The general form of the Poisson equation for an Additional Variable (non-reacting scalar) is:
| (1–177) | 
where:
- is the mixture density, mass per unit volume. 
- is the conserved quantity per unit volume, or concentration. 
- is the conserved quantity per unit mass. 
- is a volumetric source term, with units of conserved quantity per unit volume per unit time 
- is the kinematic diffusivity for the scalar.