Problem 72 - Introductory Chemistry Atoms First

Question

The development of rust spots on an automobile body is an example of a _____ change. (physical or chemical)

Step-by-Step Solution

Answer

The development of rust spots on an automobile body is an example of a chemical change.

Step 1: Define Physical and Chemical Changes

A physical change is a change in the form or state of the substance without altering its chemical composition. It is usually reversible. Examples include changes of state like solid to liquid or liquid to gas. A chemical change involves a change in the substance's chemical composition, resulting in the formation of new substances. These changes are often not easily reversible. Examples include rusting, burning, and digestion.

Step 2: Identify the Change in the Formation of Rust Spots

Rusting is a chemical reaction known as oxidation, in which iron reacts with oxygen in the presence of water or moisture to form rust (iron oxide). This process involves a change in the chemical composition of iron, so it is classified as a chemical change.

Step 3: Answer the Exercise:

Given the development of rust spots on an automobile body, we can determine that it is an example of a _chemical_ change.

Key Concepts

Physical Change

Physical changes are transformations that alter the form or state of a substance without changing its fundamental chemical structure. These changes are generally easy to spot and often reversible. For example, imagine water turning into ice. The process can easily go back by melting the ice again.

In a physical change, the molecules rearrange, but the individual molecules remain the same. It's like folding a piece of paper. Even though the paper looks different, it is still the same piece of paper. Common examples include:

• Melting of ice
• Boiling of water
• Breaking a glass

Understanding physical changes helps us identify processes that do not result in new substances.

Oxidation

Oxidation is a chemical reaction that involves the transfer of electrons between substances. It occurs when a substance loses electrons. This process is an integral part of many chemical changes and reactions we observe in everyday life.

Oxidation is vital because it often leads to the formation of new substances. For example:

• Formation of rust when iron reacts with oxygen
• Burning of wood where carbon oxidizes to form carbon dioxide

Interestingly, oxidation is not always a negative process. In our bodies, oxidation helps in energy production during cellular respiration, although excessive oxidation can lead to conditions like oxidative stress.

By knowing about oxidation, we can understand why some materials corrode or degrade over time and explore how to prevent or slow these processes.

Rust Formation

Rust formation is a specific example of oxidation, where iron reacts with oxygen and water to create a new compound called iron oxide, commonly known as rust. This process transforms iron at the molecular level, resulting in a reddish-brown flaky substance.

Rust formation is not just an aesthetic problem; it represents structural weakening. Here's how it happens:

• Iron (Fe) reacts with oxygen (O₂) in the presence of water or moisture.
• The reaction creates iron oxide (Fe₂O₃), commonly known as rust.
• This is a chemical change since the material's molecular structure is altered.

Rusting is a slow process compared to other chemical changes, but it is highly impactful on metal objects, particularly in moist environments.

Understanding rusting is crucial for preventing damage and extending the lifespan of tools, vehicles, and infrastructure.