Chemistry Reference
In-Depth Information
necessary because learning or remembering all the data that have been observed
over the ages is impossible.
One of the most important laws in chemistry is the
law of conservation of
mass.
This law states that, in any chemical reaction or physical change, the total
mass present after the change is equal to the total mass present before the
change. This law will be further explain in Section 3.1. Section 3.2 will present
John Dalton's explanation of the law, in which he proposed that the particles
that make up matter can rearrange themselves in various ways but cannot be
created or destroyed. That explanation is a theory; it explains the law. If the par-
ticles that make up the materials before and after the change are the same, the
total mass must also be the same.
The way new generalities are accepted by the scientific community as being
true has been loosely codified into a system known as the
scientific method.
The steps of this method are
1. State the problem clearly.
2. Do further experiments. Many scientists test the generality with experi-
ments, repeating each other's work and doing other experiments related to
those.
3. Interpret the results. See if the generality explains all the results, new
and old.
4. Accept the law. If all the data support the law, it is generally accepted by
the scientific community as true. If later, further experiments are in conflict
with the law, it is modified or abandoned altogether.
An example of how the scientific method works is the establishment of the
law of constant composition of compounds, also called the law of definite
proportions. The initial statement, based on the work of Antoine Lavoisier
(1743-1794), was that every sample of a given compound is composed of the
same percentage of each of its elements as any other sample of the same com-
pound. That concept was subjected to many tests, in which other scientists mea-
sured and remeasured the compositions of many samples of a wide range of
compounds. Claude-Louis Berthollet (1748-1822) showed that gaseous combi-
nations of carbon and oxygen had compositions ranging from 27.29% carbon
to 42.88% carbon; and thus the composition was not definite. He therefore stated
that the proposed law was incorrect. However, Joseph Louis Proust (1754-1826)
found that Berthollet's samples were
mixtures
of
two
compounds—carbon
monoxide and carbon dioxide—and that when either compound was analyzed
by itself, it always had the same composition. The law of definite proportions
was firmly established by his work. An explanation of the law of definite pro-
portions was proposed by John Dalton in 1803 (Section 3.2). His hypothesis
generated a great deal of additional work, all of which supported his ideas,
which made the hypothesis into a theory.
All samples of a given compound
have the same percentage of
each of its elements.
Snapshot Review
❒
Generalities about empirical observations are called
laws;
accepted
explanations of these laws are called
theories.
A. Is the law of definite proportions (Chapter 3) an explanation or a
collection of observations?
