Ask A Scientist

Why is it when you shake soda bubbles form?

Asked by: Chris Roberts
School: Chenango Forks Middle School
Grade: 7
Teacher: Mrs. Church

Answer from Jim Clum

Professor Emeritus, Watson School of Engineering and Applied Science

Research area: Materials Engineering Interests/hobbies: Travel; volunteer Wisconsin public broadcasting & UW Arboretum; gardening

The bubble formation when you shake a soda is due to a process called ‘nucleation and growth’ which happens as the molecules that make up the soda separate during the shaking.

To see how this works let’s say that the soda is made up of three different kinds of molecules:  ‘flavor’ molecules, e.g., cola, lemon-lime, etc.; water molecules; carbonation molecules, e.g., dissolved carbon dioxide.  It is also important to understand that to get the carbonation molecules into the mix they are ‘forced’ by using a high pressure (usually twice the pressure of the atmosphere).  We will come back to this idea of pressure later since it is part of the reason for the bubble formation.

When shaking occurs the mixture separates leaving some open spaces where the pressure is below that of the rest of the mixture.  In those spaces the lower pressure attracts the carbonation molecules from the surrounding mixture so that some of the carbonation molecules get together into ‘clusters’ of very tiny spherical bubbles.  This is the ‘nucleation’ stage of bubble formation. Spheres are the most efficient way for these clusters to form since they contain the largest number of molecules in an object (the sphere), which has the least surface area so that the molecules have the smallest chance to escape from the surface of the cluster. 

As the sloshing of the ‘flavor’, water and carbonation molecules moves them back and forth the smaller ‘clusters’ get swept away and re-mix into the soda assisted by the higher pressure of the mix.  However, some ‘clusters’ collect the carbonation molecules that are being lost by other smaller ‘clusters’ and those larger ‘clusters’ are now said to ‘grow’ creating the ‘growth’ part of ‘nucleation and growth’. The larger the ‘cluster’ the longer it will resist being re-dissolved and the better its chance of continuing to grow.  

Also causing the ‘cluster’ bubbles to collapse is the high pressure in the soda container, which as we said earlier is about twice the pressure of the atmosphere outside the container.  The ‘rule of thumb’ for how pressure affects the bubble collapse is that the larger bubbles are, the more they resist pressure, which causes collapse.  However, given enough time, the molecules will leave the bubbles and we are back to the original uniform mixture – until we shake it up again!

Of course, if, as often happens we open the container by popping the cap before all the bubbles have collapsed, then they will be exposed to the normal atmospheric pressure, which is only half that in the uncapped soda container.  With that lower pressure available the bubbles are now able to rapidly grow and escape the container giving the fizzing overflow with which we are familiar.



Last Updated: 3/1/17