Problem 74 - Chemistry The Central Science In Si Units

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

Pure or nearly pure substances are dry ice, distilled water, and carbon monoxide.

Step 1: Definition of Pure Substance

A pure substance is made up of only one type of particle and has a constant composition, which can be elements or compounds. A nearly pure substance can have very minimal impurities but still maintains a mostly uniform composition.

Step 2: Analyze Stomach Acid

Stomach acid is primarily composed of hydrochloric acid (HCl) mixed with various other substances like water and enzymes. This makes it a mixture rather than a pure substance.

Step 3: Examine Dry Ice

Dry ice is solid carbon dioxide (9 CO_2 9), a compound with uniform composition throughout. It fits the criteria for a pure substance.

Step 4: Evaluate Ice-Cream

Ice-cream is a mixture of various ingredients such as milk, sugar, and flavorings. Thus, it is not a pure substance.

Step 5: Assess Stainless Steel

Stainless steel is an alloy, meaning it is a mixture of iron with carbon and other elements, so it is not a pure substance.

Step 6: Study Petroleum

Petroleum is a complex mixture of hydrocarbons and other chemical compounds, and it cannot be considered a pure substance.

Step 7: Consider Distilled Water

Distilled water is water that has been purified to remove impurities and ions, making it a nearly pure substance consisting almost entirely of H₂O molecules.

Step 8: Investigate Carbon Monoxide Gas

Carbon monoxide (9 CO 9) is a compound with a fixed composition, making it a pure substance.

Step 9: Analyze Compressed Air in Balloon

Compressed air is a mixture of gases like nitrogen, oxygen, and other trace gases, which means it is not a pure substance.

Key Concepts

Mixture

A mixture consists of two or more different substances that are physically combined. Unlike a pure substance, a mixture does not have a fixed composition. Each component in a mixture retains its own properties and can often be separated by physical methods like filtration or evaporation. This makes mixtures quite versatile.
Here are some things to remember about mixtures:

• Components can be in any state of matter.
• The ratio between the components can vary.
• Each substance in the mixture retains its chemical identity.

Consider accelerating your understanding by examining everyday examples like air, salad, or ice-cream. In the case of ice-cream, even the delicious swirls and chunks of flavor showcase the diversity of a mixture.

Compound

A compound is a substance that results when two or more different elements chemically bond together in a specific ratio. The properties of a compound are distinct from the individual elements that compose it. For instance, water (H₂O) is a compound formed by the chemical bonding of hydrogen and oxygen atoms.
Important points about compounds include:

• Compounds have a definite and consistent chemical formula.
• They can only be separated into their elements by chemical methods.
• The formation of a compound involves a chemical change.

One example of a compound is carbon dioxide (CO₂), found in dry ice. It showcases the uniformity of compounds, as every molecule is identical throughout.

Element

Elements are the simplest substances in chemistry and serve as the building blocks for all other forms of matter. Each element is composed of only one type of atom and cannot be broken down into a simpler substance. The periodic table organizes and displays all known elements, highlighting their properties and atomic structure.
Key aspects about elements:

• Elements are represented by a unique chemical symbol (e.g., C for carbon).
• They participate as reactants or products in chemical reactions.
• Elements can combine to form both mixtures and compounds.

Discover the beauty of elements by exploring substances like carbon found in pencil leads or oxygen that we breathe.

Homogeneous

A homogeneous mixture is one where the composition is uniform throughout the entire mixture, providing a consistent appearance and properties throughout. Unlike heterogeneous mixtures, you cannot easily see individual components in homogeneous mixtures.
Here's what makes homogeneous mixtures special:

• The components are evenly distributed.
• Solutions, alloys, and gases can be homogeneous.
• They appear as a single phase throughout.

Examples include solutions like saltwater, where salt is completely dissolved in water, or a solid solution like stainless steel, which is an alloy where the different metals are evenly mixed at a molecular level.

Chemical composition

Chemical composition refers to the specific arrangement and ratio of elements in a compound or mixture. Understanding chemical composition is crucial because it defines the substance's properties, determining how it interacts with other substances and behaves under different conditions.
Some essential points about chemical composition:

• In compounds, it's represented by a chemical formula (e.g., H₂O for water).
• It influences the physical and chemical properties of a substance.
• Changes in chemical composition result in new substances.

For example, the chemical composition of carbon monoxide (CO) dictates its dangerous properties, showcasing the importance of knowing what makes up each substance we encounter.