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Reversible and Irreversible Processes

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The thermodynamics is the science that meant to study the energy transitions into heat and mechanical work, and flowing the energy from point to another point. It illustrates the reasons behind of many processes in nature such as fusion, freezing, evaporation, and sublimation processes, which also it relates to the matter properties. 

In the thermodynamics, there are two processes that happen when the heat flows from hot object to cool object or the reverse:

  1. Irreversible process.
  2. Reversible process.

Irreversible process   

An irreversible process is a spontaneous process that happens in all-natural processes. Heat spontaneously flows from the hot object to the cool object without going back to the hot object, which means this process walks in one direction. Once the process begins, the object state changes. Melting milk foam above the coffee is considered as irreversible process. The heat flows from coffee to milk foam. Entering milk’s foam, heat causes a disorder between milk’s molecules leading to rearranging molecules’ order. Thus, the milk foam is not being a foam anymore. Typically, this process makes a significant change in universe’s entropy because it’s more probable to happen.

Other examples

  1. Converting mechanical energy into heat to stop the car by car brakes.
  2. Crystallizing the hot magma on the earth surface to form fiery rocks. 
  3. Soaking the vegetables into water and salt solution to ferment it. 

Reversible process

The reversible process is in contrast to the irreversible process. Heat flows to the object in two reversed directions. If the process ends, the object state can back to its initial state at the same conditions that the process happened within. It is an unspontaneous process, it happens only when there is thermodynamic equilibrium between the object and its surroundings, and making an infinitesimal change in the conditions of the object or its surroundings. For example, if you place a block of ice (at 0 °C) into a closed box (at 0 °C too), then you make a very small change in the temperature of the cubic ice or the closed box, the cubic ice will melt after a while. This kind of process doesn’t influence the universe’s entropy, it only happens at infinitesimal level. 

Other examples

  1. A slow heat flow between two objects at 25 °C.
  2. A slow adiabatic compression of the air.
  3. Pushing an object on a frictionless surface.

Comparison between irreversible process and reversible process. 

Irreversible process Reversible process 
It is spontaneous, it happens in nature.   It is unspontaneous, it doesn’t happen in nature. 
Heat flows from the hotter body to the cooler body only. (One direction) Heat flows between the hot body and the cool body in reversed directions. 
It only requires different temperatures.  It requires (1) thermodynamic equilibrium (2) an infinitesimal change of the body or its surroundings conditions.
It creates a significant change of the universe’s entropy.  It does not influence the universe’s entropy. 
It increases the disorder of the molecules, particles and so on. It Doesn’t change the disorder of the molecules, particles and so on.
The object can’t return to its initial state. The object can return to its initial state.

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.


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.


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.


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.


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.


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.


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

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Last Updated : 10 Apr, 2022
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