What is meant by para hydrogen ? How is it converted into ortho hydrogen ? [1+1]

Para-hydrogen refers to the molecular form of hydrogen ($\mathrm{H}{}_2$) in which the nuclear spins of the two hydrogen atoms are antiparallel (paired). This means the spins of the two protons are in opposite directions, resulting in a net spin quantum number $I=0$. Para-hydrogen is the lower-energy state and is more stable at lower temperatures.

Conversion to ortho-hydrogen occurs when the nuclear spins of the two hydrogen atoms become parallel (both spins aligned in the same direction), giving a net spin quantum number $I=1$. This conversion can be achieved by:

• Catalytic conversion using catalysts like activated carbon, iron oxide, or platinum at elevated temperatures.
• Thermal equilibrium: At room temperature, hydrogen exists as a 3:1 mixture of ortho- and para-hydrogen. Cooling the gas shifts the equilibrium toward para-hydrogen, while heating favors ortho-hydrogen. The conversion is slow in the absence of a catalyst.