Select correct statement(s).

Options

A. `PH_(3)` reduce `AgNO_(3)` to metallic Ag
B. Organic tissues turn `AgNO_(3)` black by reducing it to `Ag`
C. `AgCN` is soluble in `KCN.`
D. `Zr` and `Ta` have almost similar size due to lanthanide contraction

Explanation

To solve the question of selecting the correct statements regarding the properties and reactions of certain compounds, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: pH 3 reduces AgNO3 to metallic Ag.**
- **Reaction:**
\[ 6 \text{AgNO}_3 + 3 \text{H}_3\text{PO}_3 + 3 \text{H}_2\text{O} \rightarrow 6 \text{Ag} + 6 \text{HNO}_3 \]
- **Analysis:** In this reaction, silver nitrate (AgNO3) is reduced to metallic silver (Ag) while the oxidation state of silver changes from +1 to 0. This confirms that pH 3 can act as a reducing agent.
- **Conclusion:** This statement is **correct**.

2. **Statement 2: Organic tissue turns AgNO3 black by reducing it to Ag.**
- **Reaction:**
\[ \text{Organic matter} + \text{AgNO}_3 \rightarrow \text{Black Ag} + \text{NO}_3^- \]
- **Analysis:** Organic tissues, particularly proteins, can reduce silver ions from AgNO3 to metallic silver, resulting in a black stain. This is a common observation in photographic processes.
- **Conclusion:** This statement is **correct**.

3. **Statement 3: AgCN is soluble in KCN.**
- **Reaction:**
\[ \text{AgCN} + 2 \text{KCN} \rightarrow \text{K}_2[\text{Ag(CN)}_2] \]
- **Analysis:** Silver cyanide (AgCN) dissolves in potassium cyanide (KCN) to form a soluble complex, potassium argento cyanide. This confirms that AgCN is indeed soluble in KCN.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: Zirconium and Ta have almost similar size due to lanthanide contraction.**
- **Analysis:** While zirconium (Zr) and hafnium (Hf) have similar sizes due to lanthanide contraction, tantalum (Ta) is not similar in size to zirconium (Zr). The statement incorrectly compares Zr with Ta instead of Zr with Hf.
- **Conclusion:** This statement is **incorrect**.

### Final Conclusion:
The correct statements are:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Correct
- Statement 4: Incorrect

### Summary of Correct Statements:
- The correct statements are 1, 2, and 3.