In situ and remote detection of chemical product is experimentally demonstrated using a continuous-terahertz (THz) wave sensing/imaging system with a Teflon pipe as a scanning arm. Based on THz-refractive-index-sensitive reflection and fast response of electronic THz detector, the reflective THz sensing system can be used to real-time trace and quantitatively analyze the ammonium-chloride aerosols produced by the chemical reaction between hydrochloric acid and ammonia vapors. With a tightly focusing terahertz beam spot, the spatial and concentration distributions of the generated chemical product are successfully mapped out by the 1D scan of the flexible pipe probe. The detection limit for the ammonium-chloride aerosol is estimated to be approximately 165 nmol/mm2. The reliable and compact THz pipe scan system is potentially suitable for practical applications, such as biomedical or industrial fiber endoscopy.