You have always noticed that an egg sinks in the fresh water or plain water while it starts floating in the salt water this because when you add salt to the water you are adding more molecules to the water. Earlier the molecules were apart from each other but now salt molecules have occupied some amount of space between the water molecules which increase the density of water. So when salt is added to the freshwater it becomes denser than the egg and the egg starts floating while the freshwater or plain water was less dense than the egg and hence it sinks.
Density= mass per unit volume.
Buoyancy principle says that the upward buoyant force of liquid on the object is equal to the weight of the liquid which is displaced by same object. More is the salt in a liquid more is the increase in density.
If you weigh the salt water and freshwater you will notice that the salt water weighs more than the freshwater. They both occupy the same amount of space in a cup but when salt molecules enter the water molecules the liquid becomes denser.
Egg sinks when its density is more. It hangs in the middle when its density becomes equal to that of the water and if we continue adding more salt to the water it will become more denser and hence the density of egg will be less than the water and it floats. If we replace the egg with the stone than this will not happen because the density of stone is far more than the density of water and adding salt to water will not match with stoneâ€™s density while the density of egg is slightly more than the density of water.
If we drop any object that is hollow from inside it will float but if it would be solid then that object will definitely sink because being hollow it gives space to the air which is less denser and hence the object remains above the surface of water.