The constituent of air necessary in the rusting process are

Analysis

The question asks for the specific components of air that must be present for the chemical process of rusting to occur on iron surfaces.

Concept Explanation

Rusting is a chemical process known as oxidation. It is the corrosion of iron in the presence of oxygen and moisture.

1. Oxygen ($O_2$): Iron reacts with oxygen to form iron(III) oxide. Without oxygen, the oxidation process cannot begin.
2. Water ($H_2O$): Water acts as a medium for the reaction. It is essential for the formation of hydrated iron(III) oxide, which is the reddish-brown substance we call "rust." The chemical formula for rust is typically represented as $Fe_2O_3 \cdot xH_2O$.

While other gases like Nitrogen, Argon, and Carbon Dioxide are present in the air, they do not participate in the fundamental chemical reaction of rusting.

Step-by-Step Solution

• Step 1: Identify the chemical requirements for rusting. Rusting is an electrochemical process that requires both an oxidizing agent (Oxygen) and an electrolyte/medium (Water/Moisture).
• Step 2: Evaluate the options: • A ($O_2$ and $H_2O$): Both are essential for the formation of hydrated iron(III) oxide. • B (Ar and $CO_2$): Argon is an inert gas and does not react; $CO_2$ is not required for rusting. • C ($CO_2$ and $H_2O$): While water is needed, $CO_2$ is not the primary reactant for rusting. • D ($O_2$ and $CO_2$): Oxygen is needed, but without water, the reaction is extremely slow or non-existent in atmospheric conditions.
• Step 3: Conclude that the combination of Oxygen and Water is the scientifically correct requirement for rusting.

Exam Tip

Think of rusting as a "partnership." Iron needs both Oxygen (the fuel) and Water (the bridge) to create rust. If you keep iron in a dry environment (no water) or a vacuum (no oxygen), it will not rust. This is why we paint iron gates or grease car parts—to create a barrier that keeps $O_2$ and $H_2O$ away from the metal!