Problem 32 - Chemistry Matter and Change

Question

Is carbon dioxide gas a pure substance? Explain.

Step-by-Step Solution

Answer

Yes, carbon dioxide gas is a pure substance because it has a uniform composition and consistent properties throughout.

Step 1: Define a Pure Substance

A pure substance is a material that has a constant composition and consistent properties throughout the sample. It consists of only one type of particle or molecule.

Step 2: Identify the Components of Carbon Dioxide

Carbon dioxide (CO₂) is composed of molecules, each containing one carbon atom and two oxygen atoms. This molecular composition is the same throughout any given sample of carbon dioxide.

Step 3: Analyze the Composition Consistency

Since carbon dioxide (CO₂) is made up of the same type of molecules throughout, its composition does not change regardless of how much of the gas you have. This uniformity is a characteristic of a pure substance.

Step 4: Verify the Property Consistency

The properties of carbon dioxide, such as its boiling and melting points, are consistent in every sample of the gas, further confirming it as a pure substance.

Key Concepts

Carbon Dioxide

Carbon dioxide, commonly abbreviated as CO₂, is a well-known gas that plays a crucial role in several natural and industrial processes. It is composed of two oxygen atoms and one carbon atom, forming a linear molecule. As a greenhouse gas, carbon dioxide is essential in trapping heat in the Earth's atmosphere, contributing to the planet's climate regulation. It is colorless, odorless, and, under normal conditions, exists as a gas. Carbon dioxide is produced naturally through processes such as respiration and volcanic eruptions. It is also a byproduct of burning fossil fuels and other industrial activities. In addition to its environmental impact, carbon dioxide is used in various applications such as carbonated beverages, industrial refrigerants, and fire extinguishers.

Molecular Composition

Molecular composition refers to the arrangement and types of atoms that constitute a molecule. In the case of carbon dioxide, it has a simple molecular structure where one carbon atom is double-bonded to two oxygen atoms, forming a linear configuration. The molecular formula of carbon dioxide is often represented as CO₂.

• The molecular weight of carbon dioxide is approximately 44.01 g/mol, calculated by adding the atomic masses of carbon and oxygen.
• Knowing the molecular composition is vital as it determines the chemical properties and reactions that the molecule can undergo.
  

In every sample of carbon dioxide, the molecular composition remains the same. This consistency ensures that its chemical behavior is predictable, emphasizing its classification as a pure substance.

Constant Properties

Constant properties of a pure substance are the characteristics that remain unchanged throughout any given sample. For carbon dioxide, these properties include:

• Boiling point: -78.5°C
• Melting point: -56.6°C
• Density at standard temperature and pressure (STP): approximately 1.977 g/L

These properties do not vary regardless of the amount of carbon dioxide you have. Whether you are dealing with a single molecule or a large volume of gas, the properties remain the same. This constancy is crucial for identifying a substance as pure, as it implies that the substance is homogenous throughout and does not contain any impurities or varying components. Hence, carbon dioxide, with its uniform properties, is a perfect example of a pure substance.

Chemical Properties

The chemical properties of a substance describe its ability to undergo chemical changes and reactions. Carbon dioxide's chemical properties reveal its stability and reactivity under certain conditions. Although it is generally stable and unreactive at ambient temperatures, carbon dioxide can react with water to form carbonic acid (H₂CO₃) in a reaction represented by the equation:

• CO₂ + H₂O ↔ H₂CO₃

This reaction is crucial in processes such as the carbonation of beverages and the natural carbon cycle. Furthermore, CO₂ can participate in reactions under high temperature or pressure, such as turning into carbon monoxide in the presence of a strong reducing agent. These chemical properties are consistent and predictable across any sample of carbon dioxide. Understanding them helps in numerous scientific and industrial applications, from biochemical pathways to manufacturing processes. This consistent reactivity further underlines why carbon dioxide is classified as a pure substance.