Zone refining is based on the principle that

Impurities are more soluble in molten metal than in solid metal

The correct answer is option 2. Impurities are more soluble in molten metal than in solid metal.

Zone refining is a method used to purify materials, especially metals, by taking advantage of the difference in solubility of impurities in the molten state compared to the solid state. Here's how it works:

1. Setup: The process starts with a rod or ingot of the material to be purified. This material is often a metal with impurities.

2. Heating: The rod is heated at one end to form a molten zone. The temperature is carefully controlled to ensure that only a small portion of the rod at the heated end melts, creating a distinct molten zone.

3. Movement of the molten zone: The molten zone is then slowly moved along the length of the rod. As it moves, it carries the impurities along with it.

4. Solidification: As the molten zone moves away from the heated end, it begins to solidify. During solidification, the impurities that were dissolved in the molten zone become less soluble and tend to segregate or crystallize out.

5. Repeated passes: The process is often repeated multiple times, with the molten zone being moved back and forth along the length of the rod. Each pass results in further purification, as impurities are gradually concentrated in one region of the rod while the rest becomes increasingly pure.

The key principle behind zone refining is that impurities tend to be more soluble in the molten state of the material than in the solid state. As a result, when the molten zone moves through the solid material, it selectively dissolves and carries the impurities along with it, leaving behind a purer material in its wake.

By carefully controlling the movement of the molten zone and repeating the process several times, zone refining can produce materials with extremely high levels of purity. This technique has been widely used in the semiconductor industry for purifying silicon, as well as in the production of high-purity metals for various applications.