Problem 65 - Chemistry The Central Science In Si Units

Question

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

Step-by-Step Solution

Answer

(a) Leaf: heterogeneous mixture, (b) 999 gold bar: pure substance, (c) Stainless steel: solution.

Step 1: Define the Terms

First, we need to understand the definitions of a pure substance, a solution, and a heterogeneous mixture. A pure substance is composed of only one type of element or compound. A solution is a homogeneous mixture of two or more substances, where the composition is uniform throughout. A heterogeneous mixture consists of visibly different substances or phases where the components are not uniform.

Step 2: Classify the Leaf

Analyze the composition of a leaf. A leaf contains various compounds like cellulose, chlorophyll, water, and other organic material, which are not uniformly distributed. Therefore, it is a heterogeneous mixture.

Step 3: Classify a 999 Gold Bar

Examine the composition of a 999 gold bar, which denotes 99.9% purity of gold with only trace impurities. This makes it a single element with high purity, classifying it as a pure substance.

Step 4: Classify Stainless Steel

Look at the makeup of stainless steel, which is an alloy mainly composed of iron, chromium, and other elements mixed uniformly to create a homogenous mixture. Thus, stainless steel is considered a solution.

Key Concepts

Pure Substance

A pure substance is a material that has a consistent and uniform composition throughout. This means that the entire sample of a pure substance is made up of only one type of element or compound. There are no other substances mixed with it.

• Elements like gold (Au), oxygen (O₂), and nitrogen (N₂) are pure substances because they contain only one type of atom or molecule.
• Compounds such as water (H₂O) or salt (NaCl) are also pure substances since they are composed of molecules that are chemically bonded together in a fixed ratio.

One of the simplest ways to identify a pure substance is to consider whether the composition is consistent throughout and whether the substance can be separated into other components through chemical means rather than physical separation. For example, a 999 gold bar is mostly composed of gold elements with very few impurities, making it a pure substance.

Solution

In chemistry, a solution is a type of homogeneous mixture. This means that the solution is the same throughout, and the dissolved components cannot be seen individually. Solutions are typically composed of a solute (the substance being dissolved) and a solvent (the substance dissolving the solute).

• An example of a solution is saltwater, where salt (solute) is dissolved in water (solvent).
• Air can also be considered a solution since it consists of a mix of gases including nitrogen and oxygen.

Solutions are important in chemistry because they allow for uniformity in reactions. Since the components are evenly distributed, the properties of the solution are consistent throughout the sample.
In the classification exercise, stainless steel is considered a solution. It is made by fusing metals like iron and chromium uniformly, resulting in consistent properties throughout the material.

Heterogeneous Mixture

A heterogeneous mixture is one where the different components are not uniformly distributed throughout the mixture. In other words, each part of the mixture may contain different amounts or types of substances. You can often see the individual components, or they can be separated physically.

• Examples include salad, rocky soil, and a leaf.
• In a salad, for instance, you can see each different ingredient, such as lettuce, tomatoes, and cucumbers, and they are not chemically bonded together.

Heterogeneous mixtures are characterized by distinct phases and can be non-uniform both in appearance and in composition. This characteristic becomes apparent in the example of a leaf, which consists of various materials like water, cellulose, and minerals, all distributed unevenly. Unlike solutions or pure substances, the different components in a heterogeneous mixture maintain their distinct properties.