Question

An element X on exposure to moist air turns reddish-brown and a new compound Y is formed. The substance X and Y are:

• A. X = Fe, Y = Fe₂O₃
• B. X = Ag, Y = Ag₂S
• C. X = Cu, Y = CuO
• D. X = Al, Y = Al₂O₃

Answer 1

The correct option is A

X = Fe, Y = Fe₂O₃

Explanation for the correct option:

(A) The exposure of element X to moist air forms a reddish-brown Y.

The reddish-brown compound is known as rust Fe₂O₃

$\underset{\left(\mathrm{Iron}\right)}{4\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{3{\mathrm{O}}_2\left(\mathrm{g}\right)}\to 2\underset{\mathrm{Iron}\left(\mathrm{III}\right)\mathrm{oxide}}{{\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{s}\right)}$

Therefore, X = Fe and Y = Fe₂O₃

Explanation for the incorrect options:

(B) Silver (Ag) reacts with Sulphur (S) present in the air to form Silver sulphide (Ag₂S) which is black in colour.

(C) Copper (Cu) reacts with moist air along with Carbon dioxide (CO₂) to form a green layer of Copper carbonate (CuCO₃) and Copper hydroxide [Cu(OH)₂]

(D) Aluminium (Al) reacts with air to form a white layer of Aluminium oxide (Al₂O₃).

Hence, option (A) is correct.