Problem 13 - Chemistry: The Central Science in SI Units, Global Edition

Question

Classify each of the following as a pure substance or a mixture. If a mixture, indicate whether it is homogeneous of heterogeneous: (a) air, \((\mathbf{b})\) chocolate with almond, \((\mathbf{c})\) alumin- (d) iodine tincture. ium,

Step-by-Step Solution

Answer

- Air is a homogeneous mixture. - Chocolate with almond is a heterogeneous mixture. - Aluminum is a pure substance. - Iodine tincture is a homogeneous mixture.

Step 1: (a) Classifying air as a pure substance or a mixture

Air consists of different gases such as nitrogen, oxygen, carbon dioxide, and others present in variable proportions. Because it contains multiple components, air is a mixture. To determine if it is a homogeneous or heterogeneous mixture, we must consider how uniformly the gases are distributed. Since the gases in the air mix uniformly, air is a homogeneous mixture.

Step 2: (b) Classifying chocolate with almond as a pure substance or a mixture

Chocolate with almonds consists of at least two different substances: chocolate and almonds. Thus, it is a mixture. To determine if it is a homogeneous or heterogeneous mixture, we must analyze the distribution of the components. In the case of chocolate with almonds, the distribution of almonds in the chocolate is not uniform; we can see regions with just chocolate and regions with almonds. Thus, chocolate with almond is a heterogeneous mixture.

Step 3: (c) Classifying aluminum as a pure substance or a mixture

Aluminum is an element that appears on the periodic table. It is composed of only aluminum atoms, and there are no other substances mixed with it. Therefore, aluminum is a pure substance.

Step 4: (d) Classifying iodine tincture as a pure substance or a mixture

Iodine tincture is a solution that consists of iodine dissolved in alcohol or another solvent. Because it has both iodine and a solvent such as alcohol, it is a mixture. Seeing as the iodine is uniformly dissolved in the solvent, it is a homogeneous mixture. In summary: - Air is a homogeneous mixture. - Chocolate with almond is a heterogeneous mixture. - Aluminum is a pure substance. - Iodine tincture is a homogeneous mixture.

Key Concepts

Pure Substances

Pure substances are materials composed of one type of particle. They have consistent properties throughout. Examples include elements and compounds.

Elements consist of a single type of atom, like aluminum or gold. Compounds are made from two or more elements that are chemically bonded, such as water or carbon dioxide.

These substances have fixed melting and boiling points. For instance, pure aluminum is just aluminum atoms with no impurities.

• Elements: single type of atom, found on the periodic table.
• Compounds: chemically bonded elements, represented by chemical formulas.

Understanding pure substances helps identify materials with uniform characteristics.

Mixtures

Mixtures involve two or more substances combined physically, not chemically. This means the components retain their original properties and can be separated by physical processes.

Mixtures don’t have a fixed composition. They can differ in proportions and can be either homogeneous or heterogeneous. Some common examples include air, chocolate with almonds, and iodine tincture.

• Physically combined: can be separated by processes like filtration or distillation.
• Variable composition: differing amounts of each component.

Knowing how to classify mixtures is crucial to understanding material composition.

Homogeneous Mixtures

Homogeneous mixtures are uniform in composition. This means the different components are evenly distributed throughout the mixture.

These mixtures are often referred to as solutions. For instance, air is a homogeneous mixture since the gases are mixed uniformly. Another example is iodine tincture, where iodine is evenly dissolved in alcohol.

• Uniform composition: can't see different components.
• Examples include vinegar, saltwater, and alloys.

Recognizing homogeneous mixtures helps in distinguishing them from heterogeneous ones.

Heterogeneous Mixtures

Heterogeneous mixtures have visibly different components or phases. These mixtures do not have uniform distribution, leading to distinct regions of different substances.

Chocolate with almonds is a classic example. You can easily see and separate the chocolate from the almonds due to the uneven distribution.

• Non-uniform composition: can see different parts.
• Examples include salad, cereal with milk, and granite.

Understanding heterogeneous mixtures is key to identifying materials that aren't uniformly mixed.