What is the main function of a semipermeable membrane in reverse osmosis?

In the context of reverse osmosis, the main function of a semipermeable membrane is to selectively allow certain molecules to pass through while blocking others. The correct answer highlights that the semipermeable membrane permits the diffusion of freshwater to salt water, facilitating the process of osmosis. In reverse osmosis specifically, the setup is designed to apply pressure to overcome osmotic pressure, pushing water from a saltwater solution through the membrane, leaving behind contaminants and salt molecules.

The purpose of the semipermeable membrane in reverse osmosis is to act as a barrier that enables freshwater to move in one direction while preventing the passage of salt and other impurities. This selective permeability is essential for effective desalination, as it ensures that purified water can be obtained from saline water.

In scenarios where the other answers imply incorrect processes, such as boiling seawater or mixing seawater and freshwater, those activities do not utilize semipermeable membranes or pertain to the mechanism of reverse osmosis. Instead, these processes mischaracterize the purpose and function of a semipermeable membrane, which is specifically aimed at separation and filtration rather than mixing or boiling.