You are working in your laboratory and decide to do some cleaning. You find a test tube with some brown substance congealed at the bottom. You take a piece of iron wool to scrub it out, but cannot remove it. You leave the iron wool in the test tube and set it aside to clean your beakers instead. You do not realize the test tube is next to the heater and after a few minutes, a brown gas starts smoking out of the test tube, followed by a reddish-brown gas. Upon observation, the iron wool is coated in a yellow-brown substance. You realize that the mystery substance was .

A. Argon (Ar)
B. Scandium (Sc)
C. Bromine (Br)
D. Iodide (I)

Explanation:

Argon (Ar) is a noble gas. Whose freezing point is -189 °C (very low), thus it cannot be the frozen substance. Also, it is not reactive, thus is would have not reacted with iron. Hence, argon is not the mystery substance.

Scandium (Sc) is a metal from group 3 of the periodic table, thus is will not react with iron. Thus, scandium is not the mystery substance.

Both bromine and iodine are halogens (group 17 of the periodic table).

The freezing point of bromine is −7.2 °C, and the freezing point of iodine is 113.7 °C. Thus, both could be solids (frozen) in the lab.

The reactivity of the halogens decrease from top to bottom inside the group. Bromine is above iodine. Then bromine is more reactive than iodine.

Bromine is reactive enough to react with iron. Iodine is not reactive enough to react with iron.

You can find in the internet that bromine vapour over hot iron reacts  producing iron(III) bromide. Also, that bromine vapors are red-brown.

Therefore, the mystery substance is bromine (Br).