# Simple question about fixed volume

## Main Question or Discussion Point

What exactly does "fixed volume" mean in thermodynamics, for a liquid? Does it mean that the container volume is fixed or that the volume of water is fixed, or that the volume of both the container and the water is fixed?

This is confusing me because if it refers only to the container having a fixed volume, then the volume of water could vary (like if you heat the water) while the volume of the container stays fixed. (for example, you have 1L of water in a 2L container).

This all started from a problem which states: "A sample of hot water is mixed with a sample of cold water in an insulated container of fixed volume..." it says the container is fixed volume, so does that mean the water fits perfectly in the container (no air space at the top?)

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The volume, fixed or variable, refers to the system under consideration, just as with any other thermodynamic variable.

The often difficult question is deciding what to include in the system. Since the point of the theory is to use the values of known variables to calculate the values of unknown ones this determines the boundaries of the system.

So if we have a perfectly insulating container and we are calculating heat changes within the system, we don't care about the container as no heat passes through it or to it. So we don't include the container in the system.

Hope this helps

go well