Key concepts
Chemistry
Reactions
Supersaturation
Nucleation

Introduction
You’ve probably seen the reaction that happens when you add Mentos candy to a bottle of diet soda. The resulting eruption can be powerful enough to be dangerous, and is the source of many online videos! Although many people are familiar with this reaction, few of them understand the science behind why it takes place. Mentos plus soda is not actually a chemical reaction but rather a physical one called nucleation. In this activity we will explore nucleation in soda at a smaller scale by adding some unusual ingredients to our soda!

Background
Carbonated drinks, such as soda, are in a state of supersaturation, meaning soda is completely saturated with carbon dioxide (CO2). Once any solution is supersaturated it generally can’t hold any more of the saturating substance. If you add sugar continuously to a glass of water, for example, eventually you’ll reach a point when the sugar just sinks to the bottom of the glass instead of dissolving into the water. If you heat the solution of sugar and water, however, the water will be able to accept more of the sugar than it could when the water was cool or at room temperature. Once the heated water has cooled to room temperature it will be supersaturated with sugar—more sugar will have been dissolved in the water than would normally be possible at room temperature.

Carbon dioxide is added to water to make soda in a similar manner. The water has been supersaturated with CO2 and then bottled and sealed to keep the carbon dioxide dissolved in the soda. The CO2 is always trying to escape from the soda, and once the soda bottle is open you see so many small bubbles forming, coming out of the solution.

Materials

  • Two cans of cold soda
  • Tablespoon of sugar
  • Tablespoon of salt
  • Tablespoon of oil
  • Four cups
  • Measuring cup (capable of measuring one-quarter cup)
  • Teaspoon


Preparation

  • Measure one-quarter cup of soda into each cup.


Procedure

  • Slowly add one tablespoon of sugar to the first cup. What happens to the soda when you add sugar? Notice the reaction and what you hear, see and even smell!
  • Move on to the next cup of soda. Slowly add one tablespoon of salt. What happens to the soda when you add the salt? Notice the reaction and what you hear, see and smell.
  • Move on to the next cup and slowly add one tablespoon of olive oil. What happens to the soda when you add the oil? Notice the reaction and what you hear, see and smell.
  • Taste the fourth cup of soda (with nothing added). This is your control soda.
  • Use a teaspoon to taste a very small amount of the soda to which you added sugar. What do you notice about the taste of the control soda compared with the sugar-added one? Is one more bubbly tasting than the other?
  • If you like, compare the taste of your control soda and sugar-added soda with the salt- and the oil-added sodas. (Taste a very small amount of the test sodas!) What do you notice about the taste of the control soda compared with the salt- and oil-added sodas? Is one more bubbly tasting than the other? How do these compare with the sugar-added soda?
  • Extra: Try adding small amounts of other edible kitchen ingredients to your soda such as flour or baking soda. Observe the reactions!

Observations and results
In this activity you added different kitchen compounds to soda and observed the reactions. You should have noticed when you added salt and sugar the soda fizzed, and lots of bubbles rose to the surface of the drink. You should also have observed the sight and sound of the gas leaving as well as fewer bubbles in the soda when you tasted it. Adding the oil, however, had little effect on the bubbles in the soda. The reason for this has to do with how the CO2 gets into soda and how it behaves once it’s there.

The soda is supersaturated with carbon dioxide that is just waiting to escape. Adding sugar and salt gives the CO2 gas the opportunity to leave the soda. Under certain conditions such as those in this activity supersaturated solutions will give up what has been dissolved into them at a far quicker rate than they would if left alone. When you added sugar or salt to soda, the CO2 in each cup latched onto the tiny bumps on the sugar or salt grains. Those tiny bumps, called nucleation sites, give the CO2 something to hold onto in the soda as it forms bubbles and escapes. The oil molecules, in contrast, have fewer rough spots than the grains of salt and sugar—and thus fewer nucleation sites. As a result CO2 escaped the soda more slowly when you added oil to it.

More to explore
Coke and Mentos—Exploring Explosive Chemistry, from Science Buddies
Salt Sculpture Stalactites, from Scientific American
How Sweet It Is—How Much Sugar Is Really in That Soda?, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies