In thermodynamics, a physical property is any property that is measurable, and whose value describes a state of a physical system. Thermodynamic properties are defined as characteristic features of a system, capable of specifying the system's state. Some constants, such as the ideal gas constant, R, do not describe the state of a system, and so are not properties. On the other hand, some constants, such as K_f (the freezing point depression constant, or cryoscopic constant), depend on the identity of a substance, and so may be considered to describe the state of a system, and therefore may be considered physical properties.

"Specific" properties are expressed on a per mass basis. If the units were changed from per mass to, for example, per mole, the property would remain as it was (i.e., intensive or extensive).

Work and heat are not thermodynamic properties, but rather process quantities: flows of energy across a system boundary. Systems do not contain work, but can perform work, and likewise, in formal thermodynamics, systems do not contain heat, but can transfer heat. Informally, however, a difference in the energy of a system that occurs solely because of a difference in its temperature is commonly called heat, and the energy that flows across a boundary as a result of a temperature difference is "heat".

Altitude (or elevation) is usually not a thermodynamic property. Altitude can help specify the location of a system, but that does not describe the state of the system. An exception would be if the effect of gravity need to be considered in order to describe a state, in which case altitude could indeed be a thermodynamic property.

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Thermodynamic properties and their characteristics

Property	Symbol	Units	Extensive?	Intensive?	Conjugate	Potential?
Activity	a	–		Y
Chemical potential	μi	kJ/mol		Y	Particle number Ni
Compressibility (adiabatic)	βS, κ	Pa−1		Y
Compressibility (isothermal)	βT, κ	Pa−1		Y
Cryoscopic constant[1]	Kf	K·kg/mol		Y
Density	ρ	kg/m3		Y
Ebullioscopic constant	Kb	K·kg/mol		Y
Enthalpy	H	J	Y			Y
Specific enthalpy	h	J/kg		Y
Entropy	S	J/K	Y		Temperature T	Y (entropic)
Specific entropy	s	J/(kg K)		Y
Fugacity	f	N/m2		Y
Gibbs free energy	G	J	Y			Y
Specific Gibbs free energy	g	J/kg		Y
Gibbs free entropy	Ξ	J/K	Y			Y (entropic)
Grand / Landau potential	Ω	J	Y			Y
Heat capacity (constant pressure)	Cp	J/K	Y
Specific heat capacity       (constant pressure)	cp	J/(kg·K)		Y
Heat capacity (constant volume)	Cv	J/K	Y
Specific heat capacity       (constant volume)	cv	J/(kg·K)		Y
Helmholtz free energy	A, F	J	Y			Y
Helmholtz free entropy	Φ	J/K	Y			Y (entropic)
Internal energy	U	J	Y			Y
Specific internal energy	u	J/kg		Y
Internal pressure	πT	Pa		Y
Mass	m	kg	Y
Particle number	Ni	–	Y		Chemical potential μi
Pressure	p	Pa		Y	Volume V
Temperature	T	K		Y	Entropy S
Thermal conductivity	k	W/(m·K)		Y
Thermal diffusivity	α	m2/s		Y
Thermal expansion (linear)	αL	K−1		Y
Thermal expansion (area)	αA	K−1		Y
Thermal expansion (volumetric)	αV	K−1		Y
Vapor quality[2]	χ	–		Y
Volume	V	m3	Y		Pressure P
Specific volume	ν	m3/kg		Y

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• Conjugate variables
• Dimensionless numbers
• Intensive and extensive properties
• Thermodynamic databases for pure substances
• Thermodynamic variable