1.2. The Scientific Method http://www.ck12.org
law, on the other hand, is a statement that is always true, but does not include an explanation as to why. The law
of gravity says a rock will fall when dropped, but it does not explain why (gravitational theory is very complex and
incomplete at present). The kinetic-molecular theory of gases, on the other hand, tells us what happens when a gas
is heated in a closed container (the pressure increases), but also explains why (the motions of the gas molecules are
increased due to the change in temperature). Theories do not get “promoted” to laws, because laws do not answer
the “why” question.
Phlogiston - The Rise and Fall of a Theory
Early chemists spent a lot of time heating things and setting them on fire (on purpose, unlike some modern-day
chemistry students). They observed that flammable materials tended to weigh less after being burned. As more
materials were studied, this observation was found to be very consistent. A seemingly reasonable explanation for
this phenomenon was that some substance was lost from the material when it was burned. This substance was named
phlogistonfrom the Greek wordφλoγιστ ́ν(transliterated as phlogistón), which means “burning up.” The phlogiston
theory was first put forth in 1667 by the German physician and alchemist Johann Joachim Becher (1635–1682, shown
inFigure1.11).
FIGURE 1.11
Johann BecherBecher had taken the four ancient Greek elements (earth, air, fire, and water) and discarded fire and air. He expanded
the “earth” category to three groups, one of which was involved in burning. In 1703, George Stahl, a German
professor of medicine and chemistry, renamed this particular fraction of Becher’s earth as phlogiston.
What was the evidence that led to the development of this theory? One obvious experiment involved the burning of
wood. The ashes remaining after the fire weighed considerably less than that original wood sample. Therefore, it
seemed that phlogiston had been released during the burning process, leaving the “dephlogisticated” ashes behind.
If wood or a candle was burned in a closed container, the fire would soon be extinguished. This was taken by
supporters of the theory as evidence that air could only absorb so much phlogiston. Later, carbon dioxide gas was
discovered and studied. An experiment was performed in 1772 that exhausted all the air in a container. Further
burning of a candle and of phosphorus were then carried out in the container. After removing the carbon dioxide
with an absorbent, a gas was found that did not support life or combustion. This gas (which we now know as nitrogen
and which comprises about 78% of the atmosphere) was believed to be phlogiston.
So far, so good. We have observations –things lose weight when they burn. We have an explanation –the original
material loses phlogiston when it burns. What we don’t know is what phlogiston is or how much of it is in a given
material. But are there other experiments that lead us in a different direction?
Other scientists started to ask questions and run experiments. They noticed some results that seemed to contradict
what would be expected if the phlogiston theory was correct. If magnesium is heated, the product (a solid) weighs