Key concepts
Chemistry
Chemical reactions
Food science
Carbonation
 
Introduction
Have you ever wondered how carbonated soda is made? As you sip it, the bubbles in soda tickle your tongue's taste buds and propel the ingredients to your palate and nose so that you get a kick of flavor. But how do the bubbles, fizz and taste get into the water? In this tasty science activity you will work with baking soda, citric acid and sugar to create your own soda! And after you develop your recipe add a few drops of green food coloring and you've got a festive drink you could share with friends and family to celebrate Saint Patrick's Day!
 
Background
Soda is essentially carbonated water with sugar added. Water becomes carbonated when a gas called carbon dioxide (CO2) has been dissolved into it under high pressure. When you open a can or bottle of soda, the pressure is released and the CO2 starts to come out of the solution. The escaping gas is what causes the bubbles in the beverage. Carbonation also occurs in nature, when water underground comes in contact with a source of carbon, such as limestone. The high-pressure reaction underground between the water and limestone forces CO2 to dissolve into the water. When the water eventually rises to Earth's surface, the pressure is released and bubbly water is the result.
 
In this science activity, instead of pressure, you'll use baking soda (also called sodium bicarbonate), citric acid and sugar to develop your own recipe for a soda drink. When you mix baking soda and citric acid together with water, a chemical reaction takes place that creates CO2, along with sodium citrate. (Sodium citrate, like the other compounds you'll be using in this activity, is a harmless substance that's safe to eat or drink.)
 
Materials

  • At least five cups
  • Baking soda
  • Citric acid (Look for it in the canning or baking section of a supermarket.)
  • Sugar
  • Water
  • Green food coloring
  • Spoon or fork for stirring
  • Measuring spoons
  • Measuring cup
 
Preparation
  • Measure out and add one-quarter teaspoon (tsp.) of citric acid to three of your cups. Leave the other two cups empty.
 
Procedure
  • To one of the cups holding citric acid add one-sixteenth tsp. baking soda. (Measure one-sixteenth tsp. by filling a one-eighth tsp. measuring spoon half full.)
  • Add one-quarter cup of cool water to the cup with citric acid and baking soda in it, mixing them together with a spoon or fork. Are there a lot of bubbles? Is the liquid mildly bubbly or is it bubbling a lot?
  • Taste a little bit of the beverage. How does it feel on your tongue? Is it too gritty? Do you find the feeling of the liquid in your mouth pleasant?
  • After tasting the beverage feel free to spit out the liquid (into one of the empty cups). It will not harm you to swallow it but it might not taste very much like soda yet, and you also don't want to over-acidify your stomach (which could give you a slight stomachache) because there is still more taste testing to do.
  • To one of the two remaining cups with citric acid add and mix together one-quarter tsp. baking soda and one-quarter cup of cool water. Are there more or less bubbles with this mixture?
  • Again taste a little bit of the beverage. How does it feel on your tongue? Is it gritty or do you find the feeling to be pleasant? Again, you can spit out the liquid into an empty cup after tasting it.
  • To the last cup with citric acid in it add and mix together one tsp. baking soda and one-quarter cup of cool water. Are there more or less bubbles with this mixture?
  • Again taste some of the beverage. How does it feel on your tongue? Is it gritty or pleasant? As with the other samples, you should spit it out afterward.
  • Decide which recipe you enjoyed the best. Make up a fresh quarter cup of it (the same way you made it before). Then mix in one-quarter tsp. of sugar. How does it taste? Does it taste like a soda pop?
  • Continue to mix in one-quarter tsp. of sugar at a time, tasting it after you add the sugar each time and keeping track of how much you have added. (Add no more than one tsp. sugar total.) What amount of added sugar makes for the tastiest soda pop?
  • After you've decided how much sugar to add to your baking soda and citric acid mixture you can scale up the recipe to make a festive, green-colored beverage! For example, if you found that one-quarter tsp. of baking soda and one-quarter tsp. of sugar worked well in one-quarter cup of water, you could make one cup of soda with one cup of water, one tsp. of baking soda, one tsp. of sugar, and one tsp. of citric acid. Then mix in just three drops of green food coloring, and your festive beverage is ready to share and enjoy!
  • Extra: Ask your friends and family to taste the final recipe that you developed. Do they enjoy the beverage as much as you do? Collect their feedback and see if you can develop a recipe that everyone enjoys. You could also try adding flavorings such as vanilla, cinnamon or a little bit of crushed fruit.
  • Extra: Do this activity again but test a wider variety of baking soda and citric acid recipes (such as including the one-eighth tsp. and one-half tsp. baking soda recipes). Then try to numerically rank (such as on a scale of 1 to 5) how gritty and bubbly each combination is. Which recipe makes the bubbliest soda pop? What about the least gritty one? Overall, which recipe do you like the best?
  • Extra: Some fruits contain citric acid. Try re-creating your soda recipe using juice from a fruit that contains citric acid—such as a lemon or a lime—instead of using food-grade citric acid. How does the amount of fruit juice compare with the quantity of food-grade citric acid in your ideal flavor combination?


Observations and results
Was the recipe that used one-quarter tsp. of baking soda and one-half tsp. of sugar the best, tasting the most like a normal soda pop?
 
In this activity, when you combined baking soda and citric acid (with water), the chemical reaction should have made fizzy CO2 bubbles, making all of the recipes you tested produce at least some bubbles. The recipe with the least amount of baking soda added (one-sixteenth tsp.) should have made the fewest, shortest-lived bubbles whereas the recipe with the most baking soda (one tsp.) should have been clearly the fizziest. This is because there is more baking soda available to react with the citric acid, making CO2 bubbles. The recipe with one tsp. of baking soda, however, was likely rather gritty and bitter, due to the large amount of baking soda. This probably made the recipe using one-quarter tsp. baking soda the ideal recipe in terms of bubbliness, flavor and texture. You may also have found that adding one-half tsp. of sugar was the ideal amount, but this can vary based on personal preference.
 
More to explore
Introduction to Pop—The History of Soft Drinks, from About.com
Shimmy, Shimmy Soda Pop: Develop Your Own Soda Pop Recipe, from Science Buddies
Sweet Science: Dancing Conversation Hearts, from Scientific American
Sugar-Sweetened Beverages: Extra Sugar, Extra Calories and Extra Weight (pdf), from the California Center for Public Health Advocacy
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies