A basic postulate of the kinetic theory of gases is that the molecules of a gas move in straight lines between collisions. This implies that

Analysis of the Question

This question tests your understanding of the Kinetic Molecular Theory of Gases. You are asked to identify what the movement of gas molecules in straight lines between collisions implies regarding the forces acting between them.

Explanation of the Concept

The Kinetic Molecular Theory makes several assumptions about ideal gases. One of the core postulates is that gas molecules are in constant, random, straight-line motion.

• Why straight lines? If there were significant forces of attraction or repulsion between gas molecules, these forces would pull or push the molecules off their straight-line paths as they moved.
• The Implication: Because they travel in straight lines until they hit something (the walls of the container or another molecule), it implies that there are no significant intermolecular forces (attraction or repulsion) acting on the molecules while they are in motion.
• The Equilibrium State: In a gas, the molecules are far apart. The assumption is that the forces of attraction and repulsion are negligible or, more specifically, that these forces are in a state of balance (equilibrium) such that they do not influence the path of the molecules.

Step-by-Step Solution

1. Evaluate the motion: The fact that molecules move in straight lines means they are not being deflected by neighboring molecules.
2. Analyze the forces: If there were strong forces of attraction, molecules would "clump" or curve toward each other. If there were strong forces of repulsion, they would veer away from each other.
3. Connect to the theory: The kinetic theory assumes that for an ideal gas, the intermolecular forces are negligible. This is mathematically represented by the idea that the forces of attraction and repulsion are effectively in equilibrium, resulting in a net force of zero on the molecules as they travel.
4. Review the options: • A: While collisions are assumed to be elastic, this is a separate postulate and does not explain the straight-line motion. • B: If forces of repulsion existed, the molecules would not move in straight lines. • C: This correctly explains why there is no net force to deflect the molecules from their straight-line path. • D: Kinetic theory assumes collisions are elastic, not inelastic.

Conclusion

The straight-line motion of gas molecules is a direct consequence of the absence of net intermolecular forces, meaning the forces of attraction and repulsion are in equilibrium.

Exam Tip: Remember the acronym "V.E.R.T." for Kinetic Theory:

• Volume of particles is negligible.
• Elastic collisions.
• Random, straight-line motion.
• Temperature is proportional to kinetic energy.
• No intermolecular forces (Attraction/Repulsion are in equilibrium).