In zone refining, silicon is purified as

Options

A. portions of Si rod are heated and cooled so that impurities are differentially evaporated
B. impurities are more soluble in liquid phase than in solid
C. impurities are less soluble in liquid phase than in solid
D. impurities are insoluble in moiten Si and can be separated

Answer

To answer the question "In zone refining, silicon is purified as," we will follow a step-by-step approach to understand the process of zone refining and how it applies to silicon purification.

### Step-by-Step Solution:

1. **Understanding Zone Refining**:
- Zone refining is a technique used primarily for purifying semiconductors and metals. It relies on the principle that impurities have different solubility in solid and liquid phases of a material.

**Hint**: Remember that zone refining is specifically used for materials like semiconductors.

2. **Process Overview**:
- In zone refining, a metal strip (in this case, silicon) is subjected to a moving heater. This heater melts a small region of the metal strip as it moves along it.

**Hint**: Visualize the setup where a heater moves along a solid strip, melting a portion of it.

3. **Solubility of Impurities**:
- The key principle here is that impurities are generally more soluble in the molten form of the metal than in its solid form. As the heater moves, the molten silicon can dissolve the impurities present in the solid silicon.

**Hint**: Focus on the relationship between temperature (solid vs. molten) and impurity solubility.

4. **Movement of the Melted Zone**:
- As the heater continues to move, the molten zone also moves. The impurities that have dissolved in the molten silicon will also move along with the molten zone.

**Hint**: Think about how the impurities are carried along with the molten silicon as it solidifies behind the heater.

5. **Separation of Impurities**:
- Eventually, the molten silicon that contains the impurities moves to the end of the strip. This portion can be removed, leaving behind a highly purified silicon strip.

**Hint**: Consider how the impurities can be easily separated from the pure silicon once they are concentrated in the molten region.

6. **Conclusion**:
- Therefore, in zone refining, silicon is purified because the impurities are more soluble in the molten silicon and can be effectively separated from the solid silicon.

**Final Answer**: The correct option is D: "impurities are more soluble in the molten silicon and can be separated."