This experiment demonstrates the power of air pressure. Before it was boiled, the can was just full of air and water. But once the water boils it changes states from a liquid to a gas that you know as water vapor. This vapor pushes the air inside the can out into the atmosphere. When the can was turned upside down and placed in the water, the water vapor condensed and turned back into the water. Water molecules in the liquid state are much denser than molecules in the gas state, so all of the water vapor that had previously filled up the inside of the can turned into only a drop or two of liquid, which took up much less space.This creates a vaccuum inside the can allowing the pressure of the air pushing from the outside of the can to crush it. The sudden collapsing of an object toward its center is called an implosion. Nature wants things to be in a state of equilibrium or balance. To make the internal pressure of the can balance with the external pressure on the can, the can implodes.
CO2 has one carbon atom and two oxygen atoms, and a molecular weight of 44 grams per mole ( a certain number of molecules). Oxygen has a molecular weight of 32. Hence, carbon dioxide has a higher density, or is heavier than oxygen.This is why CO2 can be dangerous... it can displace the oxygen in a room and lead to asphyxiation.