Key Concepts
Acids, bases, food chemistry, pigments
Three cups: the first filled with cranberry juice and lemon, the second filled with cranberry juice, and the third filled with cranberry juice and baking soda.

Introduction

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.

Background

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.  

Materials

  • 3 cups of cranberry juice (not cranberry juice cocktail – 100% cranberry juice works best)
  • Baking Soda 
  • Lemon Juice (4 tablespoons, approximately 2 lemons)
  • 4 clear glass or plastic cups
  • 1 large plastic or glass clear bowl or casserole dish (a large glass baking pan works well)
  • Measuring spoon (tablespoon)
  • Measuring cup (1 cup)

Preparation

  1. Use the measuring cup to measure 1 cup of cranberry juice into 3 of the clear cups.
  2. Use the measuring cup again to measure 1 cup of water into the 4th clear cup.
  3. If using fresh lemon juice (recommended), juice the lemon and set the juice aside in a cup or bowl.   

Procedure

  1. Place the 3 cups with juice in them side by side in the big dish. We’ll call the cup on the left Cup 1, the middle cup is Cup 2, and the cup on the right is Cup 3. The cup with water is the Water Cup. Set aside the water cup for now.
  2. Use your measuring spoon to measure 2 tablespoons of lemon juice into Cup 1. Compare the color of the juice in Cup 1, to the color of the juice in Cup 2 (with no lemon). What do you notice about the color of the juice in Cup 1? Did it change when you added the lemon juice? If so, did the lemon juice make the color get lighter or darker?
  3. Set the Cup 1 aside for now. 
  4. Using your measuring spoon, measure 1 tablespoon of baking soda and pour it into Cup 3 (on the right). What happens when the baking soda reaches the cranberry juice? What do you see? What do you hear? What happens to the juice, is there more or less in the cup? 
  5. Allow the reaction to continue for at least 2 minutes. When there is no more foam being created, look at the juice in the bottom of Cup 3. Compare it to Cup 2. What do you notice about the color of the juice in Cup 3? Is it darker, lighter, or the same as the juice in Cup 2? What about the volume of juice in Cup 3 – is there more juice in Cup 3, or less juice, compared to Cup 2? What do you think caused the changes you observe in Cup 3?
  6. Put Cup 1 back in your dish, so that all 3 cups are again lined up in order. Compare the color and volume of each cup. Which cup is the most different from the others? What makes it different?
  7. Rinse off your measuring spoon so there is no more baking soda on it. Then use it to add 2 tablespoons of lemon juice to Cup 3. What happens when you add lemon juice to Cup 3? Do you notice any differences in the reaction, compared to Step 4, when you added baking soda? How is this reaction different? How is it the same?
  8. Allow the reaction to continue for approximately 2 minutes, until no more foam is created. Did anything change about Cup 3, after you added the lemon juice? If so, what is different? What do you think caused the changes you observe?
  9. Use your measuring spoon to measure 2 tablespoons of baking soda into the Water Cup. Observe what happens when you add the baking soda. What do you notice about the water? Has anything changed? What does this tell you about the cranberry juice and baking soda reaction? Does baking soda always cause liquids to foam, or only some liquids?

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

Credits

Megan Arnett, PhD, Science Buddies

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Key Concepts
Acids, bases, food chemistry, pigments
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