Zinc oxide (ZnO) is an amphoteric oxide, meaning it can act as both an acid and a base depending on the reaction it is involved in. Let's analyze each reaction step by step.

Step 1: Analyze Reaction (a)

Reaction: $\mathrm{ZnO}+{\mathrm{Na}}_{2}O\to {\mathrm{Na}}_2\mathrm{Zn}{O}_2$

Here, ZnO reacts with sodium oxide (Na₂O), which is a basic oxide. The product is sodium zincate (Na₂ZnO₂). In this context, ZnO is accepting the oxide ion (O^2-) from Na₂O, behaving as an acid. Acids are defined as species that can accept a pair of electrons or, in oxide terms, react with bases to form salts.

Thus, in reaction (a), ZnO acts as an acid.

Step 2: Analyze Reaction (b)

Reaction: $\mathrm{ZnO}+{\mathrm{CO}}_2\to \mathrm{ZnC}{O}_3$

Here, ZnO reacts with carbon dioxide (CO₂), which is an acidic oxide. The product is zinc carbonate (ZnCO₃). In this case, ZnO is donating its oxide ion to CO₂, behaving as a base. Bases are defined as species that can donate a pair of electrons or, in oxide terms, react with acids to form salts.

Thus, in reaction (b), ZnO acts as a base.

Final Answer

Therefore, in the given reactions, ZnO is respectively acting as an acid and a base.

The correct option is: acid and base.

Related Topics and Formulae

Amphoteric Oxides

Amphoteric oxides can react with both acids and bases to form salts and water. Common examples include ZnO, Al₂O₃, and PbO.

Acid-Base Definitions

• Lewis Acid: Electron pair acceptor.
• Lewis Base: Electron pair donor.
• In oxide chemistry:
  • Basic oxides donate O^2- ions.
  • Acidic oxides accept O^2- ions.

Reaction Types

Reaction (a) is an example of an acid-base reaction where ZnO (acid) reacts with Na₂O (base). Reaction (b) is another acid-base reaction where ZnO (base) reacts with CO₂ (acid).