Question

Which of the following processes are reversible? I. Vaporization of a liquid at its boiling point. II. Expansion of gas into vacuum. III. Transformation of a solid substance into liquid at its melting point. IV. Neutralization of an acid by a base.

• A. I & III
• B. II & III
• C. II & IV
• D. I & IV

Hint:

In thermodynamics, reversible processes are idealized processes where the system is always infinitesimally close to equilibrium. Phase changes at the melting or boiling point are classic examples of reversible processes. Spontaneous, rapid processes like free expansion or chemical reactions are generally irreversible.

Answer 1

The Correct Option is A

To determine which of the given processes are reversible, we first need to understand the nature of reversible processes. In thermodynamics, a reversible process is one that can be reversed without leaving any net change in either the system or the surroundings. This typically means the process must be quasi-static and occur without entropy change. 

1. Vaporization of a liquid at its boiling point:
   • When a liquid is vaporized at its boiling point under equilibrium conditions, the process is reversible.
   • If the pressure and temperature remain constant, the system can be returned to its original state without any net change.
2. Expansion of gas into vacuum:
   • This is known as free expansion and is highly irreversible because it occurs spontaneously and rapidly.
   • No work is done, and reversing the process would require external work input.
3. Transformation of a solid substance into liquid at its melting point:
   • This process, like vaporization at the boiling point, can be made reversible by ensuring it occurs under equilibrium conditions (constant temperature and pressure).
   • The system can return to its original state without performing additional work.
4. Neutralization of an acid by a base:
   • This process involves a chemical reaction that is typically exothermic and rapid.
   • Reversing it would require additional chemical reactions with input energy, making it irreversible.

Considering these analyses, the reversible processes among the options given are:

• I. Vaporization of a liquid at its boiling point.
• III. Transformation of a solid substance into liquid at its melting point.

Thus, the correct answer is: I & III