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Practicals

The Rise and Fall of a Peanut
AKA: Carbon Dioxide Bubbles

Summary

A peanut is placed in a glass of lemonade. At first it sinks to the bottom but then rises before repeating the cycle.

Operation

This is a fun practical which may be conducted as part of a filler lesson or for younger groups of students.

READ THE SAFETY NOTE BELOW BEFORE CONDUCTING THIS PRACTICAL

It can also be conducted as part of a simple investigation by varying the object placed in the lemonade and observing any differences in outcome.

Equipment typically required (per set)

  • Glass of lemonade or soda water or other fizzy drink
  • Peanut
  • Stopclock (optional)
  • Other objects such as raisins, rice grains or apple pips (optional)
The glass (or measuring cylinder) is filled with lemonade or other fizzy drink. This must be fairly fresh because fizziness is key - the CO2 in the drink causes the effect. A peanut is dropped into the lemonade whereupon it should sink to the bottom. If the drink is fizzy enough, small bubbles of CO2 form on the surface of the nut which, when there are enough of them, will cause the nut to float back to the top. Upon reaching the surface of the lemonade, the bubbles burst and the nut sinks to the bottom of the glass once again.

Bubbles are formed at nucleation points on the rough surface of the nut for as long as there sufficient CO2 in the lemonade. Eventually the lemonade will go flat and the cycle of rising and falling will cease.

Conducting this experiment as a simple investigation may involve changing the type of drink used from lemonade to maybe soda water or cola and/or changing the object placed in it. Also students may want to measure how many times per minute the object rises and falls. This may relate to the amount of CO2 present as well as other factors.

Safety

 CautionPeanuts should not be used in the presence of anyone with a nut allergy. You may wish to replace the peanut with one of the other objects in the list above, such as a grain of rice. Seek professional advice if in any doubt. A full risk assessment should be undertaken before conducting this practical if peanuts are being used.

The contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.