At absolute zero temperature:
Ortho and para hydrogen :
A molecule of dihydrogen contains two atoms. The nuclei of both the atoms in each molecule of dihydrogen are spinning.
Depending upon the direction of the spins of the nuclei, hydrogen is of two types.
1. Ortho hydrogen
2. Para hydrogen
Ordinary dihydrogen is an equilibrium mixture of ortho and para hydrogen
ortho hydrogen⇌Para hydrogen
a. At 0 K, hydrogen contains mainly para hydrogen which is more stable.
b. At the temperature of liquefaction of air, the ratio of ortho and para hydrogen is 1:1.
c. At room temperature, the ratio of ortho to para hydrogen is 3:1.
d. Even at very high temperatures, the ratio of ortho to para hydrogen can never be more than 3:1.
Thus, it is possible to get pure para hydrogen by cooling ordinary hydrogen gas to a very low temperature (close to 20 K)
But it is never possible to get a sample of hydrogen containing more than 75% of ortho hydrogen. i.e., Pure ortho hydrogen cannot be obtained.