Sometimes it is convenient to represent the
atoms of a molecule as divided into two or more groups; this may be done
by writing the formul? of the groups, and separating each simple formula
by a full stop. Slaked lime, for instance, has the formula CaH_{2}O_{2};
or, as already explained, we may write it Ca(HO)_{2}; or, if for
purposes of explanation we wished to look on it as lime (CaO) and water
(H_{2}O), we could write it CaO.H_{2}O. A plus sign (+) has a different
meaning; CaO + H_{2}O indicates quantities of two substances, water and
lime, which are separate from each other. The sign of equality (=) is
generally used to separate a statement of the reagents used from another
statement of the products of the reaction; it may be translated into the
word "yields" or "becomes." The two statements form an equation.
Ignoring the quantitative relation, the meaning of the equation CaO +
H_{2}O = CaO.H_{2}O is: "lime and water yield slaked lime." By referring
to a table of atomic weights we can elicit the quantitative relations
thus:--
CaO + H_{2}O = CaH_{2}O_{2}
| | |
V V V
Ca = 40 H_{2} = 2 = 1?2 Ca = 40
O = 16 O = 16 H_{2} = 2 = 1?2
-- -- O_{2} = 32 = 16?2
56 18 --
74
Or, putting it in words, 56 parts of lime combine with 18 parts of water
to form 74 parts of slaked lime.
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