(a) Classify each of the following as a pure substance, a solution, or a heterogeneous mixture: a gold coin, a cup
of coffee, a wood plank. (b) What ambiguities are there in answering part (a) from the descriptions given?
a. A gold coin is probably a solid solution. Pure gold (element 79) is too soft and too valuable to be used for
coinage so other metals are usually added; A cup of coffee is a solution (if there no suspended solids like
coffee grounds) or an heterogenous mixture if there are grounds. If cream or sugar is added, the
homogeneity of the mixture depends on how thoroughly the components are mixed; A wood plank is a
heterogenous mixture of various cellulose components. The different domains of in the mixture are visible
as wood grain or knots.
b. The ambiguity in each of these examples is that the name of the substance does not provide a complete
description of the material. We rely on mental images, and these vary from person to person.
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