Iron
Explanation
Matter is classified into pure substances and mixtures based on chemical composition. A pure substance consists of a single type of particle (atoms or molecules) and possesses a fixed chemical composition and distinct physical properties. Mixtures contain two or more substances physically combined in varying proportions.
Analysis of Options:
- Brick: This is a heterogeneous mixture composed of clay, sand, lime, iron oxide, and magnesia. It lacks a specific chemical formula and fixed composition.
- Wood: This is a complex heterogeneous mixture primarily consisting of cellulose, lignin, water, and resins. Its composition varies by species and sample.
- Air: This is a homogeneous mixture of gases, primarily nitrogen, oxygen, argon, and carbon dioxide. Although uniform in appearance, the gases are physically mixed, not chemically bonded.
- Iron: This is a chemical element (Fe). It consists of only one type of atom and cannot be broken down into simpler substances by ordinary chemical means. Therefore, it is a pure substance.
Key Takeaway:
Elements (e.g., Iron, Gold) and compounds (e.g., Water, Sodium Chloride) are classified as pure substances because they have a definite and constant chemical composition. Mixtures (e.g., Air, Alloys, Wood) consist of multiple components physically combined.