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?

Short answer

(a) A gold coin is a solution, a cup of coffee is a solution, and a wood plank is a heterogeneous mixture. (b) Ambiguities may arise in the classification of a gold coin if it is pure gold, in the classification of a cup of coffee if additional ingredients are considered, and in the classification of a wood plank if considering the complex structure of wood as a macroscopic solution of plant fibers and chemicals. More information or assumptions may be necessary for accurate classification.

Step-by-step solution

Classify a Gold Coin

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.

Classify a Cup of Coffee

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.

Classify a Wood Plank

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.

Discuss Ambiguities

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.

Key concepts

Pure Substances

In the realm of chemistry, a pure substance is a material with a consistent and uniform composition throughout. It is composed of only one type of particle, which can be either atoms or molecules. Pure substances possess distinct properties that do not change, no matter the amount of the sample. They exist as either elements or compounds.
- **Elements** are the simplest chemical substances. They contain only one type of atom, which cannot be split into a smaller substance. Examples include gold (Au), oxygen (O_2), and nitrogen (N_2).
- **Compounds** are substances formed by the chemical combination of two or more elements in fixed proportions. Water (H_2O) and carbon dioxide (CO_2) are examples.
A gold coin made entirely of 24-karat gold would be a pure substance, as it consists solely of one element: gold. This can lead to potential ambiguities when classifying items, as a gold coin could either represent a pure substance or an alloy, depending on its composition.

Solutions

Solutions are homogeneous mixtures where one substance is dissolved in another. They are uniform in nature and appear as a single phase. The key components are the solute, which gets dissolved, and the solvent, which does the dissolving.
- **Characteristics of Solutions:** * Solutions are uniform throughout. * Particles in a solution are typically at the molecular or ionic level, making them not visible to the naked eye.
A common example of a solution is a cup of coffee, where coffee grounds are dissolved in water to create a homogeneous beverage. Even though additional components like sugar or cream can be added, the initial combination of coffee and water forms a solution.
Just like the gold coin example, factors such as these can sometimes lead to complexities, as added ingredients may affect the uniformity and, subsequently, the classification.

Heterogeneous Mixtures

Heterogeneous mixtures are composed of visibly different substances or phases. Unlike solutions, these mixtures are non-uniform, meaning that their composition can vary from one part of the mixture to another. This non-uniformity is usually at a macroscopic level.
- **Characteristics of Heterogeneous Mixtures:** * The different components are distinguishable by sight or with simple magnification. * They can often be physically separated into individual substances.
A typical example of a heterogeneous mixture is a wood plank, given its mixture of plant fibers and varied chemical make-up. The innate complexity and varying structure within wood make it distinct and underscore its classification as heterogeneous.
As with other classifications in chemistry, varying opinions may arise due to differing interpretations of the microscopic structures involved, yet the general consensus often regards wood as a heterogeneous mixture.