A sample of elemental chlorine gas collected from seawater electrolysis consists of 76 % 35Cl and 24 % 37Cl atoms by number. Which classification and justification are most appropriate?
a) Chlorine gas is a mixture because it contains different isotopes that could, in theory, be separated physically.
b) Chlorine gas is a pure substance because isotopes are atoms of the same element and therefore show identical chemical behavior.
c) Chlorine gas is a heterogeneous mixture because regions enriched in 35Cl differ from regions enriched in 37Cl.
d) Chlorine gas is a pure compound because Cl2 molecules always contain one 35Cl and one 37Cl atom.