X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometer (SSIMS) have been applied to characterize Si(111) surface treated with 40% NH4F solution. As-treated Si(111) surface is predominately terminated with monohydrides and free of contamination. The dynamics of etching process on ultra-clean atomically flat hydrogen-terminated Si(111) surface in 40% NH4F solution has been examined at various potentials including the open circuit potential(OCP) by using in situ electrochemical scanning tunneling microscopy (ECSTM). Two distinct mechanisms are observed: (1) dihydride terminated step silicon atoms are dissolved much faster than the defect-free monohydride terminated ones; (2) dihydrides in the corner of {110} zigzag enclosed characteristic triangular pits lead to fast erosion of silicon atoms. Our results explain dynamic mechanics in formation of triangular pits on atomically flat silicon (111) surface in solution.