Question

What do you observe when:
(i) Small piece of Mg ribbon is dropped in dil. H₂SO₄ .
(ii) When oxidizing agents are heated
(iii) CuO reacts with H₂
(iv) Red hot iron reacts with steam
(v) Copper salt exposed to flame test

Answer 1

Writing down observation for each reaction :

(i) When a small piece of Mg ribbon is dropped in dil. H₂SO₄ , bubbles of hydrogen gas are evolved.

Mg + dil. H₂SO₄ →  MgSO₄ + H₂

(ii) When oxidizing agents are heated, explosion occurs.

• Oxidizing agents are highly reactive species. On heating, they immediately catch fire and cause explosion.

(iii) When CuO reacts with H₂ , the black coating on the surface of CuO changes to reddish-brown.

• CuO + H₂ →  Cu + H₂O

CuO on reacting with hydrogen gets reduced to metallic copper which is reddish-brown in colour.

(iv) When red hot iron reacts with steam, hydrogen gas is bubbled out.

Fe (red hot) + H₂O (steam)  → Fe3O4 + H₂  

(v) When a copper salt is exposed to flame test, it burns with a bright bluish-green flame.

• On heating a salt, electrons get excited and jump from a lower energy level to a higher energy level by absorbing heat energy.

• Electrons are unstable in the higher energy levels and therefore, jump back to the lower energy levels by releasing the absorbed energy in the form of light.