Question

A piece of ice kept at room temperature melts at its own. This reaction is governed by which law?
(A) Second law of thermodynamics
(B) First law of thermodynamics
(C) Third law of thermodynamics
(D) Zeroth law of thermodynamics

Answer 1

Hint: Think about the changes which take place when ice melts at room temperature on its own. Write the statements of all the four laws of thermodynamics and then choose the law which best describes the reaction taking place during melting of ice.

Complete step by step solution:
- Let’s begin by stating the laws of thermodynamics.
- According to the first law of thermodynamics, the change in internal energy of a system is the sum of heat absorbed by the system and the work done on the system by the surroundings. It is basically the law of conservation of mass. Energy can neither be created nor can be destroyed but can only be converted from one form to another. The total energy of the system remains constant. These are some statements of the first law of thermodynamics.
- According to the second law of thermodynamics, the spontaneous flow of heat is always unidirectional, from higher temperature to lower temperature.
- According to the third law of thermodynamics, for a perfectly crystalline substance, the entropy of the system is equal to zero at absolute temperature of 0K.
- According to the zeroth law of thermodynamics, if two thermodynamic systems are in thermal equilibrium with a third system then they all are in thermal equilibrium with each other.
- When ice is kept at room temperature, it starts melting on its own to form water. Ice has a lower temperature and the surroundings have a higher temperature. So, the system that is, ice starts to absorb heat from the surroundings spontaneously and starts to melt. This phenomena is best described using the second law of thermodynamics.
- When ice melts at room temperature, heat flows spontaneously from the surroundings which have a higher temperature into the system which has a lower temperature.

Therefore, the second law of thermodynamics governs the process of melting of ice into water, which is option (A).

Note: Remember the four laws of thermodynamics. First law is the law of conservation of mass. Second law talks about spontaneous heat flow from high temperature to low temperature. Third law says entropy of a perfectly crystalline substance is zero at 0K. Zeroth law tells that when system A and system B are in thermal equilibrium with system C then, all the three systems A, B and C are in thermal equilibrium with each other. Melting of ice is in accordance with the second law of thermodynamics.