Q1: Separation of the isotopes of helium for scientific investigations is done by

ANS:B - thermal diffusion

Separation of the isotopes of helium for scientific investigations is typically done by thermal diffusion. Thermal diffusion is a process that relies on the different rates of diffusion of isotopes or molecules in a mixture due to their differing masses. In the case of helium isotopes, helium-3 (^3He) and helium-4 (^4He) have different masses, which results in different rates of diffusion when subjected to a temperature gradient. By carefully controlling the temperature gradient in a thermal diffusion column, the lighter isotope (^3He) tends to migrate towards the warmer side of the column, while the heavier isotope (^4He) migrates towards the cooler side. This differential diffusion allows for the separation of the helium isotopes, with the lighter isotope (^3He) being enriched on one side of the column and the heavier isotope (^4He) being enriched on the other side. This separation process can be repeated multiple times to achieve higher levels of isotopic purity. While reverse osmosis, leaching, and solvent extraction are important separation processes used in various applications, they are not typically used for the separation of helium isotopes for scientific investigations.