In kinetic theory, heat energy contained in a substance is primarily associated with which of the following forms of molecular motion?

Introduction / Context:
This question examines your understanding of heat from the viewpoint of kinetic theory of matter. In thermodynamics, heat is a form of energy transfer, while in kinetic theory we connect internal energy and temperature with motions of molecules. The distinction between random microscopic motion and any large scale ordered motion is important. Recognising that heat is linked to random molecular motion helps clarify what happens when a body is heated or cooled.

Given Data / Assumptions:

• We are talking about molecules in a solid, liquid or gas.
• Heat is considered as energy related to molecular motion.
• Options distinguish between random and orderly motion.
• Macroscopic ordered motion might include bulk flow, rotation or translation of the entire body.

Concept / Approach:
In kinetic theory, temperature is related to the average kinetic energy of random molecular motion within a substance. When we talk about heat content or thermal energy, we usually refer to this microscopic disordered motion: molecules moving, vibrating and rotating in all directions. Ordered motion, such as the whole body moving in one direction, corresponds to macroscopic kinetic energy, not heat. For example, a hot gas at rest in a container has high internal energy due to random motion even though there is no bulk movement. Therefore, heat energy is associated primarily with the kinetic energy of random motion of molecules.

Step-by-Step Solution:
Step 1: Recall that molecules in a substance are constantly in motion, even in solids where they vibrate about fixed positions. Step 2: Recognise that this motion is random in direction and magnitude at the microscopic level, with no overall preferred direction. Step 3: Understand that temperature is a measure of the average kinetic energy of this random microscopic motion. Step 4: When heat is supplied to a body at constant phase, the main effect is to increase the kinetic energy of this random motion, which raises the temperature. Step 5: Conclude that heat energy is most directly associated with the kinetic energy of random, disordered molecular motion, not with large scale ordered motion.

Verification / Alternative check:
Consider a block of metal that is moving across a table with some speed but at a low temperature. Its molecules have significant ordered motion because the whole block is moving, yet we do not say it is hot. If the block is heated while it is at rest, its temperature and heat content increase due to more vigorous random vibrations of the atoms, even though there is no bulk motion. Similarly, a gas confined in a container at high temperature can have intense random motion of molecules while the container itself does not move. These examples show that heat relates to random microscopic motion, not to macroscopic uniform motion.

Why Other Options Are Wrong:
Kinetic energy of perfectly orderly motion of molecules corresponds to bulk motion, such as translation of the whole body, which is mechanical kinetic energy, not heat. Total kinetic energy of both random and orderly motion mixes internal thermal energy with bulk kinetic energy; in thermodynamics, heat is associated specifically with random internal motion. None of these is incorrect because the first option correctly describes the connection between heat and random molecular kinetic energy.

Common Pitfalls:
Students sometimes blur the distinction between internal energy and total mechanical energy. They may think that if a body is moving fast, it must contain more heat, which is not necessarily true. To avoid this confusion, remember that internal energy and heat are connected to disordered microscopic motion, while macroscopic kinetic energy relates to ordered motion of the whole object. Focusing on random molecular motion will guide you to the correct interpretation in exam questions.

Final Answer:
Heat energy in a substance is primarily associated with the kinetic energy of random motion of molecules.