Pure Substance

A pure susbstance is defined as one that is homogeneous and invariable in chemical composition throughout its mass . The relative proportions of the chemical components are also constant in the chemical composition. Examples: atmospheric air, water-steam mixture and combustion products of a fuel etc.  But the mixture of air and liquified air is not a pure substance as the relative proportions of Oxygen and Nitrogen differ in gaseous and liquid phases. The state of a pure substance of a given mass can be fixed by specifying two independent intensive properties, provided the system is in equilibrium. This is known as two-property rule.

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Quasi-Static Process

Quasi means almost or near to. Quasi–Static process means very nearly static process. Let us consider a system of gas contained in cylinder. The gas held by a moving piston. A weight W is placed over the piston. Due to the weight , the gas in cylinder is compressed. After the gas reaches equilibrium, the properties of gas are denoted by p₁, v₁, t₁. The weight placed over the piston is balanced by upward force exerted by the gas. If the weight is removed, then there will be unbalanced force between the system and the surroundings. The gas under pressure will expand and push the piston upward till it touches the stops. The properties at this state after reaching equilibrium are p₂,v₂,t₂. But the intermediate states passed through by the system are non-equilibrium states which cannot be described by thermodynamic coordinates. In this case we only have initial and final states and do not have a path connecting them.

Suppose, the weight is made of large numbers of small weights. And one by one each of these small weights are removed and allowed the system to reach an equilibrium state. Then we have intermediate equilibrium states and the path described by these sates will not deviate much from the thermodynamic equilibrium state. Such a process, which is the locus of all the intermediate points passed by the system is known as Quasi-static process. It means, this process is almost near to the thermodynamically equilibrium process. Infinite slowness is the characteristic feature of quasi-static process.

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Thermodynamic Equilibrium

A system is in a state of thermodynamic equilibrium, when there is no change in any of its macroscopic properties, if the system is isolated from its surroundings. To have thermodynamic equilibrium, the following equilibriums are to be satisfied:

1 Mechanical Equilibrium: A system is said to be in mechanical equilibrium, if there are no unbalanced force within the system and also between the system and the surroundings.

2 Chemical Equilibrium: A system is said to be in chemical equilibrium, if there are no chemical reactions or transfer of matter from one part of the system to another, such as diffusion or solution.

3 Thermal Equilibrium: When a system existing in mechanical and chemical equilibrium is separated from its surrounding by a diathermic wall (a wall which allows heat energy to flow) and if there are no spontaneous change in any of the property of the system, then the system is said to exist in thermal equilibrium

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Homogeneous & Heterogeneous Systems

Matter appears in different phases at different pressures and temperatures. For example, Water at normal temperature is in liquid form. If the temperature is increased then it transforms in to gaseous form. Water also transforms into vapor if the pressure is reduced.  If the temperature is reduced below ℃ then water becomes solid ice.  So, water exists in 3 phases, solid, liquid and gas.

Any system consisting of a single phase is called Homogeneous and any system has more than one phase of the substance is called Heterogeneous.

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State, Path, Process and Cycle

1 State: When all the properties of system have a definite value, then the system is in a definite state. Whenever there is change in anyone of the property, then the system is said to have a change of state.

2 Path: If all the change of states of system are plotted and all the points are conned, then the line joining the change of states of the system is called the path.

3 Process: When the path connecting the change of states of the system is specified, then this path is called Process. Example: constant pressure process, constant volume process etc.

4 Cycle:  When a system goes through different change of states and return backs to the original state, i.e., all the properties are identical to the original state, then the system is said have gone through a thermodynamic cycle. 

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Thermodynamic Properties

Every system has some physical properties, such as volume, pressure, temperature etc. These characteristics are called properties of the system. All these properties are macroscopic. Thermodynamic properties are classified as following:

1 Intensive Property: The properties of system which are not dependent on the mass of the system are called Intensive property. Example: Pressure, Temperature. Even the system mass is reduced are increased, the temperature and pressure will not vary.

2 Extensive Property: Properties depend on the mass of the system are called Extensive Properties. Example: Volume, Weight, Energy etc. If the mass of the system increased, then the value of the extensive property will also increase.

3 Specific Extensive Properties: Extensive Properties per unit mass are called, the specific extensive property or simply specific property. Example: specific volume- Volume of a substance for unit mass. It remains same and does not depend on the mass of the system. Hence specific property is Intensive property.

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Control Volume

For thermodynamic analysis of an open system, such as turbines, compressor etc., the attention is focused on a certain volume in space. This volume is known as Control volume and surface is called Control surface. In open system, Control volume (C.V) is same as Open system. A system may have change in the boundary in some cases, where as the control volume will not change its surface.

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Types of Thermodynamic Systems

1 Isolated System
In isolated system no energy or mass cross the boundary of the system. In other words no energy transfer of mass transfer takes place across the boundary.

Example 1: Thermos flask. Since there is no heat exchange with the surroundings and no mass transfers also happening it can be taken as an isolated system.

Example 2: Universe as a total is taken as isolated system.

2 Closed System
In a closed system only energy transfer takes place across the boundary of the system. The boundary of closed system may change. But there will not be any mass flow across the boundary.

Example 1: A coolant system of an automobile can be considered as a closed system.

Example 2: A pressure cooker can also be considered as a closed system because there is no transfer of mass while cooking is happening. Only energy exchanges take place.

3 Open System
In an open system both energy and mass transfer takes place across the boundary of the system.

Example 1: Water flow through the pipes.

Example 2: Jet engines.

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Thermodynamic System

A thermodynamic system is defined as a quantity of matter in space upon which attention is concentrated in the analysis of problem. Everything outside the system is called surroundings or environment. The system is separated from surroundings by the system boundary.  The boundary of a system may be fixed or moving. A system with its surroundings is called the Universe.

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Microscopic Viewpoint

This definition is suitable for examinations. For more detailed explanation please follow the link below.

Microscopic point of view or  Statistical Thermodynamics states that a matter is composed of myriads of molecules. Each gas molecule at a given instant has certain position, velocity and energy which frequently change as a result of molecular collisions. The behavior of the gas is described by summing up the behavior of each molecule.

Detailed Explanation

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Macroscopic Viewpoint

This definition is suitable for examinations. For more detailed explanation please follow the link below.

In this view, the material or substance under consideration is comprised of a large number of molecules. The collective effects of these molecules are studied, without reference to the individual molecule.

Detailed Explanation 

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