Materials Included In Kit

Additional Materials Required

Prelab Preparation

The steel wool has a fine protective coating of oil. Before the lab, soak the wool in acetone for a few minutes to remove the oil. Allow time for the acetone to fully evaporate before the beginning of the lab.

The 30% hydrogen peroxide needs to be diluted. This is done by adding the peroxide solution to 200 mL of distilled or deionized water.

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Hydrogen peroxide is an oxidizer and a skin and eye irritant. Magnesium ribbon is a flammable solid. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or laboratory coat. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Please follow all laboratory safety guidelines.

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. Hydrochloric acid may be neutralized with base and then poured down the drain with excess water according to Flinn Suggested Disposal Method #24b. Excess hydrogen peroxide solution may be disposed of by diluting it with water to a concentration of less than 3% and rinsing it down the drain with water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Unless care is taken, the steel wool can make getting good results difficult. Make sure that it has been rinsed with acetone before the start of the lab and that all the acetone has evaporated. Students should be encouraged to take no more than 0.5 g and not pack it too tightly into the evaporating dish.
• Do not substitute a watch glass for the evaporating dish, as small pieces of burning iron spark away during combustion and the evaporating dish will contain the majority of these sparks.

Teacher Tips

• Matches will often self extinguish when only half burnt. If this happens, you can either have students record the mass as is (since the exercise is only qualitative) or have them wait for the burnt end to cool and then carefully pick up the match by the burnt end and light the unburnt end with another match.
• Matches are used in Part 1 because they have an additive that holds them together and stops them from dropping their ash.

Further Extensions

You can further extend this lab by incorporating the ideal gas law and having students calculate the volumes of gas that they could expect to be generated during Part 2.

Sample Data

Answers to Questions

Part 1. Does Phlogiston Have Mass?

1. How did the match physically change as a result of being burnt?

   The match became black and was much more fragile after being burnt. It also weighed less.

2. How did the steel wool physically change as a result of being burnt?

   The color of the steel wool changed slightly to a dull grey. Small blobs could now be seen at the ends of several strands. Its mass increased.

3. Based on your observations and measurements, what can you conclude about the mass of phlogiston?

   We cannot conclude anything about the mass of phlogiston. The match decreased in mass, which could mean that phlogiston has a positive mass. However, the steel wool increased in mass, which could mean that phlogiston has a negative mass.

Part 2. Synthesis of Dephlogisticated, Phlogisticated and Inflammable Air?

1. What did you observe about dephlogisticated air?

   The glowing splint that was lowered into the test tube reignited.

2. What did you observe about phlogisticated air?

   The lit splint that was lowered into the test tube went out.

3. What did you observe about inflammable air?

   Placing the lit splint next to the mouth of the test tube resulted in a squeaky pop sound.

Part 3. Teacher Demonstration

1. What did you observe during the teacher demonstration?

   Even though the lit splint was extinguished, the magnesium burnt brightly in the gas generator bottle. The gas that was generated behaved like the inflammable air from Part 2.

Introduction

In the 17th and 18th centuries, the prevailing wisdom was that when something was combusted it released a compound called phlogiston, and if the combustion was conducted in a confined space, the air would eventually become saturated with phlogiston and the flame would extinguish. Antoine Lavoisier conducted several experiments that challenged the phlogiston theory. In this lab, you will conduct analogous experiments and use them to scrutinize this now discredited theory.

Watch the introductory video.

Concepts

• Gases
• Scientific method
• Combustion
• Oxidation

Background

The scientific method begins with observation. Think about a burning candle, some of the observations you could make include the emission of light, smoke, and the candle melting. By simple experimentation and adjusting the conditions under which the combustion takes place we could observe that when the candle is placed inside a sealed container it burns for a while before extinguishing. Having made our observations, it is now time to put forward a hypothesis. The phlogiston theory of combustion contends that when something is burnt, it releases an invisible substance called phlogiston. The released phlogiston is absorbed by the air until it becomes saturated. Air that is saturated with phlogiston can no longer support combustion and will extinguish a flame. To aid in the discussion of this idea, several terms were coined: dephlogisticated air (air that can combine with phlogiston and thus can support combustion for a longer period of time); phlogisticated air (air that saturated with phlogiston); inflammable air (an early term for hydrogen, so named due to the ease with which it could be combusted). The phlogiston theory was applied to both organic materials, such as wood, which burnt to produce ashes, as well as inorganic materials, such as magnesium, with the products of combusted metals being referred to as calx.

While the phlogiston theory may seem silly to us today, it is important to consider the observations that led to it. We see material and energy being emitted by a burning candle, so it’s not too much of a stretch to believe that there are also things being emitted that we can’t see. This theory was also an improvement on the idea that fire was an element or substance trapped within materials. Unfortunately, the modern scientific method was only in its infancy at this time and hypothesis testing was not as rigorous as it is today.

Experiment Overview

During this laboratory, you will examine various aspects of combustion and the gases involved in an attempt to identify potential flaws with the phlogiston theory, then consider our modern oxidation theory and whether this resolves the issues you discovered during your experimentation.

Materials

Prelab Questions

View the interactive 3D presentation detailing Lavoisier’s experiments with mercury.

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Hydrogen peroxide is an oxidizer and a skin and eye irritant. Magnesium ribbon is a flammable solid. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or laboratory coat. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Please follow all laboratory safety guidelines.

Procedure

Part 1. Does Phlogiston Have Mass?

1. Weigh a clean, dry evaporating dish.
2. Weigh an unburnt match.
3. Light the match and carefully place it in the evaporating dish.
4. Record your observations in.
5. Once the match has cooled, record the new mass.
6. Calculate the change in mass.
7. Clean and dry the evaporating dish.
8. Repeat steps 1–7.
9. Weigh a clean, dry evaporating dish.
10. Accurately weigh approximately 0.5 g of steel wool.
11. Tease the strands of the steel wool apart and place it in the evaporating dish.
12. Light a match and use it to ignite the steel wool.
13. Record your observations.
14. Once the steel wool has cooled, record the new mass.
15. Calculate the change in mass.
16. Clean and dry the evaporating dish.
17. Repeat steps 9–16.

1. How did the match physically change as a result of being burnt?
2. How did the steel wool physically change as a result of being burnt?
3. Based on your observations and measurements, what can you conclude about the mass of phlogiston?

Part 2. Synthesis of Dephlogisticated, Phlogisticated and Inflammable Air?

View the gas collection procedure video.

In this section you will be forming various gases with a gas generator bottle and collecting it by water displacement (see Figure 1). The thistle tube enables the addition of reactants without losing the generated gas. The generated gas exits through the bent glass tubing, travels through the rubber tubing and displaces the water from the test tube.

1. Dephlogisticated Air

1. Place a small quantity of KI into the gas generator bottle.
2. Carefully pour 25 mL of 15% hydrogen peroxide into the thistle tube.
3. Collect the generated gas in a test tube.
4. Light a wooden splint and blow it out.
5. Place the glowing splint into the gas you collected.
6. Record your observations.

2. Phlogisticated Air

1. Put a several marble chips into the gas generator bottle.
2. Carefully pour 25 mL of 6 M HCl into the thistle tube.
3. Collect the generated gas in a test tube.
4. Light a wooden splint.
5. Place the burning wooden splint into the gas you generated.
6. Record your observations.

3. Inflammable Air

1. Place a 2-cm piece of magnesium into the gas generator bottle.
2. Carefully pour 25 mL of 6 M HCl of into the thistle tube.
3. Collect the generated gas in a test tube.
4. Light a wooden splint.
5. Place the burning wooden splint over the mouth of the test tube.
6. Record your observations.
   What did you observe about dephlogisticated air?

Questions

1. What did you observe about dephlogisticated air?
2. What did you observe about phlogisticated air?
3. What did you observe about inflammable air?

Part 3. Teacher Demonstration

For this section, your teacher will perform a demonstration, involving the combustion of magnesium. Try identifying the gas produced, then discuss with your lab partner about how this could have occurred.

Question

1. What did you observe during the teacher demonstration?

Teacher Demonstration

This demonstration involves the combustion of magnesium in steam and uses a modified gas delivery system. Instead of a thistle tube, a hook should be used to suspend the burning magnesium inside the gas generator bottle. Rather than collecting the gas by water displacement, add some liquid soap to the water in a beaker. This will trap any generated gas inside of soap bubbles.

1. Add 25 mL of water to a gas delivery bottle.
2. Heat the water to a gentle boil.
3. Light a wooden splint and lower it into the bottle to show that it gets extinguished.
4. Hang a piece magnesium ribbon from the hook attached to the stopper.
5. Place the end of the rubber hose into a soapy water solution.
6. Light the magnesium and carefully lower it into the gas delivery bottle.
7. Once the magnesium has finished burning, light a wooden splint and touch it to the bubbles that have formed on the top of the soapy water solution.
8. Have students record their observations and discuss what they saw.

In this demonstration, the magnesium ribbon reacts with the oxygen in the steam, generating hydrogen. The generated hydrogen is trapped within the soap bubbles, so when touched with a lit splint the gas will burn again. This is a good example of a discrepant event, because the students will observe the generation of a flammable gas as a result of combustion.