When we add salt to water the volume of the water expands. Why is that
Said in easy words: The salt will dissolve into ions, and in this particular case these need space between the water molecules, so the volume will expand.
However this is not always the case! As @JohnCuster points out:
There is no requirement that the volume expands. Things that hydrate (MgSO4 is one example) may actually reduce the volume under appropriate conditions.