Schematic diagram of a steam power plant


If we consider a given mass of water, we recognize that this water can exist in various forms. If it is a liquid initially, it may become a vapor when it is heated or a solid when it is cooled. Thus, we speak of the different phases of a substance. A phase is defined as a quantity of matter that is homogeneous throughout. When more than one phase is present. The phases are separated from each other by the phase boundaries. In each phase the substance may exist at various pressures and temperatures or, to use the thermodynamic term, in various states. The state may be identified or described by certain observable, macroscopic are temperature, pressure, and density.


Each of the properties of a substance in a given state has only one definite value. These properties always have the same value for a given state, regardless of how the substance arrived at the state. In fact, a property can be defined as any quantity that depends on the state of the system. Properties are independent of the path by which the system arrived at the given state. Conversely, the state is specified or described by the properties. Later we will consider the number of independent properties a substance can have. That is, the minimum number of properties that must be specified to fix the state of the substance. Thermodynamic properties can be divided into two general classes intensive and extensive. An intensive property is independent of the mass; the value of an extensive property varies directly with the mass.

Thus, if a quantity of matter in a given state is divided into two equal parts. Each part will have the same value of intensive properties as the original and half the value of the extensive properties. Pressure, temperature, and density are examples of intensive properties. Mass and total volume are examples of extensive properties. Extensive properties per unit mass, such as specific volume, are intensive properties. Frequently we will refer not only to the properties of a substance but also to the properties of a system. We necessarily imply that the value of the property has significance for the entire system, and this implies equilibrium.

Thermal equilibrium

For example, if the gas that composes the system (control mass) in Fig. The flag is in thermal equilibrium, the temperature will be the same throughout the entire system. We may speak of the temperature as a property of the system. We may also consider mechanical equilibrium, which is related to pressure. If a system is in mechanical equilibrium. There is no tendency for the pressure at any point to change with time as long as the system. The system is isolated from the surroundings. There will be variation in pressure with elevation because of the influence of gravitational forces. Although under equilibrium conditions there will be no tendency for the pressure at any location to change.

However, in many thermodynamic problems, this variation in pressure with elevation is so small that it can be neglected. Chemical equilibrium is also important and will be considered in Chapter 14. When a system is in equilibrium regarding all possible changes of state, we say that the system is in thermodynamic equilibrium.

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