Ice melts faster in saltwater compared to freshwater due to a phenomenon called freezing point depression or the lowering of the freezing point of a solvent when a solute is added. Here's how it works:
Pure water freezes at 0 degrees Celsius (32 degrees Fahrenheit). However, when salt (solute) is added to water (solvent) to create saltwater, the presence of dissolved ions disrupts the formation of the ice crystal lattice, which raises the freezing point of the solution.
In saltwater, the dissolved ions, such as sodium (Na+) and chloride (Cl-) from the salt, interfere with the hydrogen bonding between water molecules. This interference makes it more difficult for the water molecules to arrange into an orderly crystal structure and solidify into ice. As a result, the freezing point of saltwater is lower than that of freshwater.
By lowering the freezing point, saltwater needs to reach a lower temperature than freshwater in order for ice to form. Consequently, when saltwater and freshwater are exposed to the same external temperature below the freezing point of freshwater, the saltwater will remain in a liquid state for a longer period of time and will melt the ice faster.
It's worth noting that the exact extent of freezing point depression depends on the concentration of the dissolved solute. The more salt (or solute) present in the water, the greater the freezing point depression and the faster the ice will melt.