Difference Between Adiabatic and Isothermal (With Table)

Thermodynamics is a branch of science that describes work, heat, and temperature and relates them with entropy and energy. It also elaborates the relationship between work and other physical properties related to radiation and matter. The field of thermodynamics is ruled by the behavior of various parameters, which can be studied using four laws. These processes take place in specific surroundings. Two such surroundings are adiabatic and isothermal.

Adiabatic vs Isothermal 

The main difference between adiabatic and isothermal is that an adiabatic process involves no heat exchange, resulting in temperature change. In contrast, an isothermal process involves heat exchange, resulting in no temperature change. An adiabatic process involves no change in enthalpy, whereas an isothermal process involves a change in enthalpy. The adiabatic process finds applications in devices such as nozzles and turbines, whereas isothermal processes find applications in devices like heat pumps.

Adiabatic, also known as non-isothermal, means the process where no kind of heat exchange is noticed. This means that the total heat is conserved in the system, and there’s a temperature change. The heat transfer takes place only as work is done. Understanding the adiabatic process helps in decoding the first law in thermodynamics. 

In the field of thermodynamics, isothermal refers to a process where the temperature remains unchanged, thereby maintaining the thermal equilibrium. This process also finds applications in other branches of science such as space science, geology, planetary science, engineering, and others. An isothermal process takes a vertical line when represented as a phase diagram.

Comparison Table Between Adiabatic and Isothermal 

Parameters of Comparison




An adiabatic process is a process in thermodynamics wherein the heat exchange between the surroundings and the system turns out to be zero.

The isothermal process is the process in thermodynamics wherein the temperature stays unchanged throughout the process.


Adiabatic processes take place at a fast rate.

Isothermal processes take place at a slower rate.

Heat transfer

There is no kind of transfer of heat during an adiabatic process.

Heat exchange takes place in an isothermal process while temperature stays constant.


Temperature change takes place in an adiabatic process.

No temperature change is the characteristic of an isothermal process.


In an adiabatic process, enthalpy remains unchanged.

In an isothermal process, pressure is more than volume.

What is Adiabatic? 

The process of thermodynamics in which there is no involvement of heat exchange between the surroundings and the system during expansion and contraction is called an adiabatic process. For the occurrence of an adiabatic process, the fulfillment of 2 essential conditions is essential. Firstly, the perfect insulation of the system from the surrounding is necessary. Secondly, the process must not take much time and be accomplished as quickly as possible. 

The reversibility of an adiabatic process is yet unknown and depends solely on the process. Thus, adiabatic expansion of a closed system can be considered its ideal process, provided the pressure remains unchanged and the temperature gradually decreases if the air is compressed adiabatically. Its internal energy increases in proportion to the external work done upon it without any change in heat energy.

Thus, in an adiabatic process, Q comes out to be zero. Therefore, in adiabatic expansion, the work done “W” stands positive, whereas ∆U stands out to be negative. This means that the system does the job adiabatically, which leads to a decrease in internal energy. During adiabatic compression, the work done “W” stands out to be negative, whereas ∆U stands out to be positive, which leads to an increase in internal energy.

What is Isothermal? 

The process of thermodynamics in which there is no involvement of heat exchange between the surroundings and the system so that thermal equilibrium remains unaffected. The term “isothermal” takes its origin from two Greek terms, namely, “isos,” which means equal, and “therme,” which means heat. There is no change in temperature in these processes. Thus, the substance changes at that particular temperature. 

This process or situation arises only in two phenomena. Firstly, if the system is placed beside a thermal reservoir touching the system on the exterior side, the system will exchange heat with the reservoir to maintain the equilibrium. Secondly, when no heat exchange occurs, a deliberate temperature change is executed to bring up the adiabatic process.

As the temperature turns out to be the average kinetic energy of the total number of molecules in the system, the initial energy remains conserved. The isothermal process is closely related to the hypothetical ideal gases without interaction between the molecules. Instead, the molecules go through an elastic collision. Thus, according to Joule’s second law, the internal energy stays unchanged if an ideal gas undergoes the isothermal process. The study of isothermal processes has helped biologists understand how living organisms regulate the temperature in their bodies. 

Main Differences Between Adiabatic and Isothermal 

  1. In an adiabatic process, enthalpy remains unchanged. On the other hand, in an isothermal process, enthalpy change takes place.
  2. An adiabatic expansion takes place when the system absorbs heat from the surroundings. Whereas an isothermal expansion occurs when the system uses the internal energy stored in it.
  3. In an adiabatic process, the heat supplied finds applications when the work is to be done. But, in an isothermal process, the system’s internal energy finds applications when the work is to be done.
  4. An adiabatic compression takes place when the system loses heat from the surroundings. Whereas an isothermal compression occurs when heat is added to the internal energy initially present in the system.
  5. The fusion of ice is an example of an adiabatic process. But, when gas expansion takes place in a vacuum, it represents an isothermal process. 


In thermodynamics, concepts such as isochoric, isothermal, adiabatic, and isobaric processes are used. These terms are used in demonstrating the behavior of various parameters of the thermodynamic system. Generally, an adiabatic process takes place in an insulated container wherein the time is minimal. Examples of adiabatic processes include a pendulum oscillating in a vertical plane. Examples of isothermal processes include the process of melting matter.


  1. https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/direct-numerical-simulation-of-compressible-turbulent-channel-flow-between-adiabatic-and-isothermal-walls/0A43399A2ED32D0D5B981B9191339D3E
  2. https://www.sciencedirect.com/science/article/pii/S0955221913003129