From what to what does heat flow?

Heat flows from hot to cold due to the second law of thermodynamics, which states that energy tends to disperse and spread out unless constrained. This principle dictates that when two objects at different temperatures come into contact, thermal energy will naturally transfer from the hotter object (where the energy is more concentrated) to the colder object (where the energy is less concentrated).

This process continues until thermal equilibrium is reached, meaning both objects attain the same temperature. It's a fundamental concept in thermodynamics and is applicable in various scenarios ranging from everyday activities, like cooking, to complex systems in nature. Understanding this principle is crucial for grasping how heat transfer occurs in convection and other modes, such as conduction and radiation.

The other options relate to different concepts; for instance, heat transfer under conditions of pressure changes or phase changes does not describe the natural direction of thermal energy flow as accurately as the option that specifies the temperature gradient.