Is There Sodium in Bananas?

Introduction

Flame tests are used as a qualitative analysis test for the presence of various metal ions. This demonstration provides a safe, unique method for observing flame test colors of metal ions—no flammable solvents required! Demonstrate the characteristic colors produced by three different metal ions, and then let students identify which ions are present in bananas.

Concepts

• Properties of metals
• Emission spectra
• Atomic structure
• Nutrients in food

Materials

Safety Precautions

Potassium carbonate is slightly toxic by ingestion and a body tissue irritant. Sodium carbonate may be a skin irritant. Use only borosilicate glass (e.g., Pyrex^®) Petri dishes in this demonstration and check the glassware for cracks and scratches before use. Exercise caution when using a Bunsen burner. Take care that hair, clothing and hands are a safe distance away from the flame at all times. All food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any food material in the lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The resulting solutions may be disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

1. Obtain three weighing dishes. Label one Ca, one K and one Na.
2. Weigh 2 g of calcium carbonate in the Ca weighing dish.
3. Weigh 3 g each of the other two metal carbonates—potassium carbonate and sodium carbonate—in their respective weighing dishes.
4. Remove all flammable materials from the demonstration area.

Procedure

Part 1. Metal Ion Flame Test

1. Place three borosilicate glass Petri dishes in a row in front of the class. Note: The demonstration must be done on a heatresistant surface. Placing a ceramic fiber square under each of the three Petri dishes is recommended.
2. Add the measured calcium carbonate to one Petri dish, evenly distributing the metal carbonate across the bottom of the dish.
3. Repeat step 2, adding the measured amount of the other two metal carbonates to separate Petri dishes.
4. Have students observe the appearance of each metal carbonate and record their observations on the student worksheet.
5. Measure 60 mL of vinegar in a 100-mL graduated cylinder.
6. Dim the lights.
7. Light the Bunsen burner and adjust the air supply to get a blue cone-shaped flame.
8. Pour approximately 10 mL of vinegar into the first Petri dish.
9. Quickly but carefully tip the Bunsen burner so the flame is just over the Petri dish in the region of rising gas from the vinegar–carbonate reaction.
10. Have students observe the color of the flame and record their observations on the worksheet.
11. Add more vinegar (5–10 mL) if desired to continue the reaction and the observation of the flame color.
12. Repeat steps 7–10 with the potassium carbonate and sodium carbonate.

Part 2. Identifying Metal Ions in Bananas

13. Wearing gloves, obtain a banana chip. Note: Wearing gloves prevents any sodium that may be present on your hands from being transferred to the banana chip.
14. Dim the lights.
15. Light the Bunsen burner and adjust the air supply to get a blue cone-shaped flame.
16. Using tongs, hold the banana chip at the edge of the flame.
17. Have students observe and describe the yellow flame with hints of lavender to confirm the presence of both sodium and potassium, respectively.
18. See the Tips section for a fun way to end this demonstration.

Student Worksheet PDF

12822_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 25 g of calcium carbonate, 30 g of potassium carbonate, 30 g of sodium carbonate, 500 mL of vinegar, 1 lb of banana chips and 3 weighing dishes.
• This is a great integrated demonstration, with applications for lessons in chemistry, light, minerals and health.
• Students may know that bananas contain potassium, but may be surprised to see the characteristic yellow flame of sodium as well. At the end of the demonstration, ask the title question, “Is there sodium in bananas?” Then write the word, banana, on the board. Tell the students of course there is sodium in bananas, it appears twice—baNaNa!
• Be sure to use Petri dishes made of borosilicate glass. Accidents have been known to happen when evaporating dishes or Petri dishes not labeled as borosilicate glass have been used.
• A butane safety lighter, available from Flinn Scientific (Catalog No. AP8960), may be used in place of a Bunsen burner flame for Part 1. Adjust the lighter to achieve the largest flame possible. The initial burst of carbon dioxide gas may extinguish the flame—simply click the lighter again. Holding the flame of the lighter near the side of the Petri dish instead of the center may keep the flame from going out. Using a butane safety lighter for Part 2 will not give satisfactory results.
• Exact amounts of the metal carbonates and vinegar are not critical. Calcium carbonate is a fine powder; therefore less is needed by weight than the potassium carbonate or sodium carbonate. Start with a small amount of vinegar, observe the flame color, and add more vinegar until the reaction is complete.
• Copper(II) carbonate does not work well with this method. Copper(II) carbonate is in the form _Cu2(OH)2CO3.
• A student laboratory activity, the Flame Test Kit available from Flinn Scientific (Catalog No. AP5607), uses wooden splints and metal salts to demonstrate the characteristic colors of six different metal ions.
• Videos of both parts of this demonstration, Flame Test Demonstration, presented by Penney Sconzo, and Is There Sodium in Bananas? presented by Jamie Benigna, are available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. See the eLearning website at http://elearning.flinnsci.com for viewing information. Both videos are part of the Flame Tests video package.

Sample Data

Observations

Describe the appearance of each solid in the Petri dishes.

Calcium carbonate—fine white powder
Potassium carbonate—fine white crystals
Sodium carbonate—fine white crystals

List the metal ion present in each Petri dish and record the flame color it produced.

Answers to Questions

1. Compare the color of each solid to its respective flame color.

   Each solid was white. None of the flames were white, and each was a different color.

2. How do you know the metal ions were producing the characteristic flame colors and not some other substance?

   The only variable from one Petri dish to another was the type of metal ion. The other substances—carbonate ions and vinegar—were variables that remained the same.

3. Predict the color of the flame if the following materials were heated in the flame. Explain your prediction.
   1. Sodium sulfate, Na₂SO₄—yellow
   2. Potassium nitrate, KNO₃—lavender
   3. Calcium chloride, CaCl₂—red-orange

   The colors are predicted by noting the metal in each compound because it is the metal cation, and not the anion, that determines the color of the flame.

4. What color or colors were visible when the banana was placed in the flame?

   Both yellow and lavender were visible.

5. What conclusion may be drawn about the mineral ion content of bananas based on the flame test results?

   Bananas contain both potassium and sodium.

6. Metal carbonates react with vinegar, a source of H⁺ ions, to produce carbonic acid, which decomposes to carbon dioxide and water. Complete the following equation for the reaction of the carbonate ions. What is the formula of carbonic acid?

Discussion

When a substance is heated in a flame, the atoms absorb energy from the flame. This absorbed energy allows the electrons to be promoted to excited energy levels. From these excited energy levels there is a natural tendency for the electrons to make a transition or drop back down to the ground state. When an electron makes a transition from a higher energy level to a lower energy level, a particle of light called a photon is emitted (see Figure 1). The energy or wavelength of the emitted photon determines the color of light observed in the flame.

The color of light observed when a substance is heated in a flame varies from one substance to another. Because each element has a different electron configuration, the exact energy of the emitted photon and the corresponding wavelength and color are unique to each element. As a result, the colors observed when a substance is heated in a flame may be used as a means of identification.

In this demonstration, when the vinegar reacts with the carbonate ion, carbon dioxide gas is produced. The rapid evolution of gas creates an aerosol that entraps some of the small particles of solid in each dish. The metal ions from the carbonate salts are carried up and away from the Petri dishes as observed when they are heated in the flame brought over the dishes. In the presence of the flame, the electrons of the metal ions are promoted to the excited state and proceed to emit a characteristic color of light as they return to the ground state.

Calcium, potassium, and sodium ions are commonly found as mixtures in nature and are essential minerals for human health. The banana flame test can confirm what students may already know—that bananas contain potassium—and what they might not know—that a small amount of sodium is present as well.