How do specific surface area and particle size distribution change when granular media dissolve?

The measured dissolution rate of a granular medium depends on its surface area and how the changes during course measurement. Moreover, assumption that specific either remains constant or initially increases is not always valid. This paper demonstrates when particle size distribution has sufficient variance, instantaneous change in can be negative, even before smallest particles dissolve away. concept explained using spherical particles, extended for use with prismatic demonstrated experimentally gypsum powder. For commercial powder used, decreases by about 50% first 10% mass loss water, so this effect may have practical importance significant impact uncertainty reported rates batch reactors.