Question

Classify each of the following as a pure substance or a mixture: a. a soft drink b. propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) c. a cheese sandwich d. an iron (Fe) nail e. salt substitute \((\mathrm{KCl})\)

Short answer

a. Mixture, b. Pure substance, c. Mixture, d. Pure substance, e. Pure substance

Step-by-step solution

- Classify a soft drink

A soft drink is a combination of water, sugar, flavorings, and carbon dioxide. It contains multiple components that can be separated physically. Therefore, a soft drink is a mixture.

- Classify propane

Propane \(\text{C}_3\text{H}_8\) is a chemical compound consisting of carbon and hydrogen atoms. It is homogeneous and has a fixed composition. Hence, propane is a pure substance.

- Classify a cheese sandwich

A cheese sandwich consists of bread and cheese, which are distinct components that can exist separately. It is not chemically combined. Thus, a cheese sandwich is a mixture.

- Classify an iron nail

An iron (Fe) nail is composed of only iron atoms. It is homogeneous and has a fixed composition. Consequently, an iron nail is a pure substance.

- Classify salt substitute

Salt substitute, often potassium chloride (KCl), contains potassium and chlorine in a fixed ratio. It is a single compound with a homogeneous composition. Hence, salt substitute (KCl) is a pure substance.

Key concepts

pure substance

A pure substance is a type of matter that has a consistent composition and distinct properties. It can be a single element or a compound.
For example, propane \(\text{C}_3\text{H}_8\) and an iron nail (Fe).
Pure substances have specific melting and boiling points, making them easy to identify.

• Elements: These are substances that cannot be broken down into simpler substances by chemical means. For example, iron (Fe).
• Compounds: These are substances composed of two or more elements chemically combined in a fixed ratio. For example, potassium chloride (KCl).

In the exercise, propane and the salt substitute, \(\text{KCl}\), are both pure substances.

mixture

A mixture is a combination of two or more substances where each retains its individual chemical properties. Mixtures can be separated by physical means.
A soft drink and cheese sandwich are typical examples of mixtures.

• Heterogeneous Mixtures: These mixtures have visibly different parts, such as a cheese sandwich. You can see and separate the bread from the cheese.
• Homogeneous Mixtures: These mixtures appear the same throughout, like a soft drink. Though made up of different components, you can't see them separately.

In the exercise, the soft drink and cheese sandwich are classified as mixtures.

homogeneous

Homogeneous matter has a uniform composition throughout. It means the material is consistent no matter where you sample it. Pure substances and homogeneous mixtures fall under this category.
Examples of homogeneous substances from the exercise include propane and salt substitute (KCl), both appearing the same throughout.

• Homogeneous Mixtures: These mixtures have the same uniform appearance and composition. For a soft drink, you can't distinguish the water, sugar, or CO₂ distinctly.
• Pure Substances: Both elements and compounds can be homogeneous. An iron nail (Fe) and propane are homogeneous because their composition is consistent throughout.

This consistency in composition is key in distinguishing homogeneous materials.

chemical compound

A chemical compound is a substance composed of two or more different elements chemically bonded together in fixed proportions. Compounds have distinct physical and chemical properties.
For instance, propane \(\text{C}_3\text{H}_8\) and potassium chloride (KCl) are chemical compounds.

• Formed by Chemical Bonds: Compounds are formed through chemical reactions, creating bonds between different elements, resulting in a fixed ratio of atoms.
• Uniform Composition: They have a consistent and specific formula. For example, in propane, three carbon atoms (C) bond with eight hydrogen atoms (H).
• Distinct Properties: Compounds exhibit properties different from the elements that form them. For example, KCl is a solid at room temperature, unlike potassium (K) and chlorine (Cl2).

Both propane and KCl mentioned in the exercise are chemical compounds, highlighting their fixed composition and homogeneous nature.