This article does not cite any references or sources. (December 2007) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

An isobaric process is a thermodynamic process in which the pressure stays constant: Δp = 0 The term derives from the Greek isos, "equal," and barus, "heavy." The heat transferred to the system does work but also changes the internal energy of the system:

According to the first law of thermodynamics, where W is work done by the system, U is internal energy, and Q is heat. Pressure-volume work (by the system) is defined as: (Δ means change over the whole process, it doesn't mean differential)

but since pressure is constant, this means that

Applying the ideal gas law, this becomes

assuming that the quantity of gas stays constant (e.g. no phase change during a chemical reaction). Since it is generally true that

then substituting the last two equations into the first equation produces:

The quantity in parentheses is equivalent to the molar specific heat for constant pressure:

and if the gas involved in the isobaric process is monatomic then and .

An isobaric process is shown on a P-V diagram as a straight horizontal line, connecting the initial and final thermostatic states. If the process moves towards the right, then it is an expansion. If the process moves towards the left, then it is a compression.

Defining Enthalpy

An isochoric process is described by the equation Q = ΔU. It would be convenient to have a similar equation for isobaric processes. Substituting the second equation into the first yields

The quantity U + p V is a state function so that it can be given a name. It is called enthalpy, and is denoted as H. Therefore an isobaric process can be more succinctly described as

Variable density viewpoint

A given quantity (mass M) of gas in a changing volume produces a change in density ρ. In this context the ideal gas law is written

where T is thermodynamic temperature above absolute zero. When R and M are taken as constant, then pressure P can stay constant as the density-tempertature quadrant (ρ,T ) undergoes a squeeze mapping. It is this context that explains Peter Olver's use of the term isobaric group when referring to the group of squeeze mappings on page 217 of his book Classical Invariant Theory (1999).