Why do chunks of aircraft debris float? In reading about various theories of what happened to flight MH370, there's a lot of discussion about it possibly crashing into the ocean, breaking up, and pieces of the aircraft floating hundreds of miles away. 

Is it generally expected that large pieces of an aircraft will float fairly well? E.g. in the linked article, there's a picture of 4 guys carrying what looks like a big chunk of wing. How did that float? 

My assumption would have been that that chunk was hollow aluminum fuselage, maybe with some electronics and hydraulics inside, and maybe a portion of a gas tank. I don't get how it would float unless it got lucky and a subsection of gas tank that was empty and did not get pierced during the breakup landed with its inlet/outlet pointing up. 

Thanks.
Lighter materials such as plastic will naturally float.

Composites can have large sections of honeycomb core with trapped air. Even if these parts are broken, the honeycomb is sealed against both sides of the panel, still trapping air.

Large sections, whether made from composite or metal, will generally be hollow to save weight, and sealed to prevent collecting moisture which could cause corrosion. There may be small drain holes, but they wouldn't be enough to cause significant leaks.

Fuel tanks are sealed but they are also fairly large and contain baffles to allow fuel to flow within the tank. As you would guess, a catastrophic crash would be unlikely to leave a significant portion that doesn't contain holes.

The parts found suspected to be from MH370 all have these qualities.