Question

Classify each of the following as a pure substance, a solution, or a heterogeneous mixture: (a) a gold ingot, (b) a cup of coffee, (c) a wood plank.

Short answer

(a) A gold ingot is a pure substance, as it is composed of only one type of particle: gold. (b) A cup of coffee is a solution, as it is a homogeneous mixture of substances like water, coffee solutes, and sometimes sugar or creamer. (c) A wood plank is a heterogeneous mixture, as it is made of fibers from a tree composed of different substances such as cellulose, hemicelluloses, lignin, and other extracellular materials with non-uniform composition.

Step-by-step solution

Understand the Definitions

A pure substance is a material composed of only one type of particle, and its properties are consistent throughout its entire composition. Examples are elements like gold or compounds like water. A solution is a homogeneous mixture of two or more substances, where one substance has dissolved into another, and its composition is uniform throughout. Examples are saltwater or air. A heterogeneous mixture is a combination of substances that are not evenly distributed throughout its composition, and they do not dissolve into one another. Examples include salad, sand, or orange juice with pulp.

Classify the Gold Ingot

A gold ingot is a solid block of gold metal. Gold is a chemical element, so it's composed of only one type of particle. Since the entire gold ingot is composed of gold, its properties are consistent, and there's no other substance mixed into it. Conclusion: The gold ingot is a pure substance.

Classify the Cup of Coffee

A cup of coffee is a liquid in which various components like water, coffee solutes, and sometimes sugar or creamer are mixed together. The mixture is uniform throughout its composition because particles of the substances dissolved in the water are evenly distributed. Conclusion: A cup of coffee is a solution.

Classify the Wood Plank

A wood plank is made up of fibers from a tree. These fibers consist of different substances such as cellulose, hemicelluloses, lignin, and other extracellular materials. The composition of a wood plank is not uniform, as it contains different substances in various arrangements. Conclusion: A wood plank is a heterogeneous mixture.

Key concepts

Pure Substance

Understanding the characteristics of a pure substance is foundational in chemistry. A pure substance, in its essence, is a material that consists of a single type of particle. This could mean a single element, like platinum, or a compound, such as carbon dioxide. These substances have uniform properties throughout because they are made of the same building blocks. In the classroom example, a gold ingot is identified as a pure substance as it is formed from just gold atoms, showing no variation in composition.
When discussing a pure substance, think of it being so because there are no additional substances mixed in that would change its consistent composition. Furthermore, pure substances have specific boiling and melting points, which help distinguish them in the laboratory. This quality is particularly useful when trying to separate or identify substances during chemical reactions.

Solution

A solution is a key concept that we come across in everyday life without always realizing it. It is a type of homogeneous mixture where one or more substances, called solutes, are dissolved completely in another substance, known as the solvent. The resulting mixture appears uniform throughout, meaning you cannot distinguish between its components with the naked eye. In the exercise, a cup of coffee serves as a practical example of a solution. The mixture of water, coffee, and possibly sugar or milk creates a uniform blend.
Some solutions we encounter every day include the air we breathe – a solution of gases – and beverages like sodas, which are solutions of carbon dioxide in water. Solutions are integral to understanding how substances interact in both nature and industry, making them a vital topic in chemistry.

Heterogeneous Mixture

Diving into the concept of heterogeneous mixtures, we examine mixtures where the components remain physically distinct. This type of mixture has different parts that are not uniform and can usually be seen as separate substances. One example provided in the exercise is a wood plank, which is a composition of various substances such as cellulose, lignin, and others, that don't have a uniform distribution. If we look closely at a wood plank, we can see various grains and textures, indicating its heterogeneous nature.
The heterogeneous mixtures are common in daily life; examples include a bowl of cereal in milk or a box of assorted nuts. They are characterized by the fact that the different parts can often be mechanically separated from one another. This uneven composition can affect properties like boiling and melting points, which will vary within the mixture.

Chemistry Classification

Chemistry classification is the process by which substances are categorized based on their physical and chemical properties. Using the examples provided in the exercise, we can better appreciate the importance of this classification. Through understanding the nature of a substance—be it a pure substance, a solution, or a heterogeneous mixture—we gain insights into how it can be used, how it will react with other substances, and how it can be transformed through chemical processes.
Chemistry classification not only aids in scientific analysis and experimentation but also has practical applications in fields like pharmacology, environmental science, and materials engineering. By properly classifying substances, chemists can design experiments and processes that are safe, efficient, and environmentally responsible, ultimately contributing to technological advancements and knowledge expansion in the field.

Homogeneous Mixture

Homogeneous mixtures are all about uniformity. They consist of two or more substances that are so thoroughly mixed that the composition is the same throughout. This type of mixture is a single-phase system, meaning it has the same properties and appearance no matter where you sample it from within the mixture. Although the exercise does not provide an explicit example of this category, a classic example involves alloys, like steel, which is a uniform blend of iron with carbon and other metals.
In a homogeneous mixture, the particles of the different substances are intermingled at the molecular level and cannot be separated by simple physical means such as filtration or decanting. The creation of homogeneous mixtures is essential in various industries, from food production to pharmaceuticals, where consistency and uniformity are crucial.