Most of the metals, when heated to redness in contact with air, lose
their metallic lustre and become coated with, or (if the heating be
prolonged) altogether converted into, oxide. This oxide was formerly
termed a "calx," and has long been known to weigh more than the metal
from which it was obtained. For example, one part by weight of tin
becomes, on calcining, 1.271 parts of oxide (putty powder). The student
will do well to try the following experiments:--Take 20 grams of tin and
heat them in a muffle on a scorifier, scraping back the dross as it
forms, and continuing the operation until the whole of the metal is
burnt to a white powder and ceases to increase in weight.[95] Take care
to avoid loss, and, when cold, weigh the oxide formed. The oxide should
weigh 25.42 grams, which increase in weight is due to the oxygen
absorbed from the air and combined with the metal. It can be calculated
from this experiment (if there has been no loss) that oxide of tin
contains 21.33 per cent. of oxygen and 78.67 per cent. of tin. Oxidation
is performed with greater convenience by wet methods, using reagents,
such as nitric acid, which contain a large proportion of oxygen loosely
held. Such reagents are termed oxidising agents. Besides nitric acid,
permanganate of potash, bichromate of potash, and peroxide of hydrogen
are largely used for this purpose.
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