At room temperature, helium is an extremely light, colorless, odorless, and tasteless monatomic gas. It is the most difficult of all gases to liquefy and is a substance that cannot solidify under standard atmospheric pressure. Once liquefied, as its temperature drops to 2.174 K, it exhibits unique properties such as extremely low surface tension, exceptionally high thermal conductivity, and negligible viscosity. Liquid helium can be utilized to achieve cryogenic temperatures approaching absolute zero.
Chemically, helium is highly inert; however, when subjected to a low-pressure electrical discharge, it emits a deep yellow glow. Generally, it does not form chemical compounds; yet, when excited within a low-pressure discharge tube, it can transiently form ions and molecules such as He²⁺ and HeH. Under ordinary conditions, it is extremely difficult for helium to react with other substances, though under specific conditions, it can form compounds with certain metals. As liquid helium's temperature drops to 2.18 K, its properties undergo a sudden, dramatic transformation: its viscosity becomes virtually zero-becoming a "superfluid" capable of flowing upward along the walls of its container-and its thermal conductivity becomes 800 times greater than that of copper, rendering it an exceptionally efficient thermal conductor; furthermore, its specific heat capacity, surface tension, and compressibility all exhibit anomalous behaviors. This extraordinary fluid is designated as Liquid Helium II, while the normal liquid state is referred to as Liquid Helium I.
