Isotopes — Physical Separation Techniques Isotopes (of the same element) are most practically separated by which of the following processes?

Introduction / Context:
Isotopes of an element have identical chemical properties but slightly different physical properties because of their different masses. Industrial and laboratory isotope separation relies on exploiting tiny mass-dependent effects (e.g., vapor pressure differences, diffusion rates). This question asks which listed technique is relevant for separating isotopes in practice.

Given Data / Assumptions:

• Choices are common physical/chemical separation methods.
• We target a method that can exploit small vapor pressure differences between isotopic molecules (e.g., UF6 for uranium).
• Simplified exam context; advanced methods like centrifugation or mass spectrometry are not listed.

Concept / Approach:
Because isotopes behave almost identically chemically, methods based on chemical reactivity (crystallisation, filtration) are ineffective. Distillation can separate isotopic species if they form volatile compounds with slightly different vapor pressures (fractional distillation). Real-world examples include separation of hydrogen isotopes (H2, HD, D2) and oxygen/nitrogen isotopes in cryogenic air distillation, as well as enrichment steps involving UF6 in uranium processing (alongside diffusion/centrifugation).

Step-by-Step Solution:

Verification / Alternative check:
Cryogenic fractional distillation of liquid air yields slight isotopic enrichment (e.g., O-18). In nuclear industry history, gaseous diffusion and centrifugation are primary, but among given options, distillation is the best fit for isotope separation.

Why Other Options Are Wrong:

• crystallisation: Chemical identity dominates; isotopes co-crystallize.
• sublimation: Limited use; not a general isotope method.
• filtration: Physical particle size/phase separation; not applicable to isotopic mixtures.

Common Pitfalls:
Believing any standard lab separation works. Isotopic differences are subtle; only processes exploiting mass-dependent physical properties (diffusion, centrifugation, distillation) are relevant.

Final Answer:
distillation