(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?

First, we need to classify a gold coin. A gold coin is typically composed of gold and other metals, such as copper or silver. As a metal alloy, it is homogeneous in composition and therefore, a gold coin can be classified as a solution.

Second, let's classify a cup of coffee. Coffee is prepared by dissolving coffee grounds in hot water. As the coffee grounds are homogeneously dissolved, we can consider a cup of coffee as a solution.

Lastly, we need to classify a wood plank. Wood is derived from plant cells, which have varying structures and chemical compositions. This makes wood a non-uniform material. As a result, a wood plank is classified as a heterogeneous mixture.

In this exercise, some ambiguities may exist when classifying substances based on the given descriptions: 1. A gold coin can also be pure gold (also known as a 24-karat gold coin), which would classify it as a pure substance. Additionally, the proportion of gold to other metals in a gold coin may be unknown. 2. A cup of coffee can be considered a homogeneous mixture if we look at the liquid portion only, without any additives (sugar, cream). However, considering additional ingredients could complicate its classification. 3. A wood plank is generally classified as heterogeneous, but some might argue that it could be classified as a solution if considering the complex structure of wood as a macroscopic solution of plant fibers and chemicals. As a result, the classifications might change depending on the specific details and definitions. Sometimes it is necessary to have more information or make assumptions to accurately classify a substance.