During the course of the experiment, which of the following happens to the NO3- ions? — Solutions Chemistry Question

STEPS:

1. Identify the chemical reaction occurring in the experiment: The student mixes an aqueous solution of an unknown metal chloride ($\text{MCl}$) with an aqueous solution of silver nitrate ($\text{AgNO}_3$). The balanced molecular equation for this double-replacement precipitation reaction is:
$$\text{MCl}(aq) + \text{AgNO}_3(aq) \rightarrow \text{AgCl}(s) + \text{MNO}_3(aq)$$
2. Write the complete ionic equation: To see the physical state of all species in the mixture, we write them as they actually exist in the solution:
$$\text{M}^+(aq) + \text{Cl}^-(aq) + \text{Ag}^+(aq) + \text{NO}_3^-(aq) \rightarrow \text{AgCl}(s) + \text{M}^+(aq) + \text{NO}_3^-(aq)$$
3. Identify the spectator ions: Notice that the metal cation ($\text{M}^+$) and the nitrate anion ($\text{NO}_3^-$) appear in the exact same state (as dissociated, hydrated aqueous ions) on both the reactant and product sides of the equation. This indicates that they do not participate directly in the chemical reaction; they are spectator ions.
4. Understand the filtration process: During filtration, the insoluble solid precipitate ($\text{AgCl}$) is trapped by the filter paper. The liquid that passes through the filter paper is called the filtrate. Because nitrate ions ($\text{NO}_3^-$) are spectator ions and all common nitrate salts are highly soluble, they remain fully dissolved in the water and pass completely into the filtrate. Therefore, they remain dissolved in the filtrate solution, matching Option D.

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WHY_OTHERS_WRONG:

• A is incorrect: This precipitation reaction is a non-redox metathesis reaction. Neither chloride ($\text{Cl}^-$) nor nitrate ($\text{NO}_3^-$) undergoes a change in oxidation state; nitrogen remains in the $+5$ state, and chlorine remains in the $-1$ state. Furthermore, $\text{Cl}^-$ is a very weak reducing agent and cannot oxidize or reduce $\text{NO}_3^-$.
• B is incorrect: Because there is no transfer of electrons (redox) occurring in this precipitation reaction, nitrate ions are not reduced to nitrite ($\text{NO}_2^-$) or any other species.
• C is incorrect: Nitrate ions are exceptionally stable in aqueous solutions and do not decompose. They do not chemically react with the metal cation $\text{M}^+$ because all common nitrate salts are highly soluble, meaning $\text{M}^+$ and $\text{NO}_3^-$ simply coexist as separate dissolved ions in solution.