A watch glass containing some powdered substance is placed between the pole pieces of a magnet. Deep concavity is observed at the center. The substance in the watch glass is.
(A) Iron
(B) Chromium
(C) Carbon
(D) Wood

Hint:Magnet is placed in the watch. So the substance should be ferromagnetic. If it is ferromagnetic, it will tend to be attracted to a magnet.

Complete step by step answer:
Ferromagnetic materials are those materials which exhibit a spontaneous net magnetization at the atomic level, even in the absence of an external magnetic field. When placed in an external magnetic field, ferromagnetic materials are strongly magnetized in the direction of the field. According, to the data mentioned in the question, the material present in the watch glass shows concavity due to the presence of magnetic force. This means there should be a magnetically attracted material present in the glass. Now, this is only possible when the substance is ferromagnetic. And, among the four substances mentioned in the question, only the iron has such ferromagnetic properties. But chromium can show ferromagnetic property in compound form. But in option it is not mentioned that chromium is in the compound stage, that's why chromium is omitted in this case.
So, the substance in the watch glass is iron. Hence, option A is correct.

Note:Normal watch glass is flattened. Here concavity is observed. If a glass contains some ferromagnetic impurity it will curve towards the magnetic poles. Here only Iron shows ferromagnetic properties in its pure stage.