Chemistry Reference
In-Depth Information
Table 5.1
Steps of the empirical method of proving hypothesis
On the basis of considerations about the “horror vacui,” Torricelli, together with
Galilee, developed the hypothesis of air pressure, and derived the famous mercury
experiment in 1648 to prove his hypothesis (see Fig. 1.2). He also verified his
hypothesis in additional experiments: air is a substance with a specific density, the
surrounding atmosphere shows a certain air pressure, the air pressure decreases
with increasing height above earth.
Following the empirical analyses of mass ratios of elements in many organic
substances, Kekul
´
dealt with the expression of these results through formulae,
while at the same time he always thought about the structure of corresponding
molecules. In the attempt of describing the benzene molecule with his experiences,
he failed at first by taking a chain with six C atoms. But after his “famous dream”
[
3
], he established the hypothesis of the cyclic structure of the benzene molecule
in 1865 (see Fig.
5.2
) and postulated the tetrahedral model of the methane
molecule [
3
].
Kekul´ first described the benzene molecule with alternative single and
double bonds, but he knew that all six bonds between C atoms should be equivalent.
From this hypothesis, he derived many experiments concerning the substitution of
H atoms by halogen atoms. He came to the conclusion (see Fig.
5.2
) that there is
only one mono-substituted molecule (C
6
H
5
Cl), that there are three doubly
substituted molecules (C
6
H
4
Cl
2
), and three tri-substituted molecules (C
6
H
3
Cl
3
).
Many further experiments were necessary to prove the hypothesis of the cyclic
structure of the benzene molecule - all students should know this, because most of
them are thinking that one single phenomenon may be enough for proving scientific
hypotheses.
