Which one of the following statements best describes the acidic/basic/amphoteric nature of ZnO and CaO?
Show Hint
Remember the common amphoteric oxides in inorganic chemistry using the mnemonic:
"Al, Zn, Pb, Sn, Be, Sb, As" form amphoteric oxides ($\text{Al}_2\text{O}_3$, $\text{ZnO}$, $\text{PbO}$, $\text{SnO}$, $\text{BeO}$, etc.).
Most s-block metal oxides (like $\text{CaO}$, $\text{Na}_2\text{O}$) are purely basic.
Step 1 : Understanding the Question:
This question asks us to identify the acid-base nature of two metal oxides, zinc oxide ($\text{ZnO}$) and calcium oxide ($\text{CaO}$). Step 2 : Key Formulas and Approach:
The approach involves classifying metal oxides based on their reactions with acids and bases.
Typically, highly electropositive s-block metals form basic oxides, while some d-block and p-block metals form amphoteric or acidic oxides. Step 3 : Detailed Explanation:
• Zinc Oxide ($\text{ZnO}$): Zinc is a d-block transition metal.
Its oxide, $\text{ZnO}$, exhibits both acidic and basic properties, which makes it amphoteric.
It reacts with acids to form zinc salts:
\[ \text{ZnO} + 2\text{H}^+ \rightarrow \text{Zn}^{2+} + \text{H}_2\text{O} \]
It also reacts with strong bases to form zincate complex ions:
\[ \text{ZnO} + 2\text{OH}^- + \text{H}_2\text{O} \rightarrow [\text{Zn}(\text{OH})_4]^{2-} \]
• Calcium Oxide ($\text{CaO}$): Calcium is an alkaline earth metal (s-block) with high electropositivity.
Its oxide, $\text{CaO}$, reacts vigorously with water to form calcium hydroxide (an alkaline solution) and reacts readily with acids, but does not react with bases:
\[ \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca}(\text{OH})_2 \]
Thus, $\text{CaO}$ is strongly basic. Step 4 : Final Answer:
Therefore, $\text{ZnO}$ is amphoteric, while $\text{CaO}$ is basic.
This matches Option (A).
