1. The problem statement, all variables and given/known data On flat, leaned, surfaces, there are channels (rillenkarren), which are made by dissolution by rainwater (http://mafija.fmf.uni-lj.si/seminar/files/2005_2006/rillenkarren.pdf - not very important). They are 2-3 cm wide and around 0.5 m long. The formation is not yet known. Calculate how the concentration of calcium carbonate changes in the flow of the rainwater while it slides in a narrow layer on the stone. The homework is a part of basic mathematical physics course. So many simplifications have to be made and all the approximations have to be taken into account. Partial differential equation don't count.