EXPERIMENT Burning Issues

Visitors use a candle to investigate the products of combustion. The candle flame deposits carbon on a glass rod and water on the sides of a jar: Carbon dioxide gas reacts with the water in an indicator to form carbonic acid.

OBJECTIVES: Visitors learn about products of combustion reactions and how these products contribute to pollution.

SCIENCE TOPICS PROCESS SKILLS VOCABULARY

Combustion Observing Acid Chemical Reactions Inferring Chemical Reaction Indicators Investigating Molecule Pollutant

Experiment: Burning Issues Operating Guide Burning Issues

Procedure:

1. Always wear safety goggles.

2. Dry the glass rod and plastic dish with a tissue.

3. Place the candle on the plastic dish. Ask an OMSI staff person to light the candle for you CAUTION: Keep fingers and hair away from the flame!

4. Hold the glass rod by the handle. Touch the other end of the rod to the tip of the candle flame without touching the wick.

5. Remove the rod from the flame. CAUTION: Do not touch the end that was in the flame; it is hot! What do you see on the end of the glass rod now?

6. Squeeze 1 drop of bromthymol blue onto the plastic dish near the candle. What color is the drop?

7. Place the jar upside down over the candle and the drops of bromthymol blue. What do you see forming on the inside of the jar?

8. Wait one half a minute after the flame goes out; then remove the jar from over the candle. Has the bromthymol blue changed color? (Place a fresh drop near the old drop to check the color.)

9. Wipe the plastic dish and the glass rod clean with a tissue.

What chemical reaction occurs when a candle burns?

A Closer Look:

A candle’s wax molecules are made primarily of carbon and hydrogen. As the wax burns, it reacts with oxygen in the air. In this experiment, the candle’s flame went out when it used up all the oxygen in the jar. A burning reaction is called combustion. The products of combustion—carbon, water, and carbon dioxide— were all seen in this experiment. The black soot on the glass rod was carbon (C), the “fog” on the inside of the

jar was water (H2O), and the bromthymol blue drop

turned yellow in the presence of carbonic acid (H2CO3),

which formed when the carbon dioxide gas (CO2) combined with the water in the drop of bromthymol blue. The combustion reaction of a candle is similar to the burning of wood or gas. Combustion reactions are responsible for some types of environmental pollution. The carbon particles produced add to smog. Carbon dioxide is a “greenhouse gas” and contributes to global warming.

See Materials Prep (with amounts to have on hand) for more details • One votive candle (keep 3 on hand) • One petri dish • Two 4-inch glass rods • Two caps from small dropper bottles • One 30-ml dropper bottle • Bromthymol blue sodium salt (keep 5 g on hand) • One large glass jar with a neck small enough to fit inside the petri dish cover • One butane lighter or box of matches (general storage) (keep two spares on hand) • paper towels (keep 1 roll on hand) • Two plastic beakers

Setup/Takedown Procedures

Attach the cap from a small dropper bottle to one end of each glass rod to form a handle. Label the 30-ml dropper bottle “Bromthymol Blue.” Label the plastic beakers “Tissues” and “Used Tissues.” Cut paper towels up into approx. 2” x 2” squares Do not place mat at station, if one is there, remove it. The hot glass rod burns holes in mat if left there.

Refill the bromthymol blue dropper bottle with bromthymol blue solution, (Prepare more if needed; see Materials Prep.) Add spare lab tissues to the experiment tub if needed.

Set out the visitor instructions in a Plexiglas holder. Put the butane lighter or matches in a place accessible to the staff but not to visitors. NOTE: The lighter or matches are FOR STAFF USE ONLY — DO NOT PROVIDE THEM TO THE PUBLIC. Add spare lab tissues to the experiment tub if needed.

Experiment: Burning Issues Operating Guide On a tray, set out the following: • One glass rod with handle • One candle • Labeled dropper bottle of bromthymol blue solution • Large glass jar • plastic container of cut up paper towels • Petri dish with labels for candle and bromthymol blue • Labeled plastic beaker for used tissues

Empty the “Used Tissue” beaker into the trash. Tightly cap and store the bromthymol blue in an upright position in the tub. Clean and return the equipment to the tub.

Clean and leave the tray at the station. Clean the glassware and return it to the tub. Clean and return the mat to general lab storage.

◊ The bromthymol blue changes slowly from green to yellow. If a visitor has not seen an obvious color change, he or she may have removed the jar from over the candle too early.

◊ The bromthymol blue should be green at first. If it is blue, add one drop of 1M (or more diluted) HCl (hydrochloric acid). If it is yellow, add one drop of 0.5M (or more diluted) NaOH (sodium hydroxide). (These are readily available with the “Acid Rain” experiment.)

◊ Avoid getting any of the equipment wet, as this will make it hard to light the candle and may put the candle out too soon.

◊ You will need to light the candle each time someone does the experiment. The flame goes out during the experiment, so there should be little danger of uncontrolled fire play-but keep an eye on visitors you think might abuse the fire. NOTE: The lighter or matches are FOR STAFF USE ONLY — DO NOT PROVIDE THEM TO THE PUBLIC.

Experiment: Burning Issues Operating Guide

You can try other indicators-for example, the universal indicator. You can try burning a large wooden match instead of the candle. Do you get the same results?

This experiment involves an open flame; follow the safety and handling instructions.

Consult the Material Safety Data Sheets (MSDS) for additional information.

To prepare bromthymol blue solution: Weigh 0.1 g bromthymol blue sodium salt.

Add 100 ml dH2O (deionized water) and mix to dissolve the bromthymol blue. Store the solution in a labeled and dated bottle. Keep at least 50 ml on hand in the indicator cabinet.