Question

A process is reversible when:
[A] Surrounding and system change are the same.
[B] There is no boundary between system and surrounding.
[C] Surrounding is always in equilibrium with the system.
[D] System changes into surrounding spontaneously.

Answer 1

Hint: A reversible process is a process where we can obtain a product from a reactant following the forward pathway and we can also obtain the reactant from the product following the reverse pathway. Both pathways are energetically accessible by a system and its environment.

Complete step by step solution:
Before answering this question, let us discuss reversible and irreversible processes.
We know that changes can either be reversible or irreversible.
If we can get back the starting substance after it has undergone a change then the change is reversible and if we get a substance which cannot be reverted back to the original substance then the change is irreversible.
In a reversible chemical change, the reactants and the products both are at equilibrium. Once equilibrium is reached, the reaction can move either way. The reactants can change into products and the product can also change back into the reactants
However, in thermodynamics, we define a reversible change for chemical changes in terms of the system and the surroundings.
A process in which system, as well as its environment, can return to their initial states following the reverse path is a reversible process.
A process in which a system and its surrounding cannot return together to the exact states they were in initially then the process is an irreversible process.
For a system and its environment to return back to their initial states, they need to attain equilibrium.
We can understand from the above discussion that if a system is in equilibrium with its surrounding then the process is reversible.

Therefore, the correct answer is option [C] Surrounding is always in equilibrium with the system.

Note: In a reversible process, the system is in thermodynamic equilibrium with its surrounding. It would take an infinite time to complete a perfectly reversible process. Therefore, perfectly reversible reactions are impossible. For a thermodynamically reversible process, the energy from work done on the system or by the system is maximum and energy from heat is zero.