drop by drop; a white gelatinous pp. forms; add more potash, the pp. dissolves. dissolves. The white pp. is zinc hydroxide; the production of this pp., and its solubility in much caustic potash, proves the presence of a compound of zinc in the original liquid.

The production of a white pp. (barium sulphate), insoluble in bydrochloric acid, when barium nitrate solution is added to a liquid, is a test for the presence of a sulphate in that liquid.

The production of a white gelatinous pp. (zinc hydroxide), soluble in much caustic potash, when a little caustic potash solution is added to a liquid, is a test for the presence of a compound of zinc in that liquid.

You conclude that when zinc and dilute sulphuric acid interact, hydrogen and zinc sulphate are produced. This conclusion has been verified by accurate quantitative experiments.

From the experiments in this chapter you have learned, (1) how to prepare hydrogen and oxygen; (2) something regarding the chemical changes which proceed when hydrogen is prepared (a) by the interaction of zinc and dilute sulphuric acid, (b) by the interaction of water and sodium, and when oxygen is prepared by the action of heat on potassium chlorate mixed with manganese dioxide; (3) that, under ordinary conditions, hydrogen is combustible and oxygen is a supporter of combustion; (4) that hydrogen is much lighter than air, but that equal volumes of oxygen and air are nearly the same weight; (5) that a mixture of air and hydrogen explodes when a flame is brought near it; (6) that carbon burns in oxygen to form an oxide, an aqueous solution of which turns blue litmus red; (7) that sodium burns in oxygen to form an oxide, an aqueous solution of which turns red litmus blue. You have also learned incidentally how to detect (a) a chloride, (b) a sulphate, (c) a compound of zinc, in solutions; and how to purify a solid soluble in water from other substances more soluble than itself.

Water is a compound of hydrogen and oxygen; let us examine a few of its properties.

Exp. 9. To three separate portions of distilled water in basins add, (1) some powdered copper sulphate crystals, (2) powdered potassium nitrate, (3) powdered tartar-emetic; warm each basin slightly, and if the solids do not wholly dissolve add a little more water. After a time the whole of each solid has disappeared in the water. Evaporate a portion of

each solution to dryness; so far as you can judge by their appearances, the solids obtained are the same as those originally dissolved in the water. Accurate experiment confirms this conclusion.

In these three cases water has acted as a solvent. Water dissolves very many chemical compounds, and a few chemical elements, without changing their composition.

Exp. 10. To two separate portions of distilled water in basins add (1) anhydrous copper sulphate, (2) solid sulphur trioxide; each dissolves rapidly, the latter with a hissing sound. Evaporate the solutions somewhat, and allow to cool; blue crystals are obtained from the first, and a thickish, very acid, corrosive, liquid from the second. Both products are different from the substances added to the water. In one case hydrated copper sulphate, in the other case sulphuric acid, has been formed.

In these cases water has acted as a solvent, but at the same time it has interacted with the substance added to it and has produced a new substance.

Reference to "ELEMENTARY CHEMISTRY."

90 to 106.

Chap. VII. pars.

* This should be supplied in small sealed tubes; it is easily prepared by warming Nordhausen sulphuric acid and leading the vapour into a series of dry test tubes, each of which is sealed off when it contains a small quantity of the trioxide.

CHAPTER VII.

CLASSIFICATION OF OXIDES.

Exp. 1. To three separate quantities of water in basins add (1) phosphorus pentoxide, (2) sulphur trioxide, (3) sodium oxide*. When the oxides are dissolved examine the solutions

(a) to two small quantities of each add a drop of blue litmus, and a drop of red litmus, respectively;

(b) in a small quantity of each solution place a little piece of the metal zine;

(c) to a small quantity of each solution add a little sodium carbonate, and notice whether carbon dioxide is given off.

The solutions of phosphorus pentoxide and sulphur trioxide turn the blue litmus red, dissolve the zinc more or less rapidly, and dissolve the sodium carbonate with evolution of carbon dioxide.

The solution of sodium oxide turns the red litmus blue, does not dissolve the zinc, at least not so long as the solution remains cold, and does not evolve carbon dioxide from the sodium carbonate.

Exp. 2. Boil the remainder of the solution of phosphorus pentoxide for some time, and then very cautiously and slowly pour into it the solution of sodium oxide until the liquid is as nearly neutral as you can make it. To determine when this point is reached, arrange several small pieces of red and blue litmus paper on a piece of clean paper on a porcelain slab; stir the solution of phosphorus pentoxide with a clean rod as you slowly pour in the solution of sodium oxide; from time to

* Caustic soda may be used in place of the oxide.

time remove a drop of the liquid and let it fall on to one of the pieces of blue litmus paper; when a drop of the liquid produces only a slight reddening of the paper, dilute the sodium oxide solution considerably (by pouring water into it) and add the dilute solution one drop at a time. Proceed thus until a drop of the liquid scarcely affects the colour either of blue or red litmus paper. Should you add too much sodium oxide solution (i.e. should a drop of the liquid in the basin turn red litmus blue) you must dissolve a little more phosphorus pentoxide in hot water and add this drop by drop, to the liquid until a drop of it scarcely turns red litmus blue or blue litmus red.

Place the neutral solution over a low flame to evaporate. Meanwhile neutralise the solution of sulphur trioxide remaining from Exp. 2, by solution of sodium oxide, in the way already described. Place the neutral solution over a low flame to evaporate.

When both solutions have evaporated to dryness there remains in each case a white solid. Remove a little of these solids by means of clean spatulas to test tubes, dissolve in water, divide each into two parts, and to one part add a drop or two of blue litmus, and to the other part a drop or two of red litmus; the solutions are almost or quite neutral to litmus.

Dissolve in water a small portion of the white solid obtained by neutralising the solution of phosphorus pentoxide by soda; add a little nitric acid, and then heat to boiling; add a good deal of ammonium molybdate solution, and remove from the source of heat; a yellow pp. forms slowly.

The formation of a yellow pp. by adding ammonium molybdate to a hot solution containing nitric acid is a test for the presence of a phosphate in the solution. [The yellow pp. is a compound of molybdenum trioxide with ammonium phosphate.]

Dissolve in water a little of the white solid obtained by neutralising the solution of sulphur trioxide by solution of sodium oxide, and test for a sulphate.

Take a little piece of clean platinum wire; hold it in the flame of a Bunsen-lamp, near the outer edge towards the lower part of the flame; if the wire is quite clean no colour is given to the upper part of the flame; if the flame is coloured, the wire must be dipped in concentrated hydrochloric acid, held in the flame, again dipped in the acid, and again held in the flame, until it ceases to give any colour to the upper part

of the flame. Now moisten the wire, take up on it, successively, a very small particle of each of the solids you have prepared, and bring each into the lower part, near the outer edge, of the flame of a Bunsen-lamp; the upper part of the flame is at once coloured yellow. This colour proves the presence of a compound of sodium.

All compounds of sodium impart a yellow colour to a nonluminous flame in which they are volatilised; the colour is scarcely visible through a blue glass.

From these experimental results you conclude, (1) that when a solution in hot water. of phosphorus pentoxide is neutralised by an aqueous solution of sodium oxide there is probably produced a phosphate of sodium; (2) that when a solution in water of sulphur trioxide is neutralised by an aqueous solution of sodium oxide there is probably produced a sulphate of sodium. These compounds are salts.

To prove these conclusions quite satisfactorily, it would be necessary to make accurate quantitative experiments.

Oxides which resemble phosphorus pentoxide and sulphur trioxide are called acidic oxides. These oxides interact with water to form acids; or they are obtained from acids by removing water from them.

Oxides which resemble sodium oxide are called basic oxides. These oxides interact with acids to form salts and water; some of them dissolve in water to form alkalis.

Exp. 3. Zinc oxide is a basic oxide. Place some dilute sulphuric acid in a basin; warm the acid, and add zinc oxide, little by little, until some of the oxide remains undissolved (i.e. add an excess of zinc oxide); now pour the liquid through a filter into another basin, and evaporate the filtrate nearly to dryness over a low flame. Allow to cool, collect the solid which has formed, and free it from all adhering mother liquor in the manner described in Exp. 8, Chap. VI. When crystals separate from a liquid, the remaining liquid is called the mother liquor.

Finally dissolve the crystals you obtain in water and test the solution for (1) zinc, (2) a sulphate (s. Exp. 8. Chap. VI.). The crystals are zinc sulphate. But zinc sulphate is a salt; therefore zinc oxide is a basic oxide as it has reacted with sulphuric acid to form a salt.

Exp. 4. You are given an aqueous solution of nitrogen pentoxide and are asked to prove that this is an acidic oxide.