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Comparing Gas Density Mark as Favorite (6 Favorites)

DEMONSTRATION in Observations, Density, Chemical Change, Density, Inferences, Kitchen Chemistry. Last updated March 30, 2021.


In this demonstration, students will observe a reaction between baking soda and vinegar in the presence of a variety of different heights of lit candles. The initial environment has plenty of oxygen present in order to sustain the candle’s flame; however the reaction will produce carbon dioxide which will cause the lit candles to extinguish in order of height. Students will analyze and compare the presence of the gases in the container and make determinations about the densities of each.

Grade Level

Middle or high school


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

  • Determine that carbon dioxide has a higher density than air based on their observations.
  • Compare the density of gases based on their behavior.
  • Understand that oxygen is necessary for a combustion reaction to occur.

Chemistry Topics

This demonstration supports students’ understanding of

  • Density of gases
  • Chemical reactions
  • Combustion reaction
  • Observations


Teacher Preparation: 15-20 minutes

Lesson: 15-20 minutes


  • 2-gallon glass container (lid not needed)
  • 4 candles of different heights (recommendation: candlesticks cut to lengths of: 3 inches, 4 inches, 5 inches and 6 inches)
  • 1 additional candle or long reach lighter (for lighting candles in container)
  • 400 mL Vinegar
  • 30 grams Baking Soda
  • Small funnel (must fit securely in opening of tubing)
  • Plastic Tubing (approximately 3 feet in length)
  • 4 small containers of Play-Doh, or clay
  • 1 small aluminum pan (recommendation: mini loaf pan)
  • Graduated cylinder
  • Electronic scale


  • Always use caution around open flames. Keep flames away from flammable substances.
  • Always wear safety goggles when handling chemicals in the lab.
  • Vinegar vapors can be irritating. Work in a well-ventilated area. In the event of eye contact, flush with water.
  • The concentration of acetic acid in this experiment does not present any significant hazards.

Teacher Notes

  • This demonstration was inspired by and adapted from Fish Tank CO2
  • First prepare four candles for the demonstration. We suggest using candlesticks, and cutting each to the following measurements: 3inches, 4 inches, 5 inches, 6 inches.
  • Fill the aluminum mini loaf pan with Play-Doh, and place the four candles into the pan, standing each vertically in order of height. Use the Play-Doh to secure the candles in place.
  • Measure 30 grams of baking soda, and distribute it in the bottom of the glass container in a circular pattern.
  • Place the aluminum tray, containing
  • the Play-Doh and candles in the
  • center of the container.
  • Measure 400mL of vinegar.
  • Connect the funnel to the tubing. Make sure that the connection is secure.
  • Using a fifth candle of long reach lighter, light each of the candles in the glass container.
  • Place the end of the hose in the container, as close to the floor of the container as possible. Pour the 400mL of vinegar slowly through the funnel, and continuously move the end of the hose around the inside of the container so that it reacts with as much of the baking soda as possible (you may need assistance from a second person for this).
  • The reaction will begin immediately.
  • Key discussion points and explanations for this demonstration:
    • What is vinegar? What is baking soda?
    • How do you know a chemical reaction is taking place?
    • Reaction:  NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2
    • Note that one of the products of this chemical reaction is carbon dioxide (CO2)
    • Which has a higher density – CO2 or air?
    • Students should observe closely (the candles will go out sequentially from shortest to tallest and there will be fizzing/gas produced to indicate a reaction has taken place).
    • Why do you think the candles went out at different times? (CO2 is forming from the reaction in the bottom of the tank, and since it is denser than the oxygen in the container is getting pushed up higher/out of the container.  Without oxygen the flame is not sustainable).  
    • What reactants are needed for combustion? (Both oxygen and a fuel (the candle) are needed for the combustion reaction to take place.  Since CO2 has a higher density than oxygen, it builds up in the bottom of the tank, replacing the oxygen).
  • After all the candles have been extinguished, you may want to take the fifth candle, originally used for lighting, or the lighter and dip it into the container. The carbon dioxide gas will still be present and the flame will be extinguished.