Ortho hydrogen molecules are those in which the spins of both the nuclei are in the same direction. Molecules of hydrogen in which the spins of both the nuclei are in the opposite direction are called para hydrogen. At 0∘K, hydrogen contains mainly para hydrogen which is more stable. At the temperature of liquefaction of air, the ratio of ortho and para hydrogen is 1:1. At the room temperature, the ratio of ortho to para hydrogen is 3:1. Even at very high temperatures, the ratio of ortho to para hydrogen can never be more than 3:1.