Which of the following does not support the fact that air is a mixture?

Analysis

To answer this question, we must understand the fundamental difference between a mixture and a compound.

• A mixture consists of two or more substances physically combined, where the components retain their individual properties and can be present in varying proportions.
• A compound is a substance formed when two or more elements are chemically bonded in a fixed proportion by mass.

Concept Explanation

• Air as a mixture: Air is a mixture of gases (nitrogen, oxygen, argon, carbon dioxide, etc.). Because it is a mixture, it does not have a fixed chemical formula, and its components can be separated by physical methods like fractional distillation of liquid air.
• Fixed proportion by mass: This is a characteristic property of compounds (Law of Definite Proportions). In a mixture, the components can vary in concentration depending on the location or conditions (e.g., air near a busy road has more carbon monoxide than air in a forest). Therefore, saying air has a "fixed proportion by mass" contradicts the definition of a mixture.

Step-by-Step Solution

1. Evaluate Option B: "It cannot be represented with a chemical formula." This is a true statement about mixtures. Since air is a mixture, this supports the fact that it is a mixture.
2. Evaluate Option C: "The constituents of air can be separated by physical means." This is a classic property of mixtures (e.g., fractional distillation). This also supports the fact that air is a mixture.
3. Evaluate Option A: "The constituents of air are in a fixed proportion by mass." This statement describes a compound, not a mixture. Because air is a mixture, its components are not chemically bonded in a fixed ratio. Therefore, this statement does not support the fact that air is a mixture.

Conclusion

Since Option A describes a property of a compound rather than a mixture, it is the correct answer.

Exam Tip: Always remember the acronym "M-P-S" for mixtures: Mixtures have Physical separation and Separate properties (the components don't change their identity). If you see "fixed proportion" or "chemical formula," you are likely looking at a compound!