ANS:A - Thermal

In thermal diffusion, also known as the Soret effect, separation is achieved based on differences in molecular mass or isotopic mass of the components in a mixture. When a temperature gradient is applied across a mixture, the heavier molecules or isotopes tend to migrate towards the colder region, while the lighter ones move towards the warmer region. This migration occurs due to the thermal gradient inducing concentration gradients, leading to a net mass transfer. In the context of separating the isotopes of methane, thermal diffusion can be utilized to exploit the small differences in mass between isotopes such as carbon-12 (^12C) and carbon-13 (^13C). By subjecting a mixture of methane isotopes to a temperature gradient, the heavier isotopes will tend to migrate towards the colder region, allowing for their separation from the lighter isotopes. Thermal diffusion is a separation process commonly employed in isotope enrichment, particularly in cases where the differences in mass between isotopes are relatively small. It offers an alternative to other separation methods like distillation or centrifugation and can be particularly useful for isotopic enrichment in research or industrial applications.