On adding this chloride of barium to sulphate of copper solution,
we get then a change which we might represent thus: Copper sulphate,
consisting of a combination of copper oxide with sulphuric acid, yields
with barium chloride, which is a combination of barium and chlorine,
insoluble barium sulphate, a combination of barium oxide with sulphuric
acid, and soluble copper chloride, a combination of copper and chlorine.
This is called a double interchange. Now these are a few illustrations
to show you what is meant by chemical decompositions. One practical
lesson, of course, we may draw is this: We must have a care in
dissolving bluestone or copper sulphate, not to attempt it in iron pans,
and not to store or put verdigris into iron vessels, or the iron will
be acted upon, and to some extent the copper salt will become
contaminated with iron. It will now be clear to you that, as a solvent
for bodies usually soluble in water, water that is perfectly pure will
be most suitable and not likely to cause any deposition or precipitation
through chemical decompositions, for there are no salts or other
compounds in pure water to cause such changes.
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