System, Surroundings, and State

For now, we consider the system itself. How do we describe it That depends on the system. For example, a biological cell is described differently from the interior of a star. But for now, let us pick a simple system, chemically speaking. 

Consider a system that consists of a pure gas. How can we describe this system Well, the gas has a certain volume, a certain pressure, a certain temperature, a certain chemical composition, a certain number of atoms or molecules, a certain chemical reactivity, and so on. If we can measure, or even dictate, the values of those descriptors, then we know everything we need to know about the properties of our system. We say that we know the state of our system. 

If the state of the system shows no tendency to change, we say that the system is at equilibrium with the surroundings. The equilibrium condition is a fundamental consideration of thermodynamics. Although not all systems are at equilibrium, we almost always use equilibrium as a reference point for understanding the thermodynamics of a system. 

How do we define the state of our system To begin, we focus on its physical description, as opposed to the chemical description. We find that we are able to describe the macroscopic properties of our gaseous system using only a few observables They are the system s pressure, temperature, volume, and amount of matter (see Table 1.1). These measurements are easily identifiable and have well-defined units. Volume has common units of liter, milliliter, or cubic centimeter. [The cubic meter is the Systtme International (SI) unit of volume but these other units are commonly used as a matter of convenience.] Pressure has common units of atmosphere, torr, pascal (1 pascal = 1 N/m and is the SI unit for pressure), or bar. Volume and pressure also have obvious minimum values against which a scale can be based. Zero volume and zero pressure are both easily definable. Amount of material is similar. It is easy to specify an amount in a system, and having nothing in the system corresponds to an amount of zero. 

The temperature of a system has not always been an obvious measurable of a system, and the concept of a minimum temperature is relatively recent. In 1603, Galileo was the first to try to quantify changes in temperature with a water thermometer. Gabriel Daniel Fahrenheit devised the first widely accepted numerical temperature scale after 

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