Question

A Brown and bright element "x" when heated in presence of air turns into black substance “y”. If Hydrogen gas is passed-over this heating material again "x" is obtained. "x" and "'y' are-

• A. Cu & CuO
• B. S & SO₂
• C. C & CO₂
• D. Na & NaH

Answer 1

The correct option is A

Cu & CuO

Oxidation and reduction

When a metal is heated in the presence of air, it reacts with oxygen present in the air to form its corresponding oxide. This reaction is called metal oxidation. The oxide thus formed reacts with hydrogen to replenish the metal along with water. This reaction is called reduction.

In the given reaction, the metal first undergoes oxidation when heated in air to form its oxide which when reacted with hydrogen forms the metal in its elemental form. Therefore, the two compounds will be related as metal and its oxide.

(A) Copper (Cu) is a reddish brown metal. When it is heated in air, it forms Cupric oxide (CuO) which is black in colour. The Cupric oxide on reduction with hydrogen gives Copper along with Water (H₂O) as the byproduct. Therefore, option (A) is correct.

The oxidation reaction is:

$2Cu\left(s\right)+O_2\left(g\right)\to 2CuO\left(s\right)$

(Copper) (Oxygen) (Cuprous oxide)

The reduction of cuprous oxide is:

$CuO\left(s\right)+H_2\left(g\right)\to Cu\left(s\right)+H_{2}O\left(l\right)$

(Cuprous (Hydrogen) (Copper) (Water)

oxide)

(B) Sulphur is pale yellow in colour. The metal given in the question is reddish brown. Option (B) is incorrect.

(C) The oxide of carbon, Carbon dioxide (CO₂) is a gas. Therefore, when Carbon (C) is burnt in air, an effervescence is observed and not a black precipitate. Option (C) is incorrect.

(D) Sodium hydride (NaH) is not the oxide of Sodium (Na). Therefore, option (D) is incorrect.

Therefore, the correct option is (A) Cu &CuO

Hence option A is correct.