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Cola vs. Diet Cola

In this activity, students learn about the fundamental principle of density (amount of mass per volume) by comparing the buoyancy of two types of cola. 

When you place a can of cola in water it will sink. A can of diet cola, however, will float. This shows that they have different densities.

The amount of aluminum in each can is the same, as is the amount of fluid (355 ml) and the amount of carbon dioxide. What varies between cola and diet cola is the sweetener. For example, a can of Coke Classic contains 39 grams of sugar and a can of Diet Coke contains 0.1 grams of aspartame.

It takes 390 times more sugar to make a drink taste as sweet as a drink sweetened with aspartame. This additional sugar makes the regular cola more dense. 


  • Predict, test and explain the relative densities of everyday objects through investigating their interaction with water


  • Per Demo or Group:
    1 large clear container of water
    1 can of cola
    1 can of diet cola
    1 plastic bag containing 39 g of sugar
    1 plastic bag containing 0.1 g sweetener

Key Questions

  • Why does one can float and the other sink?
  • What does the amount of sugar have to do with density?

What To Do

  1. Show the students the two cans of cola. Ask them to predict whether they will sink or float.
  2. Place the regular cola into the water. It should sink. Make sure it goes in sideways, to reduce bubbles being trapped underneath.
  3. Place the diet cola into the water. It should float.
  4. Ask the students how this compares with their predictions. Can they explain why one floats and the other sinks?
  5. Point out that the similarities between the two colas: the amount of aluminum in the cans is the same, as is the amount of fluid (355 ml) and the amount of carbon dioxide.
  6. Ask the students what could be different between the two colas. Discuss the different types of sweetener used, and their relative masses.

Teacher Tip
There is some variation between individual cans, so make sure to test this with several cans each of regular and diet cola to find floaters and sinkers.


  • Measure the mass of the can of soda and the can of diet soda to make the difference quantitative.
  • Compare different brands of cola or different kinds of soda and diet soda.
  • How consistent is the mass of a particular brand of soda?