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.
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(a) A white precipitate of calcium hydroxide is formed, which is insoluble in excess sodium hydroxide.
$Ca(NO_{3})_{2} + 2NaOH \longrightarrow \underset{[\text{white ppt.}]}{Ca(OH)_{2} \downarrow} + 2NaNO_{3}$
(b) A colourless gas with the smell of rotten eggs, hydrogen sulphide (H2S), is evolved with effervescence.
FeS + 2HCl ⟶ FeCl2 + H2S ↑
(c) The amphoteric metal (e.g., zinc or aluminium) dissolves in the hot concentrated alkali showing vigorous effervescence (bubbling) due to the evolution of a colourless, odourless gas. This gas burns with a "pop" sound when a burning splinter is brought near the mouth of the test tube, confirming it is hydrogen.
$Zn + 2NaOH \longrightarrow \underset{\text{(sodium zincate)}}{Na_{2}ZnO_{2}} + H_{2} \uparrow$