(i) `A+Na_(2)CO_(3) rarr B+C`
(ii) `A overset(CO_(2))(rarr) ("Milky")C`
The chemical formula of `A` and `B` are respectively

Options

A. `NaOH and Ca(OH)_2`
B. `Ca(OH)_2 and NaOH`
C. NaOH and CaO
D. `CaO and Ca(OH)_2`

Answer

To solve the problem, we need to analyze the two given reactions step by step.

### Step 1: Analyze the first reaction
The first reaction is given as:
\[ A + \text{Na}_2\text{CO}_3 \rightarrow B + C \]

Here, we need to identify what A could be. A common reaction involving sodium carbonate (\( \text{Na}_2\text{CO}_3 \)) is with calcium hydroxide (\( \text{Ca(OH)}_2 \)).

### Step 2: Identify A
Let’s assume:
\[ A = \text{Ca(OH)}_2 \]

Now substituting A into the reaction:
\[ \text{Ca(OH)}_2 + \text{Na}_2\text{CO}_3 \rightarrow B + C \]

### Step 3: Determine the products B and C
When calcium hydroxide reacts with sodium carbonate, the products formed are:
- Sodium hydroxide (\( \text{NaOH} \))
- Calcium carbonate (\( \text{CaCO}_3 \))

Thus, we can write:
\[ B = \text{NaOH} \]
\[ C = \text{CaCO}_3 \]

### Step 4: Analyze the second reaction
The second reaction is:
\[ A \overset{\text{CO}_2}{\rightarrow} \text{(Milky)} C \]

We already identified \( A \) as \( \text{Ca(OH)}_2 \). When \( \text{Ca(OH)}_2 \) reacts with \( \text{CO}_2 \), it forms calcium carbonate (\( \text{CaCO}_3 \)), which is known to produce a milky suspension.

### Step 5: Conclusion
From the analysis:
- \( A = \text{Ca(OH)}_2 \) (Calcium hydroxide)
- \( B = \text{NaOH} \) (Sodium hydroxide)

Thus, the chemical formulas of \( A \) and \( B \) are respectively:
- \( A = \text{Ca(OH)}_2 \)
- \( B = \text{NaOH} \)

### Final Answer
The chemical formula of \( A \) and \( B \) are respectively:
- \( A = \text{Ca(OH)}_2 \)
- \( B = \text{NaOH} \)

---