It raises the density by adding more mass, to the same volume of water. (i.e. adding 4 tsps. of salt to one cup of water will raise the densitybecause the salt is adding mass to the same amount of volume, on cup of water. You can do an experiment to prove it, all you have to do is find something that doesn't float, example, egg, and then add 4-5 tsp. salt to one cup water and it will float.