_{}
-------------(1)

Where *J *is the
rate of transfer of solute per unit area or the diffusion flux.

*C* is the concentration of solute

*x* is the co-ordinate axis in the
direction of the solute flow.

*t* is the diffusion time

*D* is diffusivity

·
Equation
(1) is called **Fick's first law of
diffusion**.

·
It
states that the local rate of transfer of solute per unit area per unit time is
proportional to the concentration gradient of the solute, and defines the
proportionality constant as the **diffusivity**
of the solute.

·
The
negative sign on the right hand side of the Equation (1) states that the matter
flows in the direction of decreasing solute concentration i.e. gradient is
negative.