Astonishing Science. Spectacular museum.
Pump the air out of a sealed bottle and see what happens to the marshmallows within.
Year groups: 7–11 (ages 11–16)
Changing the air pressure in a sealed environment has an impact on objects contained within that atmosphere.
Download student activity sheet [pdf]
The direct effect of changing air pressure can be felt during flights when your ears pop. The pocket of air in the middle ear is affected by the drop in air pressure in the aircraft cabin. Pressure is equalised by allowing some air to escape through the Eustachian tubes. As a result, your eardrum is pushed outward to release that pressure, and that is when you feel the pop.
In areas of lower air pressure certain objects can expand. Amongst other effects, this is something that humans in space have to contend with. Spacesuits are worn to provide protection against this, with every single part of the body covered by 14 layers, and the bladder layer in particular providing support against the changing air pressure.
When you breathe in the pressure in the chest cavity reduces, which means that it has a lower pressure than the air in your lungs. This decreasing pressure makes lungs expand, drawing air into them. When the chest cavity relaxes, pressure increases, forcing air out of the lungs, which is experienced as breathing out. During this process gas exchange takes place in the lungs, allowing oxygenation of the blood and expulsion of excess carbon dioxide. The behaviour of the marshmallow is a simple analogy for this process.
Marshmallows are solids that contain lots of air pockets, as well as lots of sugar! Under normal circumstances the air pressure in these pockets is the same as the air pressure surrounding the marshmallows. This means the air outside the marshmallows is exerting the same force on the marshmallow as the air in the air pockets inside the marshmallow.
As a result, the different forces cancel each other out. However, when you place marshmallows in a sealed container and pump the air out of it you alter the air pressure. The air pressure inside the bottle, around the marshmallows, decreases, so that the force of the air inside the marshmallows is greater than the force acting on the outside of them. This causes the marshmallows to expand.
When the air is allowed back into the bottle the marshmallows return to their original shape and size (more or less). This is because the air pressure in the bottle increases until the pressure inside and outside the marshmallows is equal again.
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