Summary

In this demonstration, students will witness the ability of carbon dioxide to extinguish a flame and oxygen to feed a flame. They will also be introduced to the concept of catalysts. This demonstration could also be used as an opportunity to practice writing and balancing equations and classifying chemical reactions.

Grade Level

Middle School, High School

NGSS Alignment 

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• Scientific and Engineering Practices:
  • Constructing Explanations and Designing Solution

Objectives

By the end of this demonstration, students should be able to:

• Classify the types of reactions they see.
  
• Recognize that combustion reactions can only take place in the presence of oxygen.
• Understand that carbon dioxide extinguishes a flame.
• Understand the role of a catalyst.

Chemistry Topics

This demonstration supports students’ understanding of:

• Combustion
• Gases
• Chemical change
• Catalysts
• Classifying Reactions
• Balancing equations

Time

Teacher Preparation: 10 minutes

Lesson: 10-20 minutes

Materials

• 2 Erlenmeyer Flasks (250 mL)
• 10ml graduated cylinder
• Matches or a portable butane lighter
• Wooden splints
• Commercial vinegar solution
• Solid baking soda
• Hydrogen peroxide solution, 3%
• 2% yeast suspension (you can also use solid manganese dioxide or solid potassium iodide)

Safety

• Always wear safety goggles when working in a chemistry lab.
• Used matches and wood splints can be disposed in the trash as long as they have been extinguished. Immerse them in water briefly to make sure they are not smoldering.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure lose clothing.
• Sodium Bicarbonate SDS
• Vinegar SDS
• Hydrogen Peroxide SDS
• Yeast SDS
• Manganese Dioxide SDS
• Potassium Iodide SDS

Teacher Notes

• This demonstration is an easy way to demonstrate the chemical properties of two gases involved in combustion. It also shows students two examples of how gases can be produced in chemical reactions.

• One is a double replacement acid-base reaction (followed by rapid decomposition of one of the products), and the other is a decomposition reaction that involves a catalyst.
• There are a variety of ways in which to generate oxygen gas. You can add about 5 mL of a yeast suspension to the hydrogen peroxide solution. Another way to generate oxygen is to add a small quantity (0.5–1 g) of either solid manganese dioxide or solid potassium iodide to the hydrogen peroxide solution. This lab uses the yeast suspension. (If you use manganese dioxide or potassium iodide, be sure to dispose of them according to local regulations. Yeast is easiest, as it can be poured down the drain with lots of water.) Raw potatoes or raw meat also contain enzymes that catalyze the decomposition reaction. Liver works particularly well. The meat does have to be raw so be sure to thoroughly clean anything that comes into contact with raw meat if you choose that option. See examples of various catalysts in this video.
  
• Prepare yeast suspension: Add 1 gram of active dry yeast to 10ml of distilled and mix or shake well to make suspension.
• Depending on students’ familiarity with chemical equations, combustion reactions, and catalysts, you may wish to provide more detailed explanations of the chemical processes that are occurring as you do the demonstration. You can use the questions in the student handout as a way to structure the discussion with students.
  
• You could do this demonstration with just a discussion about the concepts being presented, or with the accompanying student handout (below).
  

Demonstration Procedures

1. Add 1.5 – 2 grams of solid baking soda to one of the flasks. The exact amount is not critical.
2. Measure 10ml of vinegar into a 10ml graduated cylinder.
3. Add about 15–20 mL of 3% hydrogen peroxide to the second flask.
4. Measure 5ml of yeast suspension into a 10ml graduated cylinder.
5. Pour the vinegar into the flask that contains baking soda. The goal is to produce a small quantity of carbon dioxide gas in the flask. The foam that may be produced should not overflow out of the flask.
6. Pour the yeast suspension or other catalyst into the 3% hydrogen peroxide. The goal is to produce a small quantity of oxygen gas in the flask. The foam that may be produced should not overflow out of the flask.

1. Light a wood splint and allow it to burn for a few seconds. If the tip of the wood splint begins to glow red, this will make it easier to relight the splint after it is extinguished.
2. Quickly insert the flaming wood splint into the flask with carbon dioxide gas. The splint should not touch the liquid in the flask. The flame will become extinguished because of the carbon dioxide gas. The tip of the wood splint should still glow at this point. If it is not, you need to use a match or a lighter to heat the tip of the splint until it glows.
3. Insert the glowing wood splint into the flask with oxygen gas. The splint should not touch the liquid in the flask. The glowing splint should reignite because of the presence of oxygen gas.
4. This process can be repeated over again several times. The flaming splint will go out in the carbon dioxide, and the glowing splint will relight in the oxygen. Stop when the wood splint becomes too short to use safely.
5. Before disposing of the hydrogen peroxide flask, show students that the yeast or other catalyst is still present in the flask and it was not consumed by the reaction. Explain that this is a catalyst – it increased in the rate of the reaction without getting used up itself. (In a more advanced class, you could talk about how it lowers the activation energy by providing a new mechanism or reaction path by which the reaction proceeds.) You could discuss how hydrogen peroxide decomposes over time on its own, but much more slowly. You could also mention that light hitting the hydrogen peroxide molecules speeds up the reaction too. This is why it is stored in brown bottles – the dark bottles prevent light from reaching those molecules so they don’t break down as quickly and can have a longer shelf life.
   
6. Pour the contents of the two flasks and the remaining yeast suspension down the sink and rinse with plenty of water. (If you use manganese dioxide or potassium iodide, you will need to consult local regulations for proper disposal methods before you pour the hydrogen peroxide flask down the drain.)

Lesson

Answer the following questions while you watch the demonstration and listen to your teacher’s explanations.

Baking Soda + Vinegar

The first reaction in this demonstration is between baking soda and vinegar. It proceeds according to the equation below:

NaHCO₃ (s) +    HC₂H₃O₂ (aq)  -->    H₂CO₃ (aq) +   NaC₂H₃O₂ (aq)

1. What kind of reaction is this?
2. Is this equation balanced? If not, balance it.
3. What signs indicate that a chemical reaction is occurring when these two substances are mixed together?

A secondary reaction occurs when H₂CO₃, one of the products, breaks down according to the equation below:

H₂CO₃ (aq) --> H₂O (l) + CO₂ (g)

4. What kind of reaction is this?
5. What part of this reaction caused what you observed in question #3?

Hydrogen Peroxide

The second reaction involves hydrogen peroxide, H₂O₂, breaking down into water and oxygen gas.

6. Write and balance the equation described above.
7. What kind of reaction is this?
8. Examine the flask with the hydrogen peroxide carefully. Does the substance added to the hydrogen peroxide get used up, or is it still in the flask? What is this kind of substance called?
9. Why was this substance added to the flask?

Burning Wood

This reaction involved burning a piece of wood. Observe what happens when the burning wood is exposed to the products of the first two reactions.

10. What do chemists call “burning” reactions? What is the general equation for this type of reaction?
11. What happens to the burning wood when it is put into the flask that contained the baking soda and vinegar reaction? What happens when it is put in the flask that contained the hydrogen peroxide reaction?
12. Explain why the burning wood behaved differently in the two flasks. (Hint: look at the reaction you wrote in question #10 and compare that to the products of the reactions in the two flasks.)