Question

why does water with salt in it freeze much more slower than water without it ? Explain

Answer 1

The more salt in the water, the lower the freezing point will be. When freshwaterfreezes, water molecules of hydrogen and oxygen have bonded together into a crystalline structure of ice. The presence of salt makes it harder for water molecules to bond to the ice structure, because ice naturally repels salt molecules.

Answer 2

When you add salt to water, you lower to freezing point of the substance.

So for example, normal water freezes at 0°C. But water with salt in it won't freeze at 0°C, because its freezing point is lowered. 

In answer to the question. It takes longer for water with salt in it to freeze because the substance requires a lower temperature than normal water to freeze.