Lecture VI: Respiration and Its Analogy to the Burning of a Candle

Contents of this webpage

• Outline of Faraday's Chemical History of the Candle, Lecture VI
• Reading Notes and Vocabulary for Lecture VI
• Discussion Points to Think About Before Chat
• Related Websites (Optional Reading)
• Lab for Lecture I: How different types of wax burn

In this lecture, Faraday takes the lessons learned from the candle and applies them to the world in general. Combustion of oxygen is a common chemical reaction, which not only occurs in flames, but also in the human body, although at a much slower rate.

Lecture outline

1. Continuing from last lecture: the properties of carbonic acid
2. The composition of carbonic acid
   1. Its source is in the smoke, which produces black particles (carbon)
   2. The carbon is consumed when exposed to pure oxygen
   3. The end product is carbonic acid; therefore carbonic acid is carbon and oxygen
3. Demonstrations of oxygen and carbon interactions
   1. Burning flakes of heated carbon in oxygen
   2. Burning a chunk of charcoal (no flame)
   3. Collected gas reacts with limewater: it is carbonic acid
   4. Combination by mass: 6 parts carbon to 16 parts oxygen
   5. Combined with 28 parts lime forms calcium carbonate (carbonate of lime)
   6. Charcoal burns away completely
4. Splitting carbonic acid into its constituent parts
   1. Combine phosphorus with carbonic acid gas: phosphorus can't burn
   2. Combine potassium with carbonic acid gas: heated potassium burns, producing potash and carbon flakes
   3. Generalization: carbonic acid is formed whenever carbon burns
5. Producing carbon by burning various objects
   1. Partially burnt wood
   2. Coal gas (CO, or carbon monoxide)
6. Properties of carbon
   1. Burns as a solid body, becoming a gas in combination with oxygen
   2. Comparison with lead pyrophorus, which forms a solid residue on burning
7. Living combustion (respiration)
   1. Demonstrations that exhaled breath puts out candle (not by blowing it out, but because it lacks oxygen).
   2. Demonstration with air and limewater
      1. Limewater turns milky when exposed to air from lungs: such air contains carbonic acid (CO₂)
      2. Normal air does not affect limewater
   3. Breathing is necessary: can't hold breath indefinitiely.
   4. Air drawn into body is combined with food eaten in the blood, forming carbonic acid: the oxygen in the air is consumed (i.e., food is effectively fuel that can be burned).
   5. Sugar composition (72 parts carbon to 11 parts hydrogen to 88 parts oxygen by mass): same elements, but different proportions, from candlewax.
   6. Reaction of sulfuric acid (oil of vitriol) with sugar: produces carbon
   7. Estimate of amount of carbon converted by humans due to respiration
   8. Earth could not support combustion of carbon by respiration if it produced solid
   9. Carbonic acid released into atmosphere by animal respiration is absorbed by plants, preserving balance
8. Chemical affinity: attraction of one substance for another
   1. Lead reacts with oxygen upon exposure; carbon does not.
   2. Heat is required to set coal gas aflame
   3. Gunpowder and guncotton start burning at different temperatures
   4. Respiration in humans occurs at a little above room temperature

Reading notes

oil of vitriol:

Sulfuric acid

Potash:

Ash residue produced by burning various substances

Discussion points

• How does Faraday demonstrate that human exhaled breath contains carbonic acid?
• What are the main properties of carbon?
• How do sugar and wax differ in chemical composition?

Lab

Materials

• Pennies (any year will do, but post-1982 pennies are partially zinc. If possible, do the procedure 2 times, once with pre-1982 pennies, and once with post-1982 pennies).
• Vinegar
• Salt
• Jar
• Iron Nail
• Nails or strips of other metals (zinc, nickel, aluminum, copper)
• Thread
• Pencil or similar stick

Procedure

1. Pour about 1/4 cup of vinegar into the jar, and add a tablespoon of salt.
2. Place 2-3 pennies in the vinegar solution.
3. Tie one end of the thread to the iron nail. Tie the other end to the middle of the pencil or dowel and place the pencil across the top of the jar so that the nail hangs down into the jar, half in and half out of the vinegar solution.
4. Check the nail at 15 minutes, 30 minutes, 1 hour, 2 hours, 5 hours and 24 hours. Describe the condition of the nail.
5. Pour out the vinegar solution and examine the pennies; report your observations.
6. If you can, make multiple setups and suspend different types of metals (zinc, aluminum, tin) in the vinegar/salt/copper penny solution. Be sure to use a different setup for each type of metal so that the different metal reactions do not interfer with one another.

Report

Devise a table to show the progress of the reaction over time.

• Be sure to accurately describe your apparatus. Draw a picture or take a photograph, scan and upload it as part of your report if possible.
• If you were able to use nails or strips of different metals, how did the reactions differ in amount of metal coating? In the length of time to coat the materials? Did any of your pennies get coated instead of the nail?

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