Pass the Cranberry….Volcano!
While you might enjoy cranberry sauce, if you’ve ever tasted a real cranberry you were probably surprised by the taste – definitely not as sweet as the sauce! Cranberries and cranberry juice are very tart to eat, but they can be delicious when combined with other ingredients (such as orange zest and sugar, or cranberry muffins!).
Cranberries add tartness to these foods because they are acidic, similar to lemons or limes. Foods that are acidic will usually taste sour or tart, so we use them to brighten up a meal, but we rarely eat them by themselves (can you imagine eating a whole lemon – ouch!).
In addition to being acidic, cranberries also contain a special, color-changing pigment that we can use to test whether something is an acid or a base. Who knew that cranberries were so much more than sauce?!
In this activity we will use cranberries (in this case, cranberry juice) to identify acids and bases, and to observe the chemical reactions created when you mix the two!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
If you’ve ever gotten lemon juice in a cut and felt that special sting, then you know a little bit about acids! And you probably know about bases too - when you wash your hands and get that slippery feeling from the soap – that’s a base! Most liquids are either an acid or a base, but some acids and bases are stronger than others. Lemon juice can sting when you get it in a cut, because it’s acidic. But other types of very strong acids can dissolve wood and metal. Believe it or not, the acid in your stomach (that helps dissolve your food) is a very strong acid. If you dropped a little of your stomach acid onto a block of wood, the acid would eat right through the wood! But don’t worry - the acid in our stomachs doesn’t eat through our bodies, because the inside of our stomachs are very strong – in fact, you could say that our stomachs are acid-proof!
To measure how acidic or basic a liquid is, we use the pH scale. The pH scale ranks liquids from 1-14, with 1 being the most acidic, and 14 being the most basic. We can identify how acidic or basic something is by testing how it reacts with other things. We often use pH indicators for this purpose. PH indicators turn different colors depending on whether they’re exposed to an acid or a base. So we can use a pH indicator to tell us if something is acidic or basic!
Cranberries are special because they contain a pH indicator – pigments called anthocyanins. Anthocyanins change color depending on the pH of their environment.
In this activity we will use this special ability of cranberries to observe a chemical reaction, and observe how pH changes the color of a pH indicator.
Extra: If you have enough cranberry and lemon juice, try adding baking soda to two side by side cups of cranberry juice. Allow the reaction to take place, then add lemon juice to just of the cups. Compare the color and volume of the juice, as well as the how the foam looks in the two cups.
Extra: Experiment with adding 2 tablespoons of lemon juice to a cup of cranberry juice, and then adding baking soda. How does this change the reaction?
Extra: Experiment with increasing the amount of baking soda that you add (make sure to keep this in your dish so it doesn’t get messy! Does adding more baking soda making the reaction happen faster? Does it increase the amount of foam?
Extra: With an adult helper, try microwaving 1 cup of cranberry juice for about 1 minute, making it warm (but not hot!). Add the baking soda to the warm juice. Does the temperature of the juice change the reaction?
Observations and Results
When you added baking soda to the cranberry juice, a reaction took place that released a gas. Similar to how you blow air (a gas!) into bubble gum to create a bubble, this gas creates foam (very small bubbles) and some big bubbles in the cranberry juice. Lots of them! However, when you added baking soda to the water cup- nothing really happened, no foam was created, just some cloudy water. Why doesn’t baking soda cause a reaction in water, the way it does in cranberry juice?
The difference is that, because cranberry juice is slightly acidic, it reacts with anything that is basic. Based on this activity, would you guess that baking soda is a base? If so, you would be right! Baking soda is a base, therefore when it comes in contact with an acid like cranberry juice, a reaction occurs.
You may also have noticed that the color of the cranberry juice changed when you added lemon juice or baking soda. Adding the baking soda should have made the juice turn a darker color, while adding the lemon juice made it slightly lighter. This takes place because of the amazing pigments that cranberries contain, the anthocyanins, which act as a pH indicator. Anthocyanins change color depending on whether they are reacting with an acid or a base. From this activity, would you guess that anthocyanins get darker when they come in contact with a base, or with an acid? If you predicted base, you’re right! In the presence of acids, anthocyanins become lighter in color, while in the presence of a base, they get darker.
Finally, you may also have noticed that the volume (amount of juice) was less in Cup 3, after the baking soda reaction. This is because the foam created by the reaction was made up of the juice itself (you could probably tell because the foam was a similar color). Therefore, there was less juice in the cup, because some of the juice became foam during the reaction.
More to Explore
Megan Arnett, PhD, Science Buddies
Science Buddies |
Acids, bases, food chemistry, pigments
Explore Our Science Videos
Physics and Chemistry of an Explosion Science Fair Project Idea
How to Build an ArtBot
Flower Dissection - STEM Activity