Problem 59 - Chemistry: The Central Science

Question

(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?

Step-by-Step Solution

Answer

(a) A gold coin, assuming it's pure gold, can be classified as a pure substance. A cup of coffee is a solution, as it contains multiple substances mixed uniformly. A wood plank is a heterogeneous mixture, given its non-uniform composition of various components. (b) The ambiguities in answering part (a) result from the lack of precise information about the composition of the items, such as the purity of the gold coin, the specific ingredients in the cup of coffee, or the type and quality of the wood plank.

Step 1: Understanding the definitions

A pure substance is a type of matter that consists of a single type of element or compound with a chemically constant composition. Examples include water, salt, gold, and oxygen. A solution is a homogeneous mixture, in which one or more substances are evenly distributed in a solvent. The substances don't chemically combine and can be separated by physical processes, such as evaporation or filtration. Examples include saltwater, air, and brass. A heterogeneous mixture is a type of matter that consists of visibly different substances or phases, with non-uniform composition. Examples include oil and water mixtures, sand, and granite.

Step 2: Classifying the items

(a) - A gold coin: Assuming it's made entirely of gold, it can be classified as a pure substance as its composition is chemically constant (gold). - A cup of coffee: A cup of coffee is a solution, as it contains multiple substances like water, coffee solutes, and possibly milk and sugar, all mixed uniformly. - A wood plank: A wood plank is an example of a heterogeneous mixture, as it is composed of various components (cellulose, lignin, and hemicellulose) along with air spaces, resulting in a non-uniform composition.

Step 3: Identifying ambiguities in the given descriptions

(b) The ambiguities in answering part (a) result from the lack of precise information about the composition of the items. - For the gold coin, there is no information about its purity or whether any other metals are present. Gold coins could be made of pure gold or an alloy (e.g., with copper), so it would be either a pure substance or a solution. - For the cup of coffee, the description does not mention the type of coffee or ingredients, such as sugar or milk. However, even if no sugar or milk is added, the cup of coffee would still be classified as a solution due to the solutes from coffee beans evenly distributed in water. - For the wood plank, the description doesn't provide information about the type or quality of the wood. Although it can be classified as a heterogeneous mixture, considering all of the components in it, the degree of heterogeneity may vary depending on the type and processing method of the wood.