The direction of heat flow between two objects in thermal contact depends mainly on which property of the objects?

Introduction / Context:
Heat transfer is a fundamental concept in physics and everyday life. When two objects at different temperatures are placed in contact, energy in the form of heat flows between them until equilibrium is reached. The question asks which property determines the direction of heat flow between two objects when they touch, an idea that is essential for understanding thermodynamics and daily applications such as heating and cooling.

Given Data / Assumptions:

• Two objects are in thermal contact so that heat can flow between them.
• They may have different masses, materials and heat capacities.
• We want to know what decides the direction in which heat energy moves.
• We assume no external work is done and no phase change for simplicity.

Concept / Approach:
The second law of thermodynamics states that heat naturally flows from a body at higher temperature to a body at lower temperature, and not spontaneously in the opposite direction. Temperature is the measure of the average kinetic energy of particles, while heat content depends on mass, specific heat capacity and temperature. Even if a large body at slightly lower temperature has more total heat content, heat still flows from the hotter body to the colder one. Therefore the direction of heat flow depends only on the temperature difference, not on mass or state alone.

Step-by-Step Solution:
Step 1: Identify the temperatures T1 and T2 of the two objects in contact. Step 2: If T1 is greater than T2, heat flows from object 1 to object 2. Step 3: If T2 is greater than T1, heat flows from object 2 to object 1. Step 4: When T1 equals T2, the objects are in thermal equilibrium and there is no net heat flow between them. Step 5: Conclude that temperature difference alone decides the direction of heat flow, independent of their masses or physical state.

Verification / Alternative check:
Consider placing a small hot metal spoon into a large bucket of cold water. Even though the water contains more total heat energy, heat flows from the hotter spoon to the colder water, causing the spoon to cool and the water to warm slightly. Similarly, when you touch a hot object, heat flows into your hand because the object temperature is higher, regardless of which has greater mass. These examples confirm that temperature difference governs the direction of heat transfer.

Why Other Options Are Wrong:
Their masses only is wrong because a heavier object at lower temperature does not automatically send heat to a lighter hotter object.
Their total heat contents only is incorrect, since a body with more total heat can still receive heat if its temperature is lower.
Whether they are solid, liquid or gas affects rate of heat transfer but not the direction.
Their shapes and surface areas influence how fast heat is exchanged but not the fundamental direction of flow.

Common Pitfalls:
A common misunderstanding is to think that heat flows from a body with more thermal energy to one with less. In reality, it is the temperature difference that matters. Students also confuse heat and temperature as the same quantity. Remember that temperature is an intensive property and decides the direction of heat flow, while heat is an extensive form of energy transferred due to that difference.

Final Answer:
The direction of heat flow between two objects depends on their temperatures.