- 225 ml clear glass bottle
- 4 small baking marshmallows
- Wine vacuum pump and its rubber stopper
- Take the label off the bottle before you start so students can see inside it clearly.
- You may want to roll the marshmallows in flour before you begin as this will stop them getting too sticky.
- Do not pump the air out of the bottle too much as this could end messily.
- The best way to clean the bottle after the activity is with warm water.
- This activity will not work with a plastic bottle as it will lose its shape.
- What is happening to the marshmallow?
- Why is this happening?
- If this happens to marshmallows in an environment with no air pressure, what would happen to people in a similar environment?
- Why do the marshmallows not return to their original shape after the air has been reintroduced?
- Why are marshmallows such a good material for this activity?
- What other materials would work as well as the marshmallows and why?
- Where and when might you be able to experience no/low/high air pressure?
- You could try putting other items in the bottle, such as grapes, shaving cream or honeycomb, and see what the effect is and why – but do bear in mind that this may get messy or may not work due to the structure of the object(s).
- You can do a larger, higher-impact version of this activity using a bell jar and vacuum pump.
- Link the activity with thinking about solids, liquids and gases by making your own marshmallows, as the sugar will go through all three states of matter. But remember that sugar can get very hot!
- As the activity relates to the atmosphere, providing a good link to the subject of space, you could think about other related topics such as rockets and how they land or gravity.
- This activity can be used to think about breathing mechanisms. You could explore it further by looking at balloons in a bell jar.
- You could also use the activity to talk about environments where humans experience different pressures, such as in space, under water or on high mountains. Discuss how these environments differ from each other and how people can prepare themselves.
Links to everyday life
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.
The science – an introduction
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.