Equilibrium and Soft Drinks


Related Posts:

A bottle of soda sitting on the ground is shown with a large amount of fizz-filled liquid spewing out of the top.
Figure 1. Opening a soft-drink bottle lowers the CO2 pressure above the beverage, shifting the dissolution equilibrium and releasing dissolved CO2 from the beverage. (credit: modification of work by “D Coetzee”/Flickr)

Equilibrium and Soft Drinks (OpenStax Chemistry 2e)

The connection between chemistry and carbonated soft drinks goes back to 1767 when Joseph Priestley (1733–1804) developed a method of infusing water with carbon dioxide to make carbonated water. Priestley’s approach involved the production of carbon dioxide by reacting oil of vitriol (sulfuric acid) with chalk (calcium carbonate).

The carbon dioxide was then dissolved in water, reacting to produce hydrogen carbonate, a weak acid that subsequently ionized to yield bicarbonate and hydrogen ions:

These same equilibrium reactions are the basis of today’s soft-drink carbonation process. Beverages are exposed to a high pressure of gaseous carbon dioxide during the process to shift the first equilibrium above to the right, resulting in desirably high concentrations of dissolved carbon dioxide and, per similar shifts in the other two equilibria, its hydrolysis and ionization products. A bottle or can is then nearly filled with the carbonated beverage, leaving a relatively small volume of air in the container above the beverage surface (the headspace) before it is sealed. The pressure of carbon dioxide in the container headspace is very low immediately after sealing, but it rises as the dissolution equilibrium is re-established by shifting to the left. Since the volume of the beverage is significantly greater than the volume of the headspace, only a relatively small amount of dissolved carbon dioxide is lost to the headspace.

When a carbonated beverage container is opened, a hissing sound is heard as pressurized CO2 escapes from the headspace. This causes the dissolution equilibrium to shift left, resulting in a decrease in the concentration of dissolved CO2 and subsequent left-shifts of the hydrolysis and ionization equilibria. Fortunately for the consumer, the dissolution equilibrium is usually re-established slowly, and so the beverage may be enjoyed while its dissolved carbon dioxide concentration remains palatably high. Once the equilibria are re-established, the CO2(aq) concentration will be significantly lowered, and the beverage acquires a characteristic taste referred to as “flat.”


Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/books/chemistry-2e


Related Research

Research Article: Impact of glass shape on time taken to drink a soft drink: A laboratory-based experiment

Date Published: August 27, 2018 Publisher: Public Library of Science Author(s): Tess Langfield, Rachel Pechey, Mark Pilling, Theresa M. Marteau, George Van Doorn. http://doi.org/10.1371/journal.pone.0202793 Abstract: Glassware design may affect drinking behaviour for alcoholic beverages, with glass shape and size influencing drinking speed and the amount consumed. Uncertainty remains both about the extent to which these effects … Continue reading

Research Article: Microbial Community Analysis of Anaerobic Reactors Treating Soft Drink Wastewater

Date Published: March 6, 2015 Publisher: Public Library of Science Author(s): Takashi Narihiro, Na-Kyung Kim, Ran Mei, Masaru K. Nobu, Wen-Tso Liu, Zhe-Xue Quan. http://doi.org/10.1371/journal.pone.0119131 Abstract: The anaerobic packed-bed (AP) and hybrid packed-bed (HP) reactors containing methanogenic microbial consortia were applied to treat synthetic soft drink wastewater, which contains polyethylene glycol (PEG) and fructose as … Continue reading

Research Article: Caramel Color in Soft Drinks and Exposure to 4-Methylimidazole: A Quantitative Risk Assessment

Date Published: February 18, 2015 Publisher: Public Library of Science Author(s): Tyler J. S. Smith, Julia A. Wolfson, Ding Jiao, Michael J. Crupain, Urvashi Rangan, Amir Sapkota, Sara N. Bleich, Keeve E. Nachman, Maciej Buchowski. http://doi.org/10.1371/journal.pone.0118138 Abstract: Caramel color is added to many widely-consumed beverages as a colorant. Consumers of these beverages can be exposed … Continue reading

Research Article: Consumption of Artificially-Sweetened Soft Drinks in Pregnancy and Risk of Child Asthma and Allergic Rhinitis

Date Published: February 27, 2013 Publisher: Public Library of Science Author(s): Ekaterina Maslova, Marin Strøm, Sjurdur F. Olsen, Thorhallur I. Halldorsson, Susanne Krauss-Etschmann. http://doi.org/10.1371/journal.pone.0057261 Abstract: Past evidence has suggested a role of artificial sweeteners in allergic disease; yet, the evidence has been inconsistent and unclear. To examine the relation of intake of artificially-sweetened beverages during pregnancy with … Continue reading