Which of the following is not correct about the molecules of a substance in a gaseous state. They?

Analysis of the Question

This question tests your understanding of the Kinetic Theory of Gases. The Kinetic Theory makes several assumptions about the behavior of gas molecules to explain their physical properties. We need to identify which statement among the options is not a valid assumption of this theory.

Concept Explanation

The Kinetic Theory of Gases is based on these fundamental postulates:

1. Motion: Gas molecules are in constant, random, and rapid motion.
2. Speed: At any given temperature, molecules move at different speeds (a distribution of speeds).
3. Temperature: The absolute temperature of a gas is directly proportional to the average kinetic energy of its molecules.
4. Collisions: Collisions between gas molecules, and between molecules and the walls of the container, are perfectly elastic. This means there is no loss of kinetic energy during these collisions.
5. Volume: The volume of the molecules themselves is negligible compared to the total volume of the gas.

Step-by-Step Evaluation

• Option A: "Are in a constant state of motion." This is correct according to the Kinetic Theory.
• Option B: "Have different speeds." This is correct. Due to constant collisions, molecules constantly exchange energy, leading to a range of speeds (Maxwell-Boltzmann distribution).
• Option C: "Have a temperature which is measured by the average kinetic energy." This is correct. The formula $KE_{avg} = \frac{3}{2}kT$ shows this direct relationship.
• Option D: "The collision between the gases is perfectly inelastic." This is incorrect. In physics, an inelastic collision involves a loss of kinetic energy (often converted to heat or deformation). If gas collisions were inelastic, the gas would eventually lose all its kinetic energy and settle, which contradicts the observed behavior of gases. Therefore, collisions must be perfectly elastic.

Conclusion

Since the question asks for the statement that is not correct, Option D is the right choice because it contradicts the fundamental postulate that gas collisions are perfectly elastic.

Exam Tip: Whenever you see questions about the Kinetic Theory of Gases, always remember the "Elastic" rule. Think of gas molecules like tiny, perfect billiard balls that never lose energy when they hit each other or the walls of their container!