Page 17 - Encyclopedia of Chemical Compounds 3 Vols

Reader’sGuide

the atomic weight of oxygen (2 x 16 = 32), or 44. Chemists
have been studying atomic weights and molecular weights
for a long time, and the molecular weights of most com-
pounds are now known with a high degree of certainty.
The molecular weights expressed in this book are taken
from the Handbook of Chemistry and Physics, 86th edition,
published in 2005. The Handbook is one of the oldest, most
widely used, and most highly regarded reference books in
chemistry.
Melting Point and Boiling Point: The melting point of a
compound is the temperature at which it changes from a
solid to a liquid. Its boiling point is the temperature at which
it changes from a liquid to a gas. Most organic compounds
have precise melting points and/or, sometimes, precise boil-
ing points. This fact is used to identify organic compounds.
Suppose a chemist finds that a certain unknown compound

melts at exactly 16.5 C. Reference books show that only a
small number of compounds melt at exactly that temperature
(one of which is capryllic acid, responsible for the distinctive
odor of some goats). This information helps the chemist
identify the unknown compound.
Inorganic compounds usually do not have such precise
melting points. In fact, they may melt over a range of tem-

peratures (from 50 Cto55 C, for example) or sublime with-
out melting. Sublimation is the process by which a substance
changes from a solid to gas without going through the liquid
phase. Other inorganic compounds decompose, or break
apart, when heated and do not have a true melting point.
Researchers often find different melting points and boil-
ing points for the same compound, depending on the refer-
ence book they use. The reason for this discrepancy is that
many scientists have measured the melting points and boil-
ing points of compounds. Those scientists do not always get
the same result. So, it is difficult to know what the ‘‘true’’ or
‘‘most correct’’ value is for these properties. In this book, the
melting points and boiling points stated are taken from the
Handbook of Chemistry and Physics.

Some compounds, for a variety of reasons, have no spe-
cific melting or boiling point. The term ‘‘not applicable’’ is
used to indicate this fact.

Solubility: The solubility of a compound is its tendency
to dissolve in some (usually) liquid, such as water, alcohol, or

xvi CHEMICAL COMPOUNDS