[vanderwaals]

2. What is the first change that occurs when I₂(s) is heated slowly at one atmosphere pressure?
(A) The solid melts.
(B) The solid vaporizes.
(C) The solid breaks into atoms.
(D) The solid becomes darker in color.

I do not understand why the answer is (B) and not (A), because on a phase diagram of iodine, if you increase temperature but keep pressure constant (at 1 atm) by going horizontally, the solid seems to melt first. What am I missing?

[AWK]

the first change ...heated slowly

You increase temperature slowly from RT (default), and up to 113.6 C  iodine sublimes, though partial pressure of iodine vapors is much lower than 1 Atm.

The first change for ice heated from -20 C (up to 0.01 C - triple point) is also sublimation.

[vanderwaals]

Thank you! It's more clear to me now. However, why does the problem state that the change occurs at 1 atm if it actually occurs at a lower pressure?

[AWK]

This is true for iodine up to 115 Atm (critical point). But then melting point is about 100 C (not at the triple point).
Boiling point of iodine is 184.3 C at 1 Atm. From the triple point temperature to boiling point  phase changes from liquid to gas.

[Borek]

IMHO question is poorly worded. "Heated at 1 atm" is ambiguous. If it is intended to mean "heated in an open vessel when the atmospheric pressure is 1 atm" it should state it clearly. It doesn't say what has the pressure of 1 atm. If you put a large iodine crystal in a closed vessel (no free space around) pressurized to 1 atm (say with a piston) there will be no sublimation.