Key concepts
Chemistry
Acids
Bases
Reactions

Introduction
You might enjoy cranberry sauce, but if you’ve ever tasted a real cranberry, you were probably surprised by the taste—definitely not as sweet as the sauce or sweetened dried berries! Pure 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).

Cranberries 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. 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. Who knew that cranberries were so much more than sauce?!

Background
If you’ve ever gotten lemon juice in a cut and felt that 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 stings 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 it!

To measure how acidic or basic a liquid is, we use the pH scale. This scale ranks liquids from 0 to 14, with 0 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, which turn different colors depending on whether they’re exposed to an acid or a base.

Cranberries are special because they contain a natural pH indicator—pigments called anthocyanins, which 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 basicity or acidity changes the color of a pH indicator.

Materials

  • Three cups of pure, 100 percent cranberry juice (not cranberry juice cocktail)
  • Baking soda
  • Lemon juice (four tablespoons, approximately two lemons, as fresh-squeezed juice works best)
  • Four clear glass or plastic cups
  • One large plastic or glass clear bowl or casserole dish (A large glass baking pan works well.)
  • Measuring spoon (tablespoon)
  • Measuring cup (one cup size)
  • A flat workstation that can be exposed to some spills (including cranberry juice)

Preparation

  • Use the measuring cup to measure one cup of cranberry juice into three of the clear cups.
  • Use the measuring cup again to measure one cup of water into the fourth clear cup.
  • If using fresh lemon juice (recommended), juice the lemon and set the juice aside in a cup or bowl.
  • Place the three cups with juice in them side by side in the big dish. We’ll call the cup on the left cup 1, the middle one is cup 2, and the cup on the right is cup 3. The cup with water is the water cup, which should be set aside for now.

Procedure

  • Use your measuring spoon to measure two tablespoons of lemon juice into cup 1. Compare the color of the cranberry juice in cup 1, with the color of the juice in cup 2 (with no lemon). What do you notice about the color of the liquid in cup 1? Did it change when you added the lemon juice? If so, did the lemon juice make it lighter or darker?
  • Set cup 1 aside for now.
  • Using your measuring spoon, measure one 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?
  • Allow the reaction to continue for at least two minutes. When there is no more foam being created, look at the juice in the bottom of cup 3. Compare it with cup 2. What do you notice about the color of the liquid in cup 3? Is it darker, lighter or the same as that in cup 2? What about the volume of juice in cup 3? Is there more or less in cup 3 compared with cup 2? What do you think caused the changes you observe in cup 3?
  • Put cup 1 back in your dish, so that all three 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?
  • Rinse off your measuring spoon so there is no baking soda on it. Then use it to add two 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 with when you added baking soda? How is this reaction different? How is it the same?
  • Allow the reaction to continue for approximately two minutes, until no more foam is created. Did anything change in cup 3 after you added the lemon juice? If so, what is different? What do you think caused the changes you observe?
  • Use your measuring spoon to measure two 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. Now add lemon juice to just one of the cups. Compare the color and volume of the juice as well as the how the foam looks in the two cups. What is different about the two cups?
  • Extra: Try adding two tablespoons of lemon juice to a cup of cranberry juice, and then adding baking soda. How does this change the reaction?
  • Extra: Try 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 one cup of cranberry juice for about one 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
You might 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 whereas 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! And in the presence of acids anthocyanins become lighter in color.

When you added baking soda to the cranberry juice, a reaction took place that released a gas, which creates big bubbles as well as little bubbles (foam) in the juice. When you added baking soda to the water cup, however, 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. Baking soda is a base, therefore when it comes in contact with an acid like cranberry juice, a reaction occurs. But when the liquid is pH neutral, as water is, no reaction occurs.

Finally, you might 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 liquid juice left in the cup because some of the juice became foam during the reaction.

More to explore
Acids, Bases and the pH Scale, from Science Buddies
Cabbage Chemistry—Finding Acids and Bases, from Scientific American
Measuring the Amount of Acid in Vinegar by Titration with an Indicator Solution, from Science Buddies
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies