Given below are the steps involved in the process of zone refining for removing impurities which are more soluble in the melt than solid state of the metal.

1. Impurities get concentrated at one end
2. As the heater moves forward, the pure metal crystallizes out of the melt left behind and the impurities pass to the next zone
3. A mobile heater surrounding the rod of impure metal is fixed at its one end.
4. The end is cut off.
5. The molten zone moves along with the heater which is moved forward.

Choose the correct answer from the options given below:

C, E, B, A, D

The correct answer is option 3. C, E, B, A, D

Zone refining is a highly effective method used to purify metals, especially when the impurities in the metal are more soluble in the molten (liquid) state than in the solid state. Here’s a step-by-step breakdown of the zone refining process with an explanation of each step:

Step C: A mobile heater surrounding the rod of impure metal is fixed at one end.

The process begins by setting up a special heating device around the metal rod to be purified. This heater is placed initially at one end of the rod, and it’s designed to move slowly along the length of the rod.

The purpose of the heater is to create a narrow zone of molten metal, which will melt the impurities as it progresses.

Step E: The molten zone moves along with the heater, which is moved forward.

As the heater moves forward, it creates a “molten zone” – a narrow band of metal that is melted and moving along the rod. The rest of the rod remains in the solid state.

This movement is critical because it allows impurities to stay in the molten region, while the surrounding metal solidifies in a purer form behind the moving molten zone.

Step B: As the heater moves forward, the pure metal crystallizes out of the melt left behind, and the impurities pass to the next zone.

As the molten zone advances, the metal left behind cools and crystallizes into a solid form. Because the impurities are more soluble in the molten state, they tend to stay in the liquid phase rather than solidifying.

This results in the metal crystallizing in a purer form, with most of the impurities being “pushed” forward to the next molten section along the rod.

Step A: Impurities get concentrated at one end.

With each pass of the molten zone, more impurities are concentrated in the molten phase and pushed towards one end of the metal rod. By the end of the process, a significant amount of the impurities will have accumulated at one end of the rod.

This repeated concentration process purifies the remainder of the rod, as impurities are progressively separated out and pushed away from the pure metal.

Step D: The end is cut off.

Once the impurities are gathered at one end of the rod, this section is removed by cutting it off. What remains is a rod of highly purified metal, as most of the impurities have been isolated in the end section and removed.

Thus, the correct order of steps is: option 3: C, E, B, A, D.

Zone refining is especially useful for purifying metals and semiconductors (like silicon and germanium) because it leverages the different solubility of impurities in molten and solid states. By concentrating impurities in a small section of the rod, the rest of the metal achieves high purity, which is crucial for applications in electronics and other sensitive fields.