no gas is evolved in the second beaker†.

Prove that the gas evolved during the interaction of the zinc and the acid is hydrogen; and that the gas evolved in the interaction of the zinc carbonate and the acid is carbon dioxide (s. Exp. 6, Chap. V.). Warm the three liquids, and allow the reactions to proceed until all visible change has ceased; now add a little more zinc, zinc oxide, and zinc carbonate, respectively; warm again for a minute or two, and filter into basins. Evaporate the three filtrates to dryness over low flames; collect the three solids, press them between porous paper, and crystallise each from water two or three times (s. Exp. 8, Chap. VI.). Prove that each solid (1) is a compound of zinc, (2) is a sulphate.

The solids have the same composition; each is zinc sulphate. Note that the results of your experiments do not prove this conclusively; accurate quantitative analysis is required. But we shall assume that such analyses have been made; the result is that each solid has the same composition, and that each is zinc sulphate.

When zinc reacts with dilute sulphuric acid, zinc sulphate and hydrogen are formed; when zinc oxide reacts with dilute

* About 100 c.c. of a mixture of 1 part strong acid with 8-10 parts water may be used.

+ If the zinc oxide contains a little carbonate, as most specimens of the oxide do, a little carbon dioxide will be evolved.

sulphuric acid, zinc sulphate and water are formed (your experiments do not indicate or even suggest the production of water); when zinc carbonate reacts with dilute sulphuric acid, zinc sulphate, carbon dioxide, and water are formed.

[Note carefully what your own experiments have actually proved, and what further data would be required to prove these statements.]

Exp. 2. Perform an experiment similar to No. 1, but use a solution of hydrochloric acid in place of sulphuric acid, and magnesium, magnesium oxide, and magnesium carbonate, respectively, in place of zinc, zinc oxide, and zinc carbonate, respectively.

Prove that hydrogen is evolved in the reaction with magnesium; carbon dioxide in the reaction with magnesium carbonate; and no gas in the reaction with magnesium oxide.

Prove that the white solid obtained by evaporating each solution just to dryness at 100° is (1) a chloride, and (2) a compound of magnesium.

The production of a white pp. (magnesium-ammonium phosphate) on adding sodium phosphate solution to a solution to which a large quantity of ammonium chloride and then excess of ammonia have been previously added, shews the presence of magnesium.

Assuming-and this has been rigorously proved by quantitative analysis—that each of the solids obtained in this experiment is magnesium chloride, it follows that the reactions between hydrochloric acid and magnesium, magnesium oxide, and magnesium carbonate, are similar to the reactions between sulphuric acid and zinc, zinc oxide, and zinc carbonate.

These experiments enable us to gain a notion of the characteristic property of acids, viz. that they interact with metals, metallic oxides, and metallic carbonates, to form salts.

Zinc and magnesium are metals; zinc sulphate and magnesium chloride are salts; these salts were produced in your experiments by the interaction of sulphuric and hydrochloric acid, respectively, with a metal, a metallic oxide, and a metallic

carbonate.

To understand more completely what is meant by saying that salts are formed by the interaction of metals, &c. with acids, it would be necessary to determine the compositions of the acids used, and of the salts obtained. Such determinations have been made. The results of the quantitative examination

of the reactions used in Exps. 1 and 2 may be represented in chemical equations as follows* :—

Reactions between zinc &c. and sulphuric acid.

(1) Zn + H2SO,Aq

ZnSO4Aq + 2H.

(2) ZnO + H.,SO,Aq = ZnSO,Aq + H,O.

(3) ZnCO,+H,SO,Aq= ZnSO,Aq+ H2O + CO

Reactions between magnesium &c. and hydrochloric acid. (1) Mg + 2HClAq

(2) MgO + 2HCl Aq

(3)

MgCl, Aq + 2H.
- MgCl2Aq + H2O.

MgCO,+2HC1Aq=MgCl,Aq+H,O + CO.

3

A salt may at present be regarded, then, as a compound of a metal with the elements of an acid except hydrogen.

In the last chapter you learned that an acid reacts with an alkali to form a salt; alkalis are compounds of certain metals with oxygen and hydrogen; they are hydroxides of certain metals (s. Chaps. VII. and X.).

If an acid reacts with a metal, the oxide, and the carbonate, of a metal, to form a salt; if an acid also reacts with an alkali to form a salt; and if an alkali is the hydroxide of a certain class of metals, it is probable that an acid will react with any metallic hydroxide, whether it be an alkali or not, to form a salt. Let us put this hypothesis to the test of experi

ment.

Exp. 3. Add (1) excess of barium hydroxide to nitric acid, and (2) excess of ferric hydroxide to sulphuric acid, and proceed as described in Exp. 1. After purifying the salts obtained prove that one is (1) a compound of barium (s. Exp. 5, Chap. VII.); (2) a nitrate (s. Exp. 4, Chap. VII.); and that the other is (1) a compound of iron, (2) a sulphate (s. Exp. 8, Chap. VI.).

The production of a reddish-brown flocculent pp. (ferric hydroxide) when excess of ammonia is added to a solution is a test for a ferric compound.

A salt is one of the products obtained by the interaction of acids with (1) metals, (2) metallic oxides, (3) metallic hydroxides, (4) metallic carbonates. The salt obtained is in each case a compound of the metal with the elements of the acid except hydrogen, or, as is usually said, with the acid radicle.

* It is assumed that the student has become acquainted with the use of chemical formulæ. 8. ELEMENTARY CHEMISTRY, Chap. VI.

But more than one salt is sometimes obtained by the reaction between a specified acid and the same metallic oxide or hydroxide.

Exp. 4. You are given two exactly equal quantities of a solution of oxalic acid in water, and a quantity of a solution of

Fig. 17.

potassium hydroxide*. To one of the quantities of oxalic acid add the potash solution very cautiously, from a graduated tube with a stopcock (a burette) (Fig. 17) until the liquid is exactly neutral (s. Exp. 2, Chap. VII.). Read off, on the burette, the number of cubic centimetres of potash solution added. To the other quantity of oxalic acid add exactly half as much potash as you have added to the first quantity of the acid. Evaporate both solutions over low flames until a drop taken out on a rod solidifies on cooling; allow to cool; collect, dry, and recrystallise the solids from as little water as possible; collect, dry, and recrystallise each again.

Prove that each solid is (1) a compound of potassium, (2) an oxalate.

The production of a white pp. (calcium oxalate) when calcium chloride is added to a solution, which pp. is insoluble in acetic acid, is a test for the presence of an oxalate in that solution.

Dissolve the solids in distilled water, noting that one is much more soluble than the other, and test each solution with blue and red litmus; the oxalate of potassium obtained by exactly neutralising oxalic acid with potash is neutral to litmus; the other oxalate of potassium turns blue litmus red; this is often expressed by saying that an aqueous solution of the salt has an acid reaction.

Quantitative analyses of these oxalates of potassium shew that the composition of the neutral salt is expressed by the formula K,C,O,, and the composition of the other salt is expressed by the formula KHCO

A salt may then be formed from an acid by replacing a part, or the whole, of the hydrogen of the acid by a metal.

*Note to Demonstrator. About 10 grams oxalic acid in 250 c.c. water; 100 c.c. is enough for each experiment: about 20 grams caustic potash in 250 c.c. water; 50 c.c. for each experiment.

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