Soal Latihan dan Jawaban Reading Comprehension 62
A seventeenth-century theory of burning proposed that anything that burns must
contain material that the theorists called “phlogiston”. Burning was explained as the
release of phlogiston from the combustible material to the air. Air was thought
essential, since it had to provide a home for the released phlogiston. There would be a
(5) limit to the phlogiston transfer, since a given volume of air could absorb only so much
phlogiston. When the air had become saturated, no additional amounts of phlogiston
could leave the combustible substance, and the burning would stop. Burning would
also stop when the combustible substance was emptied of all its phlogiston.
Although the phlogiston theory was self-consistent, it was awkward because it
(10) required that imaginative, even mysterious, properties be ascribed to phlogiston.
Phlogiston was elusive. No one had ever isolated it and experimentally determined its
properties. At times it seemed to show a negative weight: the residue left after burning
weighed more than the material before burning. This was true, for example, when
magnesium burned. Sometimes phlogiston seemed to show a positive weight: when,
(15) for example, wood burned, the ash weighed less than the starting material. And since
so little residue was left when alcohol, kerosene, or high-grade coal burned, these
obviously different materials were thought to be pure or nearly pure phlogiston.
In the eighteenth century, Antoine Lavoisier, on the basis of careful experimentation,
was led to propose a different theory of burning, one that required a constituent of
(20) air-later shown to be oxygen-for combustion. Since the weight of the oxygen is
always added, the weight of the products of combustion, including the evolved gases,
would always be greater than the weight of the starting material.
Lavoisier’s interpretation was more reasonable and straightforward than that of the
phlogiston theorists. The phlogiston theory, always clumsy, became suspect, eventually
(25) fell into scientific disrepute, and was replaced by new ideas.
Reading Comprehension 62
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Limitations of seventeenth-century scientific theories
The chemical composition of phlogiston.
Attempts to explain what happens when materials burn
The characteristics of the residue left after fires
natural limits on the total volume of phlogiston
release of phlogiston into the air from burning material
ability of phlogiston to slow combustion
absence of phlogiston in combustible material
assumed to be true of
analyzed and isolated in
returned to their original condition in
seemed to have phlogiston with a negative weight
leaves no residue after burning
was thought to be made of nearly pure phlogiston
was thought to contain no phlogiston
were more mysterious than phlogiston
required more heat to burn than other substances did
burned without leaving much residue
contained limited amounts of phlogiston
Both theories are considered to be reasonable and straightforward.
Both theories have difficulty explaining why residue remains after burning.
Both theories recognize that air is important to combustion.
Both theories propose that total weight always increases during burning.