Tap water freezes much more quickly than salt water. The freezing point of tap water is approximately 32 degrees Fahrenheit. It takes longer for salt water to freeze because it has a higher solute concentration than tap water.
Ice is created by restricting the movement of water molecules at extremely cold temperatures. In tap water, the only thing that has to be restricted is the movement of water molecules. However, to form ice out of salt water, the movement of water molecules and the movement of sodium have to be restricted. This phenomenon results in salt water having a lower freezing point than tap water.
The freezing point of salt water is directly related to the salt concentration in the solution; a higher concentration of salt requires a colder temperature for freezing, as low as -6 F.