Properties/State Of A System OR Thermodynamic Properties

Each system has certain characteristics by which its physical conditions are described. These behaviour/characteristics of the system are called the "Properties Of The System". And the value of these properties of the system describes the state of a physical system. Properties of a system includes pressure, temperature, volume, density, heat capacity, internal energy, enthalpy, entropy, weight, mass, Gibbs free energy, specific heat, specific volume, etc. 
These properties are classified into three different categories namely "Reference Properties", "Energy Properties" & "Derived Properties". 
First we understand about the state of thermodynamic system then we will understand the properties of the system which describe its state.

Thermodynamics State Or State Of A System
Since, we have seen above that we can define the thermodynamics state or state of a system only when the system is in thermodynamic equilibrium. 
The thermodynamic state can be described completely by specifying any two of the three quantities pressure(P), volume(V) & temperature(T). These quantities are known as thermodynamic parameters or thermodynamics variables of the system. 

Let Us Now Define The State Of The System
Assume that the system is in thermodynamic equilibrium then the state of a system is defined as- "When the condition of the system is completely described at a specific time by the properties of the system such as temperature, pressure, volume, etc, it is called the state of the system". Any two of these quantities are sufficient to describe the thermodynamic state completely.
The simplest example of a system to which thermodynamic can be applied is a single chemically defined homogeneous substance.
For Examples- 
A human body is an open system and its states are described by parameters like pressure, temperature, etc. at a particular time.

Let Us Now Understand Thermodynamics Properties
As we know that thermodynamics properties are the characteristics of a system by which the system can be specified. These thermodynamic properties are broadly classified into three types namely;
(1) REFERENCE PROPERTY
These properties are the ones that are used to define the state of the system. All types of extensive and intensive properties are fall into this category. 
For Example 
Pressure, Temperature, Volume, Mass, 
(2) ENERGY PROPERTY
These properties are dependent on reference properties. These properties are the fundamental properties of thermodynamics because these are related to the fundamental laws of thermodynamics.
For Example
• Internal Energy (U) is dependent on the pressure, temperature and volume of the system 
Similarly,
• Enthalpy (H)
• Helmholtz free energy (A) 
• Gibbs free energy (G)
(3) DERIVED PROPERTY 
These properties are partial derivatives of reference properties or energy properties. 
For Example
Joule-Thomson coefficient, Specific Heat, Coefficient of expansion, Coefficient of compressibility, etc.

Types Of Reference Properties
Reference properties are further categorize into three types namely: "Extensive Properties", "Intensive Properties" & "Specific Properties". Let us now look at the types of reference properties in detail;

(A) Extensive Properties
"Extensive properties of a system are those properties whose value depends on the size/mass of matter present in the system". 
These properties are used for describing a sample because they change the internal structures. 
The values of comprehensive/extensive properties are not uniform throughout the system. 
Extensive properties are additive: 
this means that when we divide the system into several subsystems then the total value of the property for the whole system is the sum of their values for each subsystem (therefore, these properties are additive in nature). 
For Example- 
Volume, mass, energy, enthalpy, entropy, heat capacity, weight, Gibbs free energy, Helmholtz free energy, electrical resistance, internal energy, length, etc.
          
Mass is a extensive property. Let us understand it through an example. A cube, which is divided into 8 small pieces. When we measure its weight, we find that the mass of that big cube is equal to the sum of the weights of all these 8 smaller cubes.

(b) Intensive Properties
"Intensive properties of a system are those properties whose value does not depend on the size/mass of matter present in the system". 
These properties are used for the identification of samples because they involve the physical change that can be observed easily.
The values of the intensive properties are the same throughout the system. This is the exact opposite of extensive property. 
These are sometimes called "Point Properties", because these properties are defined at a point in space. Therefore these properties may vary from one place to another in space.
Extensive properties are not additive: 
This means that when we divide the system into several subsystems then the total value of the property for the whole system is not the sum of their values for each subsystem. 
For Example- 
Temperature, Pressure, Viscosity, Density, Melting Point, Freezing Point, Flammability, Refractive index, Color (in solution), Thermal Conductivity, Odor, Malleability, Ductility, Surface tension, hardness, etc.

Temperature is an intensive property because the temperature of one drop of water is the same as the temperature of one glass of water meaning it is independent of amount.

(C) Specific Properties
"Specific properties are an example of intensive properties because, like intensive properties, the values of specific properties do not depend on the mass/size of matter present in the system".
Specific properties show a relationship between extensive and intensive properties. With the help of specific properties, we can convert extensive properties into intensive properties.
       
"Specific properties are defined as extensive properties per unit mass" and are denoted by lower case letters.  
For example-
      specific volume (v) ➝ V/m＝ v
      specific enthalpy (h) ➝ H/m＝ m
      specific energy (e) ➝ E/m＝ e
                   and so on.......
The above relations are valid if the specific properties are the same throughout the system. For example, to calculate the total volume of a system we need to know the value of the specific volume and mass of the system as well as the value of the specific volume must be the same throughout the system.

Important Points
☛ All the properties of a system in a given state have fixed values and when the value of one of these properties changes, the state of the system also changes.
☛ Whenever the properties of the system become the same as their previous values, we can say that the system has returned to its previous state. 
☛ Specific property is a type of intensive property which is always given on a unit mass basis.
☛ It is important to note that Properties describe states only when the system is in a state of equilibrium. So unless the system is in a state of equilibrium, properties cannot describe states.
☛ Extensive properties are those whose value fluctuates in response to any alteration in the size or shape of the system, otherwise known as intensive properties.
☛ Molar Properties
"The properties are referred to as molar properties rather than specific property when the extensive property is divided by the number of moles (i.e., the extensive property per unit mole)". Molar property refers to the properties of one mole of a substance within the system.

FAQs
Why is the ratio of two extensive properties an intensive property?
The ratio of two extensive properties is an extensive property. This is because both these properties cancel each other’s amount dependency and we get an intensive property.
For example, density is the ratio of mass and volume. Both these quantities are extensive properties but density is an intensive property because the ratio of two extensive properties is an intensive property.
Is time an intensive property?
Time is always an intensive property because it never depends on mass or amount.

Hope you have found this article helpful!!
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