Problem 65 - Chemistry: The Central Science in SI Units, Global Edition

Question

Classify each of the following as a pure substance, a solution, or a heterogeneous mixture: \((\mathbf{a})\) a leaf, \((\mathbf{b})\) a 999 gold bar, (c) stainless steel.

Step-by-Step Solution

Answer

a) A leaf can be classified as a \(\underline{\text{heterogeneous mixture}}\) since its components are not uniformly distributed. b) A 999 gold bar can be classified as a \(\underline{\text{solution}}\) as it contains 99.9% gold and only 0.1% impurities mixed at the molecular level. c) Stainless steel is classified as a \(\underline{\text{solution}}\) since it is an alloy with various elements mixed uniformly at the atomic level.

Step 1: Classification of a Leaf

A leaf is a part of a plant and consists of various cells, including chlorophyll and other components. Since the different components of a leaf are not uniformly distributed, we can classify a leaf as a \(\underline{\text{heterogeneous mixture}}\).

Step 2: Classification of a 999 Gold Bar

A 999 gold bar is a gold bar that is made up of 99.9% gold, meaning it only contains 0.1% of impurities. Although it is not completely pure, this high level of gold concentration can be considered as a solution. Therefore, we classify a 999 gold bar as a \(\underline{\text{solution}}\).

Step 3: Classification of Stainless Steel

Stainless steel is an alloy composed mostly of iron along with varying amounts of carbon, chromium, and other elements. These elements are mixed at the atomic level and distributed uniformly throughout the material. Therefore, stainless steel is classified as a \(\underline{\text{solution}}\).

Key Concepts

Pure Substance

In chemistry, a pure substance is basically a material that consists of only one type of particle. These particles can be atoms or molecules that are all identical to each other.
A pure substance has a constant composition, meaning that no matter where you sample it, you'll find the same makeup and characteristics. Some common examples include:

• Elements, like oxygen
• Compounds, like water

When we focus on the exercise's example of a gold bar (999 gold), it is almost entirely composed of gold atoms with only a tiny amount of impurities. This high purity makes it *nearly* a pure substance.
However, technically, because of the tiny amount of other elements present, it doesn’t completely fit the strict definition, which requires complete uniformity.

Solution

A solution is a homogeneous mixture, which means the composition is consistent throughout. In a solution, solute particles are distributed evenly within a solvent, forming a single-phase mixture.
This makes solutions visually uniform even under a microscope. Key characteristics of solutions include:

• Uniform distribution of particles
• Usually involve a solute and a solvent
• Can be gases, liquids, or solids

Examining the 999 gold bar more closely, while it is highly pure, the small presence of other atoms within the gold make it similar to a solution due to its uniform atomic distribution.
Stainless steel is another example of a solid solution, where atoms like iron and chromium blend at the atomic level, creating an even distribution throughout the alloy.
This gives stainless steel its strength and corrosion resistance, illustrating the advantages of solutions in material science.

Heterogeneous Mixture

A heterogeneous mixture contains components that are not uniformly distributed. You can usually see the different substances or phases when looking at it, as they are unevenly mixed.
This is in contrast to solutions, which are homogeneous. Heterogeneous mixtures can include:

• Oil and water
• Concrete
• A salad

In the case of a leaf, it is a good example of a heterogeneous mixture. A leaf contains various cells and particles, such as chlorophyll, water, and other small structures that are not evenly mixed.
Due to this non-uniform composition, a leaf exhibits distinct areas with different substances and hence, demonstrates the classic traits of a heterogeneous mixture.