Thermodynamics deals with macroscopic systems, typically consisting
of 
microscopic objects such as atoms or molecules.
A system is defined as the part of the Universe we are interested in.
This could be a test tube in which we are observing a chemical reaction,
a cup of water we are heating, an automobile engine we are fixing etc.
The rest of the Universe is called the surroundings. This may be
illustrated with the "formula"
In everyday practice we do not consider the influence remote parts of the Universe on our system, and only look at the more immediate neighborhood, e.g. only the bench-top apparatus and air in the lab for the test tube, the rest of the automobile and surrounding atmosphere for the engine etc. Usually we are able to draw a real or imaginary boundary that separates the system from the surroundings.
The simplest kind of system is the isolated system which does not exchange energy or matter with the surroundings. A hypothetical example of an "almost isolated" system would be a gas enclosed in a sealed double-walled steel container with thick thermal insulation between the steel walls.
The closed system is able to exchange energy but not matter with surroundings. In our gas example above we would make one of the walls thin and able to conduct heat, and allow it to move so that gas may be compressed or expanded. We would add some sort of gaskets to make sure no substance can enter or leave the container.
The open system exchanges both energy and matter with the surroundings. In the gas example we would punch a hole in the double walls, allowing mater to flow.
| System | Exchange energy | Exchange matter |
| Isolated | no | no |
| Closed | yes | no |
| Open | yes | yes |
In practice, all systems are open to some degree,
but sometimes we can neglect
the effects of energy or matter flow. For example we can consider
hot coffee in a thermos flask to be an isolated system over some short
period of time, since heat loss from a thermos is relatively slow.
Similarly, we can usually think of a glass of water or
open test-tube in a laboratory to be closed systems, neglecting the
evaporation of liquids to the atmosphere and mixing of atmospheric
gases with our samples.
Examples of interesting kinds of open systems are the human body
or our planet Earth.
Two other kinds of system/surroundings boundaries are especially interesting:
the adiabatic boundary, in which case the
system cannot exchange heat with the surroundings and the opposite, or
diathermic boundary through which
exchange of heat with surroundings can take place.
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