Question

Give one relevant observation for the following: 

(a) Sodium hydroxide is added dropwise to Calcium nitrate solution. 
(b) Dilute Hydrochloric acid is added to Iron (II) sulphide. 
(c) An amphoteric metal is added to hot concentrated alkali. 
 

Hint:

To distinguish between Calcium and Magnesium salts using NaOH: Calcium forms a white precipitate that is sparingly soluble, whereas Magnesium forms a white precipitate that remains strictly insoluble in excess NaOH!

Answer 1

(a) Sodium hydroxide is added dropwise to Calcium nitrate solution:
A white precipitate of calcium hydroxide is formed.
The precipitate is slightly soluble in excess sodium hydroxide.

(b) Dilute Hydrochloric acid is added to Iron (II) sulphide:
A gas with a rotten egg smell (hydrogen sulphide gas, H₂S) is evolved.
The gas is colourless but has a characteristic unpleasant odour.

(c) An amphoteric metal is added to hot concentrated alkali:
Hydrogen gas is evolved with brisk effervescence.
The metal dissolves forming a complex salt (such as sodium aluminate or zincate).

Final Observations:
(a) White precipitate formed
(b) Rotten egg smelling gas evolved
(c) Hydrogen gas evolved with effervescence