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DEMONSTRATION in Density, Chemical Change, Density, Combustion, Chemical Change, Kinetic Molecular Theory, ChemMatters Teaching Resources. Last updated April 17, 2023.

Summary

In this demo, students will carry out and witness the burning of a substance in its gas and in its liquid states. They will compare the results.

Grade Level

High school

Objectives

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

  • Understand the difference between a burning liquid and a burning gas.

Chemistry Topics

This lesson supports students’ understanding of

  • Combustion
  • Physical changes
  • Chemical changes

Time

Teacher Preparation: 10 minutes

Lesson: 45 minutes

Materials

  • Methanol or isopropyl alcohol (95%) in a small bottle (in the hood)
  • Graduated cylinder
  • Large watch glass
  • Plastic 1-gal (methanol) or 1-L (isopropyl alcohol) empty milk jug with lid
  • Candle taped to end of meter stick
  • Matches

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • An operational fire extinguisher should be in the classroom.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure lose clothing.
  • Do not reuse the jug more than once (after two demonstrations, discard the jug). You could wrap it in duct tape for extra reinforcement.
  • Methanol should be kept in the fume hood, with the cap on, away from the demonstration area.
  • If the methanol does not light, redo the experiment only once.

Teacher Notes

  • See Chemistry Solutions article Demonstrations and Good Pedagogy
  • Before the demo homework, assign for homework
  • Post demo homework: write a compare and contrast paragraph about what you witnessed in class today.
  • Students may be surprised that there is no explosion in the second experiment, especially if they are reminded that there is MORE alcohol left in the watch glass than there was in the jug.
    • Going to standard KMT model pictures for gases, liquids, and solids, the low-density gas is easily mixed with air (oxygen) and burns quickly. The liquid only burns at the surface where it contacts air.
  • Possible Socratic Questions:
    • What is your hypothesis before each lighting?
    • What’s the difference between the ChemMatters and the Bakersfield materials?
    • Was the bottle empty after the explosion (if they did not observe it afterwards, remind them to, and tell them to turn it upside down for the water to run out if they did not)?
    • Where did the water come from?
    • Why did the teacher turn the jug over and over?
    • Which is less dense, methanol or air (or isopropyl alcohol and air)? (Remind them of the KMT if needed).
    • Why did it make that sound? (for the jug experiment)
    • Why did it emit light?
    • Could you make the light different colors by adding different things to the alcohol?

For the Student

Lesson
  1. Put on safety goggles (entire class).
  2. Tape a candle to a meter stick.
  3. The reader reads the instructions for the demo twice. Those who are not the reader should carefully observe, diagramming or sketching the apparatus, and perhaps making hypotheses. The second time the instructions are read, the doer performs the actions.

Demonstration #1: Alcohol mixed with air and ignited

  1. Pour not more than 4 mL of methanol or 7 mL of isopropyl alcohol from the bottle into a graduated cylinder in the hood.
  2. Put the cap back on the alcohol bottle and bring the graduated cylinder to the front.
  3. Pour the alcohol into the jug, putting the graduated cylinder on another table away from the jug.
  4. The teacher should cap the jug and turn the jug over and around.
  5. Light the candle far away from the jug.
  6. The teacher will place the jug on the desk, take off the lid, and turn out the lights.
  7. Staying far away from the jug, hold the flame near the top of the jug. Make observations.

Demonstration #2: Methanol or isopropyl again, but liquid

  1. Measure about 5 mL of methanol or isopropyl alcohol from the bottle into the graduated cylinder in the hood.
  2. Put the cap back on the alcohol bottle and bring the graduated cylinder to the front.
  3. Pour it into a large watch glass.
  4. Step away, light the candle, and staying far away, bring the flame to the alcohol. Make observations.