Sodium zincate and water is obtained on reaction of ............... with concentrated caustic soda.

Options

A. Lead
B. Sodium
C. Zinc
D. Zinc hydroxide

Explanation

To solve the question, we need to identify the reactant that, when reacted with concentrated caustic soda (sodium hydroxide, NaOH), produces sodium zincate and water.

### Step-by-Step Solution:

1. **Identify the Reactants**: The question mentions that sodium zincate and water are formed when a certain substance reacts with concentrated caustic soda. The key reactant we need to identify is one that can react with sodium hydroxide.

2. **Understand Sodium Zincate Formation**: Sodium zincate (Na2ZnO2) is a compound that can be formed from zinc compounds. Since we are looking for a reactant that can react with sodium hydroxide, we should consider zinc compounds, particularly zinc hydroxide (Zn(OH)2).

3. **Recognize the Nature of Zinc Hydroxide**: Zinc hydroxide is amphoteric, meaning it can react with both acids and bases. In this case, it will react with the base (sodium hydroxide).

4. **Write the Reaction**: The reaction between zinc hydroxide and sodium hydroxide can be represented as follows:
\[ Zn(OH)_2 + 2NaOH \rightarrow Na_2ZnO_2 + 2H_2O \]
In this reaction, zinc hydroxide reacts with sodium hydroxide to produce sodium zincate and water.

5. **Balance the Reaction**: To ensure the reaction is balanced:
- On the left side, we have 1 Zn, 2 Na, and 4 H (2 from Zn(OH)2 and 2 from NaOH) and 2 O from Zn(OH)2.
- On the right side, we have 2 Na from Na2ZnO2, 1 Zn from Na2ZnO2, 4 H from 2 H2O, and 2 O from Na2ZnO2.
- The equation is balanced as both sides have equal numbers of each type of atom.

6. **Conclusion**: The reactant that reacts with concentrated caustic soda to produce sodium zincate and water is zinc hydroxide (Zn(OH)2).

### Final Answer:
Sodium zincate and water are obtained on reaction of **zinc hydroxide** with concentrated caustic soda.

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