[philonossis]

"Specify the physical state of a pure substance at the following condition or indicate that the state determination is not possible from the information given."

(d) at its boiling point

My answer: physical state is a gas.

Book answer: not possible to determine.

My question is: is the book's answer because the pure substance could be a compound that is decomposing due to the heat into more basic substances, e.g., could both a gas and a solid residue be forming at boiling point?


Definition of pure substance:

"Matter can be classified in terms of its chemical composition as a pure substacne or a mixture.  A pure substance is a single kind of matter that cannot be separated into other kinds of matter using physical means. All samples of apure substance contain only that substance and nothing else. Pure water is water and nothing else. A substance always has a definite and constant composition. . All samples of a pure substance, no matter what their source, have the same properties under 5the same conditions...."
--- source: Introduction to Chemical Principles, Stoker.

[Yggdrasil]

The definition of boiling point is the temperature at which the liquid and gas phases of a substance are at equilibrium.  Therefore, at a substance's boiling point, you could have a liqiud, a gas, or a mixture of the two.