The ability to selectively chemically functionalize silicon nitride (Si3N4) or silicon dioxide (SiO2) surfaces after cleaning would open interesting technological applications. In order to achieve this goal, the chemical composition of surfaces needs to be carefully characterized so that target chemical reactions can proceed on only one surface at a time. While wet-chemically cleaned silicon dioxide surfaces have been shown to be terminated with surficial Si-OH sites, chemical composition of the HF-etched silicon nitride surfaces is more controversial. In this work, we removed the native oxide under various aqueous HF-etching conditions and studied the chemical nature of the resulting Si3N4 surfaces using infrared absorption spectroscopy (IRAS), x-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and contact angle measurements. We find that HF-etched silicon nitride surfaces are terminated by surficial Si-F and Si-OH bonds, with slightly subsurface Si-OH, Si-O-Si, and Si-NH2 groups. The concentration of surficial Si-F sites is not dependent on HF concentration, but the distribution of oxygen and Si-NH2 displays a weak dependence. The Si-OH groups of the etched nitride surface are shown to react in a similar manner to the Si-OH sites on SiO2, and therefore no selectivity was found. Chemical selectivity was, however, demonstrated by first reacting the -NH2 groups on the etched nitride surface with aldehyde molecules, which do not react with the Si-OH sites on a SiO2 surface, and then using trichloro-organosilanes for selective reaction only on the SiO2 surface (no reactivity on the aldehyde-terminated Si3N4 surface).