Dow's process of extraction of Mg involves extraction of Mg from sea water. Sea water is concentrated in sun-light and is then treated with slaked lime. Magnesium hydroxide is heated in a stream of HCl to give `MgCl_(2)` which is electrolysed to discharge Mg. The mixture is in the ratio 35% `MgCl_(2)+50% NaCl+15% CaCl_(2)`. NaCl and `CaCl_(2)` are added to lower the fusion temperature and to increases the conductance.
`Mg^(2+) +Ca(OH)_(2) to Mg(OH)_(2)+Ca^(2+)`
`Mg(OH)_(2)+2HCl to MgCl_(2)+2H_(2)O(l)`
Electrolysis of fused `MgCl_(2) hArr Mg^(2+)+2Cl`
`Mg^(2+)+2e^(-) to Mg`( At Cathode)
`2Cl^(-) to Cl_(2)+2e^(-)` (At Anode)
Mg electrolysed is protected from atmospheric oxidation by a blanket of inert gases.
Molten mixutre of NaCl and `CaCl_(2)` is added to the heated `MgCl_(2)` because :

Options

A. `MgCl_(2). xH_(2)O+ " dry " HCl overset(973-1223K) to ` Partially dehydrated `MgCl_(2)` and molten mixture `(NaCl+CaCl_(2))` makes it fully dehydrated
B. `CaCl_(2)` is dehydrating agent
C. `(CaCl_(2)+ NaCl)` lowers the melting point of `MgCl_(2)`
D. None of these

Answer

To solve the question regarding the role of NaCl and CaCl₂ in the Dow's process of extracting magnesium (Mg) from seawater, we need to break down the process step by step.

### Step-by-Step Solution:

1. **Understanding the Dow's Process**:
- The Dow's process involves extracting magnesium from seawater, which contains magnesium ions (Mg²⁺). The seawater is concentrated and treated with slaked lime (Ca(OH)₂) to form magnesium hydroxide (Mg(OH)₂).

2. **Formation of Magnesium Hydroxide**:
- The reaction can be represented as:
\[ \text{Mg}^{2+} + \text{Ca(OH)}_2 \rightarrow \text{Mg(OH)}_2 + \text{Ca}^{2+} \]
- Magnesium hydroxide precipitates out as a white solid.

3. **Conversion to Magnesium Chloride**:
- The magnesium hydroxide is then heated with hydrochloric acid (HCl) to produce magnesium chloride (MgCl₂):
\[ \text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O} \]
- This MgCl₂ can be in hydrated form initially, but it needs to be dehydrated for electrolysis.

4. **Electrolysis of Magnesium Chloride**:
- The anhydrous MgCl₂ is then subjected to electrolysis, which dissociates into magnesium ions (Mg²⁺) and chloride ions (Cl⁻):
\[ \text{MgCl}_2 \rightleftharpoons \text{Mg}^{2+} + 2\text{Cl}^- \]
- At the cathode, magnesium ions gain electrons to form magnesium metal:
\[ \text{Mg}^{2+} + 2e^- \rightarrow \text{Mg} \]
- At the anode, chloride ions lose electrons to form chlorine gas:
\[ 2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^- \]

5. **Role of NaCl and CaCl₂**:
- The mixture used for electrolysis consists of 35% MgCl₂, 50% NaCl, and 15% CaCl₂.
- NaCl and CaCl₂ are added to:
- **Lower the melting point** of MgCl₂, making it easier to melt and electrolyze.
- **Increase the conductance** of the molten mixture, allowing for more efficient electrolysis.

6. **Conclusion**:
- The primary reason for adding NaCl and CaCl₂ is to enhance the electrolysis process by lowering the fusion temperature and increasing the conductance of the molten MgCl₂.

### Final Answer:
NaCl and CaCl₂ are added to the molten mixture of MgCl₂ to lower the fusion temperature and to increase the conductance during the electrolysis process.