Thermodynamics explains how heat and energy behave during physical processes and how energy is transferred from one body to another. Many natural phenomena, such as melting, freezing, evaporation, condensation, and sublimation, occur because of the exchange of heat and energy between systems and their surroundings.

When heat transfer takes place in a thermodynamic system, the process is generally classified into two types:

1. Reversible Process
2. Irreversible Process

These processes describe whether a system can return to its original state after the transfer of heat, work, or energy.

1. Reversible Process

A reversible process is an ideal thermodynamic process in which the system and surroundings can be restored completely to their original states without leaving any net change in the universe. The process occurs extremely slowly so that the system remains in thermodynamic equilibrium at every stage.

In a reversible process, even a very small change in pressure, temperature, or volume can reverse the direction of the process.

Characteristics of a Reversible Process

• The process occurs very slowly (quasi-static process).
• The system remains in thermodynamic equilibrium throughout the process.
• There is no loss of energy due to friction, viscosity, or resistance.
• The process can be completely reversed.
• The entropy change of the universe is zero.
• Maximum possible work is obtained.

Conditions for a Reversible Process

For a process to be reversible, the following conditions must be satisfied:

• The process should occur infinitely slowly.
• There should be no friction or dissipative forces.
• The temperature difference between the system and surroundings must be extremely small.
• The pressure difference during expansion or compression should be infinitesimally small.
• Heat transfer must take place under near-equilibrium conditions.

Comparison between irreversible process and reversible process. 

Sample Problems 

Question 1: Which one of the following is considered reversible process.

1. Roasting the nuts at 85 °C.  
2. Burning the papers.
3. A slow isothermal expansion of the Helium gas.

Answer: 

3. A slow isothermal expansion of the Helium gas.

Question 2: determine whether flowing the heat very slowly between two objects at thermal equilibrium reversible or irreversible process.

Answer: 

Reversible process. The heat flows so slowly, and both objects at thermal equilibrium. 

Question 3: determine whether Igniting the wood into the fireplace to warm the room reversible or irreversible process.

Answer: 

Irreversible process. After the wood has become ash, you can’t reverse the process to get the wood back.

Question 4: determine whether combining a 1 kg of water at 0 °C into 1 kg of water at 100 °C reversible or irreversible process.

Answer:

Irreversible process. There is a significant difference of temperatures between a 1 kg of water and a 1 kg of water, so the heat will spontaneously flow from the hot water to the cool water.

Question 5: Which one of the following is considered reversible process.

1. Mixing the cake’s ingredients.
2. Melting the ice cream on hot weather. 
3. Quasi-static compression of Methane gas.

Answer: 

3. Quasi-static compression of Methane gas.

Question 6: Determine whether flowing the heat to a pot at 15 °C placed into an oven at 150 °C reversible or irreversible process.

Answer: 

Irreversible process. The heat will spontaneously flow in the pot.