When water evaporates, the salt stays behind in the water. If all the water evaporates, the salt will be left behind as white stuff in the pan. Try it! Put salt in water and stir it until it all dissolves. Then boil the water away. The salt is left behind. So the water that evaporated doesn't have much salt in it. By cooling the evaporated water, you can turn it back into liquid water and it won't have any salt in it.