MRS. B.
In the first case, the metal merely combines with a portion of oxygen taken from the acid, which is thus partly deoxygenated, as in the instance you have just seen; in the second case, the metal, after being previously oxydated, is actually dissolved in the acid, and enters into a chemical combination with it, without producing any further decomposition or effervescence.—This complete combination of an oxyd and an acid forms a peculiar and important class of compound salts.
EMILY.
The difference between an oxyd and a compound salt, therefore, is very obvious: the one consists of a metal and oxygen; the other of an oxyd and an acid.
MRS. B.
Very well: and you will be careful to remember that the metals are incapable of entering into this combination with acids, unless they are previously oxydated; therefore, whenever you bring a metal in contact with an acid, it will be first oxydated and afterwards dissolved, provided that there be a sufficient quantity of acid for both operations.
There are some metals, however, whose solution is more easily accomplished, by diluting the acid in water; and the metal will, in this case, be oxydated, not by the acid, but by the water, which it will decompose. But in proportion as the oxygen of the water oxydates the surface of the metal, the acid combines with it, washes it off, and leaves a fresh surface for the oxygen to act upon: then other coats of oxyd are successively formed, and rapidly dissolved by the acid, which continues combining with the new-formed surfaces of oxyd till the whole of the metal is dissolved. During this process the hydrogen gas of the water is disengaged, and flies off with effervescence.
EMILY.
Was not this the manner in which the sulphuric acid assisted the iron filings in decomposing water?
MRS. B.