Final answer:
In the desalting phase, salt is removed from seawater, primarily through reverse osmosis, where pressure forces water molecules through a semipermeable membrane, leaving dissolved salts behind.
Step-by-step explanation:
During the desalting phase of the experiment, the substance that is removed is salt, specifically the sodium and chloride ions that make up the bulk of the salinity in seawater. This process is typically referred to as desalination, which is essential for producing fresh water from seawater for human use. Various methods are used in desalination, including evaporation and condensation, freezing, and reverse osmosis. Reverse osmosis is a widely-used technology in which pressure is applied to seawater forcing water molecules through a semipermeable membrane, effectively separating the pure water from dissolved salts. This process is valuable, especially in regions where freshwater is scarce, such as in the Middle East and parts of California.