When the temperature of liquid helium drops to 2. 18K, its properties suddenly change, its viscosity is extremely low, and it becomes superfluid, which can flow upward along the container wall. Its thermal conductivity is 800 times that of copper, and it will become a thermal conductor with excellent thermal conductivity, with abnormal specific heat capacity, surface tension and compressibility. This abnormal liquid is called liquid helium ⅱ, and normal liquid helium is called liquid helium ⅰ.
Physical and chemical properties:
At room temperature, helium is a very light, colorless, odorless and tasteless monoatomic gas. It is the most difficult to liquefy among all gases and cannot be solidified at standard atmospheric pressure. When the temperature drops to 2. 174K after liquefaction, it has special properties such as small surface tension, strong thermal conductivity and extremely low viscosity. Liquid helium can be used to obtain a low temperature close to absolute zero.
The chemical properties are stable, and it is dark yellow when discharged at low voltage. Generally, compounds are not generated, but excited in a low-voltage discharge tube, and He2+, HeH plasma and molecules can be formed. Generally, it is difficult to react with other substances and can form compounds with some metals under certain conditions.