When iron rusts in moist air, should this process be classified as a chemical change or a physical change?

Introduction / Context:
Rusting of iron is one of the most familiar real life examples used to distinguish between chemical and physical changes. A physical change affects the form of a substance but does not create a new substance, while a chemical change produces one or more new substances with different properties. The question asks whether rusting should be treated as chemical or physical, so we must examine what happens to iron at the molecular level when it rusts.

Given Data / Assumptions:

• Rust forms when iron reacts with oxygen and water from the environment.
• The main product is hydrated iron(III) oxide, commonly called rust.
• Rust has different colour, texture, and properties compared with pure iron.
• Physical changes are usually reversible by simple physical methods, while chemical changes are often difficult to reverse.

Concept / Approach:
During rusting, iron atoms combine with oxygen and water molecules to form new compounds. This process involves breaking old bonds and forming new chemical bonds, which is the hallmark of a chemical reaction. The original metallic iron is converted to a flaky, brittle oxide that has different properties, such as reduced strength and conductivity. Therefore, rusting cannot be viewed as mere physical change like melting or cutting; it is a chemical change, more specifically a slow redox reaction and an example of corrosion.

Step-by-Step Solution:
Step 1: Identify what happens to iron during rusting. Iron reacts with oxygen and water to form iron oxides, especially hydrated iron(III) oxide. Step 2: Check whether a new substance is formed. Rust is chemically different from metallic iron in both composition and properties. Step 3: Compare with definitions. Chemical change involves formation of new substances, while physical change does not. Step 4: Conclude that rusting is a chemical change.

Verification / Alternative check:
One way to verify is to ask if the process is easily reversible by a simple physical method. Melting or freezing water can be reversed by changing temperature, so these are physical changes. In contrast, once iron has rusted, simply changing temperature or pressure cannot restore metallic iron. Removing rust requires either strong chemical treatment or complete replacement of the rusted portion. This difficulty of reversal and the change in composition both confirm that rusting is a chemical change.

Why Other Options Are Wrong:
Option B: Physical change is incorrect because rusting does not merely change the form; it creates new chemical substances. Option C: Both A and B cannot be correct because the process clearly fits the definition of a chemical change and not a typical physical change. Option D: None of the above is not valid since chemical change is a correct and standard description of rusting.

Common Pitfalls:
Some learners think that any change that happens slowly or can be scraped off must be physical, which is not accurate. Others confuse rusting with temporary surface tarnishing. Remember that rusting involves actual chemical reaction between iron, oxygen, and water, and it leads to loss of metallic properties. Keeping the rule in mind that new substances with new properties mean chemical change will help avoid these errors.

Final Answer:
Rusting of iron is a Chemical change.