A general equation was derived for the rate of reaction of a porous solid with liquid reactant. Spherical porous are assumed to be distributed uniformly within the spherical particle. The equation takes into consideration the intrinsic rate constant, intra-particle diffusion, and the specific surface area of pores, which increases as the reaction progresses. For the highly porous solid, the general equation holds until the pores overlap and sub-grains form within the particle.