Its constituents can be separated by physical methods like fractional distillation.
Explanation
Matter is broadly classified into pure substances (elements and compounds) and mixtures. The fundamental distinction lies in how the constituents are combined. In a compound, elements are chemically bonded in fixed proportions. In a mixture, substances are physically mixed without chemical bonding, retaining their individual properties.
Option Analysis
- It is invisible and odourless. is incorrect: Physical characteristics such as visibility or odour do not determine whether a substance is a mixture or a compound. Many pure compounds (e.g., water vapour, carbon dioxide) are also invisible and odourless.
- Its constituents can be separated by physical methods like fractional distillation. is correct: The defining characteristic of a mixture is that its components can be separated by physical means. The gases in air (Nitrogen, Oxygen, Argon, etc.) are not chemically bonded and have distinct boiling points. Therefore, they can be separated using fractional distillation of liquid air, a physical process.
- It supports combustion. is incorrect: The ability to support combustion is a chemical property of Oxygen, one of the constituents of air. While this proves Oxygen is present, it does not define the structural classification of air as a mixture versus a compound.
- It occupies space and has mass. is incorrect: Occupying space and having mass is the fundamental definition of matter. This property applies universally to elements, compounds, and mixtures, and therefore cannot be used to distinguish between them.
Key Takeaway
Air is classified as a mixture because its constituents are not chemically combined, do not exist in fixed mass ratios, and can be separated using physical methods such as fractional distillation.