Problem 74 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which of the following would you characterize as pure or nearly pure substance? (a) stomach acid; (b) dry ice; (c) ice-cream; (d) stainless steel; (e) petroleum; (f) distilled water; \((\mathbf{g})\) carbon monoxide gas; \((\mathbf{h})\) compressed air in balloon.

Step-by-Step Solution

Answer

Among the given substances, (b) dry ice, (f) distilled water, and (g) carbon monoxide gas can be characterized as pure or nearly pure substances. Dry ice is the solid form of carbon dioxide (CO2), distilled water is composed of mostly water molecules (H2O), and carbon monoxide gas consists of CO molecules. These substances have a uniform composition and are composed of only one type of particle or molecule, making them pure or nearly pure.

Step 1: (a) Stomach Acid

Stomach acid is a mixture of hydrochloric acid (HCl), potassium chloride (KCl), and sodium chloride (NaCl). It also contains various enzymes and secretions from the stomach lining. Since stomach acid has multiple components, it is not a pure or nearly pure substance.

Step 2: (b) Dry Ice

Dry ice is the solid form of carbon dioxide (CO2). It is composed of only CO2 molecules and has a uniform composition, making it a pure or nearly pure substance.

Step 3: (c) Ice-Cream

Ice-cream is a mixture of milk, sugar, and flavorings such as fruits, chocolate, or vanilla. It has multiple components and a non-uniform composition, so it is not a pure or nearly pure substance.

Step 4: (d) Stainless Steel

Stainless steel is an alloy made of iron, chromium, and other elements such as carbon, nickel, and molybdenum. Since it is a mixture of multiple elements, it is not a pure or nearly pure substance.

Step 5: (e) Petroleum

Petroleum is a mixture of various hydrocarbon compounds, such as alkanes, cycloalkanes, and aromatic compounds. It also contains other elements like nitrogen, sulfur, and oxygen. Because of its complex composition, petroleum is not a pure or nearly pure substance.

Step 6: (f) Distilled Water

Distilled water is a nearly pure substance composed of water molecules (H2O). The distillation process removes most impurities and dissolved solids, resulting in a uniform composition throughout. However, it is tough to obtain 100% pure water, but it is still considered nearly pure.

Step 7: (g) Carbon Monoxide Gas

Carbon monoxide gas is formed from carbon (C) and oxygen (O) atoms, and each molecule has the formula CO. It has a uniform composition and is considered a pure or nearly pure substance.

Step 8: (h) Compressed Air in a Balloon

Compressed air in a balloon is a mixture of gases, primarily composed of nitrogen (N2), oxygen (O2), argon (Ar), and carbon dioxide (CO2). Since it has multiple components and no uniform composition, it is not a pure or nearly pure substance.

Key Concepts

Mixtures

When talking about mixtures, we refer to a combination of two or more substances where each retains its individual chemical properties. Mixtures come in two main forms: homogeneous and heterogeneous. A homogeneous mixture has a uniform composition throughout, like saltwater, whereas a heterogeneous mixture does not, like a salad. In mixtures, components can be separated by physical means, such as filtration or evaporation. Examples include:

• Stomach acid, containing multiple acids and enzymes.
• Ice-cream, made from milk, sugar, and flavors.
• Stainless steel, an alloy of different metals.
• Compressed air, which consists of various gases.

Understanding mixtures can help us manipulate their properties for different applications, like combining metals to make steel stronger or using mixed gases for industrial purposes.

Composition

Composition refers to the nature and arrangement of parts or elements in a substance. For chemical analysis, knowing the composition helps determine purity and suitability for particular uses. In substances, the composition affects characteristics like boiling point, melting point, and reactivity. Take, for instance:

• Dry ice, composed solely of carbon dioxide (CO2), and thus has a uniform composition.
• Petroleum, which has a vast composition of hydrocarbons, affecting its refinement process.
• Distilled water, preferred in laboratories for its reduced impurities, signifying near-uniform composition.

Essentially, composition tells us what a substance is made of, impacting everything from its physical attributes to how it interacts with other substances.

Chemical Compounds

Chemical compounds form when two or more elements chemically bond together in fixed proportions. The result is a substance with properties distinct from its component elements. Compounds have uniform compositions and can occur naturally or be synthesized. In our examples:

• Carbon monoxide ( CO ), a compound formed by one carbon and one oxygen atom.
• Dry ice ( CO_2 ), the compound form of carbon dioxide.

Such compounds have consistent chemical formulas and specific characteristics, which define their applications in industries and research. A compound's study helps understand their reactivity and potential uses across various fields.

Substance Classification

Classifying substances into pure and mixtures helps in the study and application of materials. Pure substances might be elements or compounds with a definite and uniform composition. On the other hand, mixtures consist of various substances that are not chemically combined.

• Elements like oxygen in compressed air are pure when isolated.
• Compounds like distilled water and carbon monoxide qualify as pure substances due to their consistent chemical composition.
• Mixtures such as petroleum show the variability in composition, marking them as complex substances.

Substance classification allows chemists and industries to decide how to utilize a material, whether in chemical reactions, manufacturing, or daily use.