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A system refers to a part of the universe being studied.

You can define any areas or parts of the universe as a system. A system can be as large as the rain forest in the Amazon or as small as a kettle of water in your kitchen. A system refers to a defined area you specifically want to put your focus on. Thermodynamics is the study of the quantity of heat exchange between a system and its surrounding.
The system is subject to surrounding factors such as air temperature, humidity and atmospheric pressure.

There are three types of thermodynamics systems. Based on the possible heat and matter transfer, they are classified as open, closed or isolated systems.

Diagram of types of thermodynamics system - open, closed and isolated

Open Systems

openYou may have heard of open systems and closed systems. An open system is one that freely allows energy and matter to be transferred in an out of a system.

Boiling water without a lid is an open system. Steam and heat can both escape freely. Matter is lost and it is transferred out of the system. An open system is one that freely allows energy and matter to be transferred from the system into the environment.

Closed Systems

closedsystemA closed system, on the other hand, does not allow the exchange of matter but allows energy to be transferred.

It allows heat to be transferred from the stove to the water

Heat is also transferred to the surroundings

Steam is not allowed to escape

Example of a closed system – a pressure cooker


Isolated Systems

This system is completely sealed
Matter is not allowed to be exchanged with the surroundings
Heat cannot transfer to the surroundings


In reality, there are no 100% isolated systems. However, in practice, a system many be classified as isolated over a short period of time or distance. Everything over time will eventually exchange matter and heat with the surrounding. The universe is considered an isolated system because it is assumed to have no surrounding.