The chemical state of hydrogen-terminated (100) Si surface during storage in water (H2O) and heavy water (D2O), has been investigated in situ and in real-time using infrared spectroscopy in the multiple internal reflection geometry. We have examined how infrared absorption spectra in the Si-H stretch vibration region change while the Si surface is immersed into H2O and D2O. We demonstrate that hydrogen exchange reaction takes place on the H-terminated Si surface during storage in water; that is, the hydrogen atom in the surface Si-H bond is replaced with a hydrogen atom of the water molecule. We propose that the hydrogen exchange reaction proceeds through the formation of a negatively charged state of the surface Si atom and a hydronium ion, H3O+:Si-H + H2O --> H3O+ + Si-.