A new model is proposed that explains the anisotropy of the etch rate of single crystalline silicon in certain etchants. It is inspired from theories of crystal growth. We assume that the (111)-face is flat on an atomic scale. Then the etch rate should be governed by a nucleation barrier of one atomic layer deep cavities. The origin of the nucleation barrier is that the formation of a too small cavity increases the free energy of the system due to the step-free energy. The step-free energy and the undersaturation governs the activation energy of the etch rate. Having the largest step-free energy, the (111)-face etches the slowest. The model explains qualitatively why the etching is isotropic in certain etchants and anisotropic in others