Question

Which one of the following statements best describes the acidic/basic/amphoteric nature of ZnO and CaO?

• A. ZnO is amphoteric, while CaO is basic.
• B. ZnO is basic, while CaO is amphoteric.
• C. Both ZnO and CaO are amphoteric.
• D. ZnO is acidic, while CaO is basic.

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.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Metal oxides can be classified as basic, acidic, amphoteric, or neutral based on how they react with acids and bases.
Most s-block metal oxides are strongly basic, whereas certain d-block and p-block metal oxides show amphoteric characteristics.
Step 2: Detailed Explanation:
Let us analyze the chemical properties of both oxides:
- Calcium Oxide ($\text{CaO}$): Calcium is a Group 2 alkaline earth metal. Its oxide reacts vigorously with water to form a strong base, calcium hydroxide, and dissolves readily in acids. It does not react with bases. Thus, $\text{CaO}$ is a purely basic oxide.
- Zinc Oxide ($\text{ZnO}$): Zinc is a d-block transition metal. Its oxide reacts with both strong acids and strong bases to form salts and water.
Reaction with an acid ($\text{HCl}$):
\[ \text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O} \]
Reaction with a base ($\text{NaOH}$):
\[ \text{ZnO} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2\text{O} \]
Because $\text{ZnO}$ exhibits both basic and acidic behaviors, it is classified as an amphoteric oxide.
Step 3: Final Answer:
The statement that best describes their nature is: ZnO is amphoteric, while CaO is basic.