Which of the following statements in not correct regarding preparation of NaOH/

Options

A. NaOH is prepared by electrolysis of sodium chloride in castner-kellner cell.
B. sodium metal discharged at cathode cmbines with mercury to form sodium amalgam,.
C. Chlorine is evolved at anode.
D. Amalgam is heated to separate Na and Hg.

Explanation

To determine which statement is not correct regarding the preparation of NaOH, we will analyze each option step by step.

### Step 1: Understand the Process of NaOH Preparation
NaOH (sodium hydroxide) is primarily prepared through the electrolysis of brine (a concentrated solution of sodium chloride, NaCl). This process occurs in a specialized cell known as the Castner-Kellner cell.

### Step 2: Analyze Each Statement
1. **Statement A**: "NaOH is prepared in electrolysis of sodium chloride in Castner-Keller cell."
- This statement is **correct**. The Castner-Kellner cell is indeed used for the electrolysis of sodium chloride to produce NaOH.

2. **Statement B**: "Sodium metal discharge at cathode combined with mercury to form sodium amalgam."
- This statement is also **correct**. During electrolysis, sodium ions gain electrons at the cathode to form sodium metal, which then reacts with mercury to form sodium amalgam.

3. **Statement C**: "Chlorine is evolved at anode."
- This statement is **correct**. At the anode, chlorine gas is produced by the oxidation of chloride ions (Cl⁻).

4. **Statement D**: "Amalgam is heated to prepare Na and Hg."
- This statement is **not correct**. Heating sodium amalgam does not yield sodium and mercury directly. Instead, sodium amalgam is typically hydrolyzed in water to produce NaOH and mercury.

### Conclusion
The statement that is not correct regarding the preparation of NaOH is **Statement D**.

### Final Answer
**The incorrect statement is D: "Amalgam is heated to prepare Na and Hg."**

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