CHAPTER ITHE QUANTITATIVE ASPECTS OF CHEMISTRYSo many things happen when substances undergo a chemical
change that it is no wonder the student is astonished and even
bewildered by his observations. He soon learns that there is an
entire change in physical properties of the reacting substances and
finds that this phenomenon is usually the easiest to observe.
Also he is able to discover the transformations of energy which
always accompany a chemical change, although this is usually
confined to observations of the evolution of heat.
He learns, usually from the text book, that no mass is lost or
gained during a chemical change and that the total weight of the
substances before and after is the same. This rule is known
as the law of the conservation of matter, and it is more difficult
for the student to convince himself of the truth of this character-
istic of chemical change by direct observation because the ex-
periments must be quantitative and require apparatus for meas-
uring and weighing.
When hydrogen combines with oxygen to form water it is easily
observed that a great deal of heat is given off, also that a liquid
substance is formed and that the gases taken decrease in volume
and in fact entirely disappear when exactly the right mixture is
used. When measuring tubes are used it is observed that, when
2 volumes of hydrogen and 1 volume of oxygen are mixed, the
gases entirely disappear after the reaction has taken place.
When more than 2 volumes of hydrogen are taken to 1 of oxygen,
the excess of hydrogen over the 2 volumes is found to remain
unaffected; and likewise when more than 1 volume of oxygen is
taken to 2 volumes of hydrogen, the excess of oxygen over the
1 volume is found to remain unchanged after the reaction. How-
ever, to determine that 1.008 parts by weight of hydrogen com-
bine with exactly 8.00 parts by weight of oxygen requires elabo-
rate apparatus and very painstaking measurements. This pro-
portion, 1.008 :8.00, is known as the combining ratio. Similar
exact ratios exist in all chemical changes. When oxygen and zinc
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