water is a kind of substance & freshwater is a kind of water ->  freshwater is a kind of substance  freshwater is a kind of substance & saltwater has a higher density than freshwater & saltwater is a kind of substance ->  saltwater is a substance that has a higher density than the substance freshwater  saltwater is a substance that has a higher density than the substance freshwater & if a substance has a higher density than another substance , then the molecules in the substance will be closer than those of the other substance ->  the molecules in the saltwater are closer than the molecules in the freshwater  the molecules in the saltwater are closer than the molecules in the freshwater & as the distance between molecules decreases , the speed of the sound will increase -> sound will travel faster in saltwater than freshwater