The general equation in conservative form is
where
* is the variable of interest (species concentration for mass transfer, temperature for heat transfer),
* is the diffusivity (also called diffusion coefficient), such as mass diffusivity for particle motion or thermal diffusivity for heat transport,
* is the velocity field that the quantity is moving with. It is a function of time and space. For example, in advection,  might be the concentration of salt in a river, and then  would be the velocity of the water flow as a function of time and location. Another example,  might be the concentration of small bubbles in a calm lake, and then  would be the velocity of bubbles rising towards the surface by buoyancy (see below) depending on time and location of the bubble. For multiphase flows and flows in porous media,  is the (hypothetical) superficial velocity. 
* describes sources or sinks of the quantity , i.e. the creation or destruction of the quantity. For example, for a chemical species,  means that a chemical reaction is creating more of the species, and  means that a chemical reaction is destroying the species. For heat transport,  might occur if thermal energy is being generated by friction.
For example, if  is the concentration of a molecule, then  describes how the molecule can be created or destroyed by chemical reactions.  may be a function of  and of other parameters. Often there are several quantities, each with its own convection–diffusion equation, where the destruction of one quantity entails the creation of another. For example, when methane burns, it involves not only the destruction of methane and oxygen but also the creation of carbon dioxide and water vapor. Therefore, while each of these chemicals has its own convection–diffusion equation, they are coupled together and must be solved as a system of simultaneous differential equations.
* represents gradient and  represents divergence. In this equation,  represents concentration gradient.