Answer: Option (3) is the correct answer.
Explanation:
The statement heat always flows from warmer to cooler substances is true.
When a hot substance is placed in contact with a cooler substance then exchange of heat takes place from warmer to cooler substance until thermal equilibrium is reached.
The molecules of warmer substance collide rapidly with each other. When a cooler substance is placed in touch with it then the molecules of warmer substance starts to collide with the molecules of cooler substance.
As a result, the molecules of cooler substance also gain energy. Hence, there will be exchange of energy from warmer substance to cooler substance.
This exchange of energy will continue till the temperature of both warmer and cooler substance becomes equal. After that there will be no exchange of heat.
Therefore, we can conclude that heat always flows from warmer to cooler substances.
