Question

Which of the following are irreversible processes?
(This question has multiple correct answers)
A. Mixing of two gases
B. Evaporation of water at $373K$ and 1 atm in a closed system.
C. Dissolution of $NaCl$ in water
D. ${H_2}{O_{(s)}}$ at $ - {4^\circ }C$

Answer 1

Hint: In thermodynamics, we have a theory that the reversible process is the one in which both the system and the environment can be restored back to its original state. And irreversible processes are those where the system and the environment both cannot be retained back to its original state. Using this phenomena we will identify the process taking place in each reaction given above.

Complete step by step answer:
Mixing of two gases
When we mix two gases, they will either react and form a new compound. The entropy will increase (spontaneous process). And gages will remain in the newly formed state and will not retain back. Hence, this example is an irreversible process. Therefore, the option A is correct.
Evaporation of water at $373K$ and 1 atm in a closed system.
The given temperature and pressure are standard conditions of the environment around living beings. We know that water in room temperature exists in liquid form. When freezed in low temperature it exists in solid form and when boiled it evaporates (gaseous state).
When the evaporation takes place in an open system, the water droplets will evaporate in the air and deposit on the clouds which form rains. But when water is evaporated in a closed system, the water droplets will deposit on the surface of the container and it will condense back in the liquid state.
Hence this process is naturally reversible. Therefore, the option B is incorrect.
Dissolution of $NaCl$ in water
Sodium chloride also called as common salt exists in solid crystalline state in standard conditions. When dissolved in water, the sodium and chlorine atom in water are in aqueous state. The entropy of the liquid state is much greater than in solid state. The process is irreversible. Because if we want to separate it back we need to undergo certain procedures. The water and salt cannot be separated on its own.
${H_2}{O_{(s)}}$at $ - {4^\circ }C$
Water at $ - {4^\circ }C$ temperature will be in the solid state only. It can only convert to a liquid state in high temperature or even the standard room temperature.
Hence, the given example is an irreversible process.

So, the correct answer is Option A,C,D.

Note: The basic idea behind the reversible and irreversible process is that, a reversible process takes place in a natural environment where the system and environment both can react and retain back to its original state on its own without involving any other process to reverse the reaction. And an irreversible process is the one where the system and environment once changed cannot retain back to its original property on its own. It needs some reaction or process to do so. For example, salt dissolved in water. If we want to separate them two we need to evaporate the salt water solution in a closed system. The water droplets will evaporate leaving behind the salt.