How can I demonstrate how air is used to lift submarines ?

You can demonstrate this by completing this experiment.

What will I need to do this?

1 bowl, 2 quart (liter)
1 clear plastic drinking glass, 7 0z.
Flexible drinking straw.

How do I do this ?

Pour water into the bowl until its fill.
Put the glass on its side and push it underneath the water in the bowl. You should fill the glass with as much water as possible.
Keep the top of the glass underneath the water as you turn the glass so that it sits upside down on the base of the bowl.
Lift the glass slightly with your hand and put the end of the straw under the brim of the glass.
Hold the glass with your hand, but do allow the glass to move.
Blow into the straw.

What did you observe ?

As you blow through the straw, water escapes out of the glass and the glass moves to the top of the water.

What caused this to happen ?

Filling the glass with water makes it heavy enough to sink and rest on the base of the bowl. When you blow into the glass you are forcing the water out of the glass. The gas in your breath weighs less than that of the water. The glass is more buoyant (tending to float) when filled with gas from your breath than when filled with water. Subs have tanks that when filled with water causes the ship to sink. To go to the surface, the water in the tanks is substituted with air, making the ship more buoyant.