Why do we sink when we let our breath out under water?

Question

Why do we sink when we let our breath out under water?

Answer 1

When our lungs are full of oxygen, they act like buoys helping to keep us afloat.  When the oxygen is released, the buoys are deflated thus causing us to sink.

Answer 2

Because air is lighter then water, and so when it still contained in our body's it keeps us afloat. SO when we realease the air, we no longer consist of an element lighter then water.

Answer 3

It's all a matter of buoyancy.

You float when the your weight is less than the weight of the amount of water you displace. Water displacement is the amount of water you shove aside when getting in, similar to how the water level rises when you get in a bathtub.

After exhalation, we (for the most part) weigh more than the weight of the water we displace, because our body mass is dense (density is mass over volume). So we sink.

With a big breath of air, however, our body volume increases some, and our body weight does not increase (since air weighs little to nothing). Therefore, the amount of water we displace increases a little, as does its total displaced weight. On the other hand, our weight does not change, and our density decreases with the expanding volume.

All in all, it just comes to a matter of volume. Think of yourself lying flat on the surface of the water vs standing upright in the water. The more area of water you cover, the more buoyant you'll be. Another example is a slice of bread vs that same slice rolled into a ball. The ball sinks, as it's more dense (packed together)