Most of industrial or natural gaseous flows involve density heterogeneities that may be responsible for specific physical mechanisms: local accelerations, instabilities, segregation processes, etc. To be accurate enough, models used for numerical simulations must take these mechanisms into account. Some of these physical processes are the consequence of the mass conservation law that imposes a direct correlation between density and the velocity field. Besides, in low Mach number flows, the density field may also be expressed from scalar quantities such as temperature and/or species mass fractions via the equation of state. Accordingly, such a velocity/scalar interaction leads to direct relationships between turbulent transport terms of scalars and momentum in turbulent flows. Therefore, the objective of this work is to introduce a simple strategy able to evidence these relationships, which may lead to closures for turbulent transport terms taking large density variations effects into account. The strategy presently developed holds for flows where the value of the density can be obtained through the knowledge of a unique scalar quantity. This quantity can be a passive scalar, as pertaining for mixing between two flows featuring different density. It can also be a reactive scalar, such as the progress variable often retained to characterize fully premixed combustion. The present study considers a situation that involves both phenomena: mixing and reactive processes, as encountered in diffusion flames.