Respuesta :
The pink color in the solution fades. Some of the colored indicator ion converts to the colorless indicator molecule.
Explanation
What's the initial color of the solution?
[tex]\text{NH}_4\text{Cl}[/tex] is a salt soluble in water. [tex]\text{NH}_4\text{Cl}[/tex] dissociates into ions completely when dissolved.
[tex]\text{NH}_4\text{Cl} \; (aq)\to {\text{NH}_4}^{+} \; (aq) +{\text{Cl}}^{-} \; (aq)[/tex].
The first test tube used to contain [tex]\text{NH}_4\text{OH}[/tex]. [tex]\text{NH}_4\text{OH}[/tex] is a weak base that dissociates partially in water.
[tex]\text{NH}_4\text{OH} \; (aq) \rightleftharpoons {\text{NH}_4}^{+} \;(aq)+ {\text{OH}}^{-} \; (aq)[/tex].
There's also an equilibrium between [tex]\text{OH}^{-}[/tex] and [tex]{\text{H}_3\text{O}}^{+}[/tex] ions.
[tex]{\text{OH}}^{-}\;(aq) + {\text{H}_3\text{O}}^{+} \;(aq) \to 2\; \text{H}_2\text{O} \;(l)[/tex].
[tex]\text{OH}^{-}[/tex] ions from [tex]\text{NH}_4\text{OH}[/tex] will shift the equilibrium between [tex]\text{OH}^{-}[/tex] and [tex]{\text{H}_3\text{O}}^{+}[/tex] to the right and reduce the amount of [tex]{\text{H}_3\text{O}}^{+}[/tex] in the solution.
The indicator equilibrium will shift to the right to produce more [tex]{\text{H}_3\text{O}}^{+}[/tex] ions along with the colored indicator ions. The solution will show a pink color.
What's the color of the solution after adding NH₄Cl?
Adding [tex]\text{NH}_4\text{Cl}[/tex] will add to the concentration of [tex]{\text{NH}_4}^{+}[/tex] ions in the solution. Some of the [tex]{\text{NH}_4}^{+}[/tex] ions will combine with [tex]\text{OH}^{-}[/tex] ions to produce [tex]\text{NH}_4\text{OH}[/tex].
The equilibrium between [tex]\text{OH}^{-}[/tex] and [tex]{\text{H}_3\text{O}}^{+}[/tex] ions will shift to the left to produce more of both ions.
[tex]{\text{OH}}^{-}\;(aq) + {\text{H}_3\text{O}}^{+} \;(aq) \to 2\; \text{H}_2\text{O} \;(l)[/tex]
The indicator equilibrium will shift to the left as the concentration of [tex]{\text{H}_3\text{O}}^{+}[/tex] increases. There will be less colored ions and more colorless molecules in the test tube. The pink color will fade.