Lecture V: The Nature of the Atmosphere

Contents of this webpage

• Outline of Faraday's Chemical History of the Candle, Lecture V
• Reading Notes and Vocabulary for Lecture V
• Discussion Points to Think About Before Chat
• Related Websites (Optional Reading)

So far, we have concentrated on the chemical properties of two products of combustion, oxygen and hydrogen. In this lecture, Faraday looks at a mix of ideas: the properties of the atmosphere, its overall composition, the characteristics of nitrogen, and the production of yet another gas by the burning candle.

Lecture outline

1. Seting up the discussion of nitrogen
   1. Why does the candle burn better in oxygen than in the atmosphere?
   2. Review of the test for oxygen: Improved combustion in a smoldering stick
   3. Demonstration of reaction of oxygen with dinitrogen oxide N₂O to form the red-brown gas nitrogen dioxide NO₂.
      1. Reaction with pure oxygen produces redder gas than reaction with air
      2. Nitrogen dioxide dissolves in water, releasing another gas: nitrogen
2. Properties of nitrogen
   1. Colorless, oderless, tasteless
   2. Doesn't burn, support combustion
3. Uses of nitrogen
   1. Problems with pure oxygen atmosphere
   2. Inactive; will disperse burning substances without causing fire
4. Composition of atmosphere
   1. By bulk, O:N is 20:80
   2. By weight, O:N is 22.3:77.3
      1. Method of weighing gas
5. Properties of atmosphere: weight
   1. Low-pressure air weighs less than high-pressure air
   2. Absolute weight of air is 1.2 ounces per cubic foot
   3. Weight of atmosphere holds hand to partially evacuated tube
   4. Weight of atmosphere depresses and breaks rubber bladder which is unsupported by equal amount of atmosphere from below
   5. Weight of atmosphere holds evacuated hemispheres together
   6. Weight of atmosphere holds sucker (like the clear plastic hangers used to fasten hooks to glass)
   7. Weight of atmosphere holds paper to bottom of water-filled glass
   8. Popgun demonstration
   9. Air under pressure can lift and move solid objects (egg)
6. Properties of atmosphere: elasticity
   1. Removal of air causes expansion of bladder filled with air due to internal pressure
7. New product from combustion of candle: gas exhausted above burning candle (carbonic acid, or carbon dioxide)
   1. Gas does not support combustion or itself burn: it is not oxygen or hydrogen
   2. Reaction of gas with quicklime in water (limewater) to form milky liquid
   3. Test of nitrogen with limewater does not form liquid; therefore new gas is not nitrogen
   4. Milky solid is chalk: same gas produces by burning chalk
   5. Other sources of gas
      1. Seashells (source of name, "fixed gas") mixed with acid
      2. Marble mixed with sulfuric acid
      3. Gas collection: note heavy gas collection apparatus on p. 121 differs from light gas (hydrogen) collection apparatus on p. 65.
      4. Test of collected gas
         1. Doesn't support combustion
         2. Produces milky substance when in contact with limewater
         3. Dissolves in liquid
         4. Weighs more than hydrogen, oxygen, or air
         5. Can be poured in air like water--will sink to low part of container
         6. Supports bubble like liquid

Reading notes

Quicklime

Calcium oxide, CaO, forms calcium carbonate, CaCO₃ (chalk), when in contact with CO₂. A mixture of CaO dissolved in water is often used as a test solution for carbon dioxide.

Discussion points

• Note how Faraday creates standard tests for each of the substances he needs to recognize. We now have tests for hydrogen, oxygen, nitrogen, and carbon dioxide which will uniquely identify those gases. Sometimes more than one test is necessary to eliminate several candidates.
• How does collection of a "heavy" gas differ from collection of a "light" gas?
• Why is the 20:80 balance of oxygen to nitrogen so important for life on earth?

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