Which of the following equations represents an oxidation-reduction (redox) reaction?

• A. HCl + NaOH ightarrow NaCl + H_2O
• B. CaCO_3 ightarrow CaO + CO_2
• C. AgNO_3 + NaCl ightarrow AgCl + NaNO_3
• D. Zn + Cu^{2+} ightarrow Zn^{2+} + Cu

Answer: (D)

Verified step by step guidance

1. Step 1: Understand what an oxidation-reduction (redox) reaction is. A redox reaction involves the transfer of electrons between species, resulting in changes in their oxidation states.
2. Step 2: Examine each given equation and identify if any atoms change their oxidation states from reactants to products.
3. Step 3: For the first equation, HCl + NaOH \rightarrow NaCl + H_2O, recognize this as an acid-base neutralization reaction with no change in oxidation states.
4. Step 4: For the second equation, CaCO_3 \rightarrow CaO + CO_2, this is a decomposition reaction where no oxidation states change; it is not a redox reaction.
5. Step 5: For the last equation, Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu, identify that Zn goes from 0 to +2 (oxidation) and Cu goes from +2 to 0 (reduction), confirming it is a redox reaction.