Question

Classify each of the following as a pure substance or a mixture. If a mixture, indicate whether it is homogeneous or heterogeneous: (a) air, (b) tomato juice, (c) iodine crystals, (d) sand.

Short answer

(a) Air is a homogeneous mixture, (b) Tomato juice is a homogeneous mixture, (c) Iodine crystals are a pure substance, and (d) Sand is a heterogeneous mixture.

Step-by-step solution

Air

Air is a mixture of different gases, such as nitrogen, oxygen, and carbon dioxide, and their distribution is even. Therefore, air is considered a homogeneous mixture.

Tomato Juice

Tomato juice is a mixture of water, tomato solids, and other components like vitamins and minerals. These components are evenly distributed and cannot be distinguished visually. Thus, tomato juice is a homogeneous mixture.

Iodine Crystals

Iodine crystals are made up of only iodine molecules, and there are no other components or elements mixed in. As a result, iodine crystals are a pure substance.

Sand

Sand is made up of small particles of rocks and minerals, such as quartz, feldspar, and mica, and these particles are easily distinguishable. Therefore, sand is considered a heterogeneous mixture.

Key concepts

Homogeneous Mixtures

Homogeneous mixtures are fascinating because they appear to be uniform throughout. This means that if you take a sample from any part of the mixture, it will have the same composition as any other sample from the same mixture. They are often referred to as solutions.
Examples of homogeneous mixtures include air and tomato juice.
In the case of air, the gases such as nitrogen, oxygen, and carbon dioxide are perfectly mixed. This even distribution gives air its homogeneous quality.
Similarly, tomato juice is another great example of a homogeneous mixture. The water and solids are blended so seamlessly that you cannot see or pick apart the individual components.
When identifying homogeneous mixtures, it's crucial to look for mixtures where individual parts are not visible and the mixture has the same properties throughout.

Heterogeneous Mixtures

Unlike homogeneous mixtures, heterogeneous mixtures have visibly different parts. The components in a heterogeneous mixture are not uniformly mixed, making it possible to see the distinct elements with the naked eye.
For instance, let's consider sand. Sand is a mixture of tiny rock and mineral particles. When you look closely, you can see these different particles distinguished from one another. This is what makes sand a heterogeneous mixture.
Other examples of heterogeneous mixtures include salads, granite, and trail mix. In each of these, you can notice the different ingredients or components, which are not evenly distributed throughout.

Classification of Matter

Classifying matter is essential for understanding its diverse forms and behaviors. Matter is typically categorized into pure substances and mixtures.
Pure substances include elements and compounds, with a consistent composition. For example, iodine crystals are pure substances because they're composed solely of iodine molecules.
On the other hand, mixtures are combinations of two or more different substances, each retaining its own chemical properties. Mixtures can be further classified into homogeneous and heterogeneous mixtures.
Recognizing these classifications can help to predict how matter will interact under different conditions and is fundamental in chemistry and materials science.

• Pure substances: Consistent composition (e.g., iodine crystals)
• Mixtures: Combination of substances, can be homogeneous (like air) or heterogeneous (like sand)