What describes comproportionation in chemical reactions?

Comproportionation is a specific type of redox reaction where two species containing the same element in different oxidation states react to form a product where that element is present in a single intermediate oxidation state. When considering this definition, the correct choice highlights that two species of the same element combine, ultimately resulting in a single product.

For example, when a lower oxidation state of an element, such as copper(I) [Cu(I)], reacts with a higher oxidation state, such as copper(II) [Cu(II)], it can yield a common species such as Cu(I) or Cu(II). This illustrates how two different oxidation states of the same element come together in the product.

The other choices do not accurately capture the essence of comproportionation. The formation of two species with different oxidation states does not describe the unique aspects of this reaction because comproportionation yields a single product with a specific oxidation state. The conversion of elements to their gaseous forms and the reaction of two different elements to form a compound also stray from the definition, as they do not involve any change in oxidation states of a single element. Thus, the defining characteristic of involving two species of the same element leading to a common product makes the correct choice clear.