Question about the derivative of Gibbs Energy

[yungwun22]

Homework Statement

There is a formula which says the change (delta) of volume of a mixture is equal to the partial derivative of delta G with respect to pressure at constant temperature. I don't understand how there can be a derivative and the delta used at the same time on G. I thought that derivatives were used for infinitesimal changes and that delta was used for larger ones.

Homework Equations

The Attempt at a Solution

 

[Mapes]

Perhaps the delta signifies not the finite difference between values of G at different pressure or volume (which, as you said, would be improper to combine with differentials), but the finite difference between values of G for two separate phases of the same material.