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→States and thermodynamic properties
Any substance of engineering interest can have '''states'''. For example, water can be ice, liquid water, or water vapour. If you are asked the question "What is the state of water at room temperature?" you may intuitively answer "liquid water". However, believe it or not, by reaching this conclusion you are making the assumption that the pressure that this water is at is at everyday atmospheric level. If the same water is put into vacuum while maintained at the same temperature, it will boil.
Therefore, to fully define the state that a substance is at, two thermodynamic quantities properties must be specified. The most common thermodynamic quantities properties can be:
*Temperature \(T\)
*Pressure \(p\)
*Density \( \rho \)
and a bunch of others. At least two must be known to fully define the state of a substance.
It is '''very important''' to understand that, by calling a thermodynamic property a '''property''', it is implied that it belongs to the substance. It does '''not''' depend on the way you look at it, or, in physical language, it is independent of the frame of reference. For example, when you stand still you think the air is still at a temperature of 25°C, but if you drive past in a car you will think the air is moving. This does not, however, affect the fact that the air is still at 25°C. We shall see later that we can define some other things that depends on motion, known as the stagnated '''quantities''', sometimes incorrectly referred to as "stagnation properties".
=== Ideal gas ===
