Question

__________gas is used to obtain very low temperature.

Answer 1

Helium:

1. Helium is a colorless, tasteless, and odorless gas with an atomic number $2$.
2. Helium has a very low boiling point compared to other elements because of weak van der Waal's forces and small size.
3. As we go down the group the boiling points of a noble gas increase, as atomic size increases.

Noble gas	Atomic symbol	Boilling point$(°\mathrm{C})$
Helium	$\mathrm{He}$	$-269$
Neon	$\mathrm{Ne}$	$-246$
Argon	$\mathrm{Ar}$	$-186$
Krypton	$\mathrm{Kr}$	$-152$
Xenon	$\mathrm{Xe}$	$-107$

Among all elements, helium is known to possess the lowest boiling point. Therefore, Helium gas is used to obtain a lower temperature.