Write one chemical equation each for the chemical reaction in which the following have taken place :

(a) Change in colour

(b) Change in temperature

(c) Formation of precipitate

Mention colour change/temperature change (rise/fall)/compound precipitated along with equation.

(a) Change in colour reaction :

$\underset {\text {Blue}}{\text{CuSO}$4 (\text {aq})} + \text {Fe(s)} \longrightarrow \underset {\text {Pale green}}{\text{FeSO}4 (\text {aq})} + \underset {\text {Reddish-brown}}{\text {Cu(s)}}

Observation : $\text{CuSO}$4 solution is blue; after the reaction the solution becomes pale green (due to $\text{FeSO}4$) and reddish-brown copper metal is deposited.

(b) Change in temperature (exothermic neutralisation) reaction :

$\text {NaOH(aq)} + \text {HCl(aq)} \longrightarrow \text {NaCl(aq)} + \text H_2 \text {O(l)} + \text {Heat}$

Observation: The mixture becomes warm (temperature rises) because neutralisation releases heat.

(c) Formation of precipitate reaction:

$\text {AgNO}$3 (\text {aq}) + \text {NaCl (aq)} \longrightarrow \underset {\text {White precipitate}}{\text {AgCl(s)}} + \text {NaNO}3 (\text {aq})

Observation: A white precipitate of silver chloride (AgCl) is formed.