According to the law of heat flow, where does heat tend to flow?

The law of heat flow states that heat naturally moves from areas of higher temperature to areas of lower temperature. This principle is rooted in the second law of thermodynamics, which indicates that energy tends to disperse and spread out rather than concentrate. When a hot substance is in contact with a cooler substance, the thermal energy from the hot substance will flow toward the cooler one until thermal equilibrium is reached—that is, until both substances are at the same temperature.

This fundamental concept is key in understanding heat transfer in various scenarios, such as in heating systems, cooking, or even natural phenomena like weather systems. The other options do not accurately represent this principle, as heat does not flow from cold to hot, does not pertain specifically to fluids, and does not indicate a specific phase change from solid to gas, which are unrelated to the general behavior of heat transfer.