In the electrolysis of alumina to obtain aluminium metal, cryolite is added mainly to:

The correct option is: Lower the melting point of alumina

Explanation:
In the metallurgy of aluminium, pure aluminium metal is extracted from alumina (Al₂O₃) by a process known as the Hall-Héroult process (electrolytic reduction).

Alumina is a very stable oxide and has an extremely high melting point of about 2050 °C (approximately 2323 K). Carrying out electrolysis at such high temperatures presents several major practical challenges:
1. It requires an enormous amount of electrical energy, making the process highly expensive.
2. At this temperature, the liberated aluminium metal (which melts at 660 °C and vaporizes at higher temperatures) can easily volatilize or oxidize.

To overcome these challenges, cryolite (Na₃AlF₆) along with fluorspar (CaF₂) is mixed with alumina. The addition of cryolite serves two main purposes:
• Lowers the Melting Point: It acts as a flux and reduces the melting point of the electrolyte mixture from 2050 °C to about 950 °C (around 1223 K), making the process energy-efficient and manageable.
• Improves Conductivity: Pure alumina is a poor conductor of electricity. The addition of cryolite provides mobile ions in the molten state, which significantly increases the electrical conductivity of the cell mixture.

Although cryolite also increases conductivity, its primary and most critical role is to lower the melting point of the alumina to a temperature at which electrolysis can be practically and economically carried out. Therefore, the main reason cryolite is added is to lower the melting point of alumina.