Lecture III: Products: Water from the Combustion
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In our first experiment, we looked at the appearance of the candle and at the disappearance of wax (or oil) over time as the candle, noting that candles of different materials and sizes burned at different rates. In the second experiment, we noted that the candle flame used up something in the air, decreasing the volume of the air by some amount before the flame died. You may also have noticed that the burning candle produces carbon (the black soot in smoke), especially if the candle is not burning very steadily. Obviously, the activity of combustion is causing a change in materials, which affects both the substances in the candle and in the atmosphere around the candle, and which results in new substances. In chemistry terms, the reactants (the wax and whatever it is in the air--we haven't explicitly identified it yet, although you probably all realize that it is oxygen) undergo a chemical change to become other things--the carbon soot, and, as Faraday shows us in this lecture, water, which has some interesting properties of its own.
- Products from combustion include carbon, gases (to be discussed later) and water
- General observations on water from combustion
- Captured by condensation (cooling vapor from the candle):
- Identified by its reaction with zinc
- Is a product of combustion of many substances; formed by something in fuel combining with something in the atmosphere
- Composition of water is constant (made of same stuff in same proportions)
- Physical state change does not change identity of substance
- Ice properties: less dense than liquid, occupies greater volume
- Steam: much less dense than liquid, occupies much greater volume
- Changes due to change in temperature (amount of heat) in water
- Chemical properties of water
- Water does not exist as such in the candle; it is a product of combustion activity
- Water reacts with potassium, zinc, and iron to form new solid substances (oxides) and some kind of gas
- Demonstration of steam passing over heated iron
- Iron combines with something in steam (oxygen) to form iron oxides
- Iron now weighs more than it did before exposure to steam
- Resulting gas has interesting properties
- Doesn't condense, so isn't water vapor
- Ignites explosively when exposed to flame (not oxygen)
- Ligher than the atmosphere
- Gas with same properties produced by reaction of zinc with water
- Gas identified as hydrogen
- Burning hydrogen ALWAYS produces water (hydrogen + something in air) = water and NOTHING ELSE
- Water therefore is hydrogen plus some gas found in the atmosphere
- An element (K) which combines violently with water to release hydrogen gas
- salt and ice:
- Salt lowers the freezing point of water. It is harder for the individual water molecules to form bonds with each other than with the sodium and chlorine ions of the dissolved salt. As a result, salty water must be colder than fresh water before it will freeze--the exact amount of "freezing point depression" depends on the concentration of salt in the water. The more salt, the lower the temperature.
- The effect of salting your sidewalk to prevent ice buildup is cumulative. You put a little salt on the sidewalk. The temperature drops enouhg below 32 F° and some of the water eventually freezes--but since water freezes as a pure substance, the salt ions are left behind in the remaining liquid water. This now has a higher salt concentration, and therefore a still lower freezing temperature, so that the temperature must now drop even more before the remaining water will start to freeze.
- In Faraday's day, one theory was that heat was a kind of substance called caloric which flowed from hotter to colder objects. The identification of heat as a form of energy occured late in Faraday's life, through the experiments of James Joules and Benjamin Thompson. Faraday's insistence that electricity, magnetism and gravity were all similar expressions of some force lay the ground work for realizing that energy had many forms.
- Pouring hydrogen (page 64):
- Imagine Faraday putting the jar containing the hydrogen UNDER the jar containing the (heavier) atmosphere, which then pours into the bottom jar. The hydrogen "floats" to the top and is captured in the upper jar.
- nitrocellulose (pyroxylin) in a mixture of alcohol and either, used to coat objects, including skin after surgery or to clean optics.
- How do vapor and gas differ?
- What does constant composition mean? Why is it important?
- Describe what happens in Faraday's demonstration where steam passes over heated iron filings.
- Why does heating the iron speed up the rate of the reaction?
- What is Faraday's test for the identify of water?
- What is his test for the identify of hydrogen gas?
- Why are such tests important?
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