Amazing Marshmallows

The effects of air pressure are investigated in this highly visual demonstration.

Ingredients:

  • marshmallows (+ cornflour, depending on the type of marshmallows you use)
  • wine bottle (must be clean and dry, and preferably light coloured)
  • vacuum wine saver pump and stopper (available from most supermarkets)

Instructions

  1. Roll the marshmallows gently between your hands until they are small enough to pass through the opening of the wine bottle. Note: if the marshmallows become at all sticky then roll them in cornflour, otherwise they will just stick to the sides of the bottle and make a big mess.
  2. Insert the vacuum stopper into the neck of the wine bottle. Explain to the audience that the vacuum pump removes air from the bottle. Instruct them to carefully watch the marshmallows.
  3. Pump the vacuum pump a few times – the marshmallows inside the bottle begin to expand. Shake the bottle gently up and down to distribute the marshmallows throughout the bottle, then pump again. Repeat until the marshmallows are no longer expanding. Discuss with your audience.
  4. Tell your audience that you are going to let the air back into the bottle. Remind them to watch the marshmallows closely as you do so.
  5. Release the valve on the vacuum stopper – the marshmallows will rapidly shrink back to their normal size.

How Does it Work?

Marshmallows have small bubbles of air trapped inside them. These bubbles are at atmospheric pressure. When the air inside the glass container is sucked out, the volume of the container remains the same although there is much less air inside – so the pressure is reduced. The air bubbles inside the marshmallows are therefore at a much higher pressure than the air surrounding the marshmallows, so those bubbles push outwards, causing the marshmallows to expand. When air is let back into the glass container, the surrounding pressure increases again, and the marshmallows deflate back to their normal size.

Tips for Success

Don't try to re-use the same marshmallows too much or they'll stretch and stop working properly. Long thin marshmallows work best because they are less sticky than the traditional sort, so easier to insert into the wine bottle. If you have access to a vacuum pump and bell jar, this trick works beautifully. Try using matchsticks to make marshmallow shapes – e.g. animals or a person – which the audience can see grow and shrink as you decrease and increase the pressure.

Serving Suggestions

This demonstration will impress audiences of all ages. It is most suitable when you have already gathered a small audience.

Did You Know?

Although aircraft cabins are pressurised, they are not kept at sea level pressure. A similar effect to the marshmallow experiment can be observed by drinking half a bottle of water during a flight. When the aircraft lands you will see that the sealed plastic bottle is slightly crushed by the higher atmospheric pressure at about the same time as your ears pop on the approach to landing.