You can try this fun experiment at home-
Fill a clear glass about half full of water. Pour vegetable oil into the glass and watch the two liquids immediately separate into two distinct layers. The oil is less dense than water and it rises to the top of the glass. Next we put in a few drops of food coloring because this helps us see a little better what happens next. Each person needs a little cup or bowl of salt.
Using your fingers drop a pinch of salt on top of the food coloring and watch what happens. Salt is more dense than oil and water and sinks to the bottom of the glass.
On its way down in pulls some of the oil with it. But as the salt dissolves the oil is freed and makes its way back up to the top. Try pouring in more or less salt to see how that affects the oil bubbles that go floating to the top of the glass.