Simple inorganic reactions in gels, such as NaOH+CuCl2, NaOH+Cu(NO3)(2), and NaOH+AgNO3, are used to obtain complex spatial patterns. When the reactions are running in a thin gel sheet, trapezoid-shaped precipitate regions emerge behind shrinking reaction fronts. Our experimental results suggest that the reaction fronts, which are segments of the borders of the precipitate regions, act as reactive surfaces where an intermediate compound is formed, and the progression of the fronts require a critical concentration of this compound. This phenomenon was modeled by the combination of reaction-diffusion equations and a generalized cellular automata. The most important qualitative features of the patterns have been reproduced by computer simulations.