Final answer:
A warm substance becomes cooler due to the flow of heat energy from itself to cooler surroundings, reaching thermal equilibrium through conduction, convection, radiation, and phase changes.
Step-by-step explanation:
Why would a warm substance become cooler? A warm substance becomes cooler due to the process of heat transfer where heat energy flows from the warmer substance to the cooler surroundings until thermal equilibrium is reached. This is in accordance with the second law of thermodynamics, which tells us that it is impossible for heat to transfer energy spontaneously from a cooler to a hotter object without external work being done on the system. Convection, radiation, conduction, and phase changes are some of the mechanisms through which a warm substance can lose heat.
Heat Transfer Mechanisms
In the case of conduction, a warm substance, such as a cup of hot coffee, will transfer heat to a cooler object it's in contact with, such as a spoon, resulting in the coffee cooling down. Similarly, convection allows warmer material to rise and colder material to sink, which can lead to cooling, such as when heating soup, and the steam cooling when exposed to cooler air. Furthermore, phase changes at certain temperatures cause a substance to change from one state to another, absorbing or releasing heat in the process without changing temperature, such as water boiling into steam or freezing into ice.
Examples of Cooling
When a pot of boiling water's lid is removed, allowing the steam to expand, the steam cools down. The same concept applies to celestial bodies, such as stars that expand and cool, leading to changes in their color due to temperature changes. Overall, as substances or objects cool down, they move towards their preferred phase which is solid at low temperatures and gas at high temperatures.