The metallic copper produced separates in the
form of a red coating on the iron scraps. But we may also, relying on
the fact that oxide of copper is insoluble in water, arrange for the
deposition of the copper in that form. This we can do by adding caustic
soda to a hot solution of copper sulphate, when we get the following
change: Copper sulphate, consisting of a combination of copper oxide
with sulphuric acid, yields with caustic soda, sulphate of soda, a
combination of soda with sulphuric acid and oxide of copper. Oxide of
copper is black, and so in this decomposition what is called a "black
precipitate" of that oxide is produced on adding the caustic soda. But
it might not suit us thus to deposit the copper from our solution; we
might desire to remove the sulphuric acid from the copper sulphate, and
leave the copper dissolved, say in the form of a chloride. We select,
then, a compound which is a chloride, and a chloride of a metal which
forms an insoluble combination with sulphuric acid--chloride of barium,
say.
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