In the second typedef, a template class pureMixture is used. This says, that our fluid is a single specie, not a mixture, and is most common of mixture types. Different classes are used for multiphase simulations, see following list:

• pureMixture
• reactingMixture
• homogeneousMixture
• inhomogeneousMixture
• veryInhomogeneousMixture
• egrMixture
• multiComponentMixture
• singleStepReactingMixture

Source code for class pureMixture can be found found in

$FOAM_SRC/thermophysicalModels/basic/mixtures/pureMixture

in files

pureMixture.C
pureMixture.H

Other classes representing type of mixture have code in
$FOAM_SRC/thermophysicalModels/reactionThermo/mixtures/

in subdirectories named after respective mixture types, eg.

homogeneousMixture/homogeneousMixture.C
homogeneousMixture/homogeneousMixture.H

In the figure  is the inheritance diagram for pureMixture. Note, that this class does not inherit from its template parameter. This breaks an inheritance chain, which can be followed from the base class (i.e. specie in this case) up to sutherlandTransport. As a consequence of this, pureMixture and classes derived from it cannot directly access members of sutherlandTransport. Instead, pureMixture and other mixture classes contain member function to do so.