Q.State, with reasons based on oxidation-number change, whether each of the following is a redox reaction:

[!TLDR] Check whether any element's oxidation number changes between reactants and products. [!ANSWER] (a) $\text{NaCl}+\text{AgNO}_3\to\text{AgCl}+\text{NaNO}_3$ is NOT a redox reaction (no oxidation number changes);

(b) $\text{Zn}+2\text{HCl}\to\text{ZnCl}_2+\text{H}_2$ IS a redox reaction (Zn: $0\to+2$; H: $+1\to0$).

(a) In $\text{NaCl}(aq) + \text{AgNO}_3(aq) \to \text{AgCl}(s) + \text{NaNO}_3(aq)$, assign oxidation numbers: Na is $+1$ on both sides, Cl is $-1$ on both sides, Ag is $+1$ on both sides, and the $\text{NO}_3^-$ group is unchanged throughout. No atom changes oxidation number anywhere in this equation — it is simply an ion-exchange (double-displacement/precipitation) reaction, so it is not a redox reaction.\n\n(b) In $\text{Zn}(s) + 2\text{HCl}(aq) \to \text{ZnCl}_2(aq) + \text{H}_2(g)$, zinc goes from oxidation number $0$ (free element) to $+2$ in $\text{ZnCl}_2$ — oxidation. Hydrogen goes from $+1$ (in HCl) to $0$ (free $\text{H}_2$ gas) — reduction. Since two elements change oxidation number in a paired, compensating way, this is a redox reaction (zinc is the reducing agent, $\text{H}^+$ from HCl is the oxidizing agent). [!ANSWER] (a) $\text{NaCl}+\text{AgNO}_3\to\text{AgCl}+\text{NaNO}_3$ is NOT a redox reaction (no oxidation number changes); (b) $\text{Zn}+2\text{HCl}\to\text{ZnCl}_2+\text{H}_2$ IS a redox reaction (Zn: $0\to+2$; H: $+1\to0$).

Assign oxidation numbers to every atom on both sides of each equation and compare; any change identifies a redox reaction.

Assuming any reaction between an acid and a metal, or any reaction involving ions, is automatically redox — precipitation/exchange reactions like (a) are common non-redox counter-examples.