100. We have seen that familiar substances may undergo
great changes. The burning candle changes from an opaque
white solid to a translucent liquid, and then to a mixture of
invisible gases. Salt, upon being mixed with water, becomes
a clear liquid not distinguishable from water itself. The
solid carbon of wood and the gas oxygen unite to form car-
bon dioxide, a gas quite different from oxygen ; and when
the action is over, a handful of gray ashes is all the solid
substance left. Heat readily changes ice to water, and
water to steam.
We are so used to these happenings that they excite in us
no wonder ; yet, for hundreds of years men have been study-
ing these and similar changes without finding out all there
is to be known about them. So important is the part they
play in our everyday work, especially in cooking, that a
50 THEORY AND PRACTICE OF COOKERY
knowledge of the simpler facts about them is a great help
to housekeepers.
101. Physical and chemical changes. — Liquid candle
grease returns to the solid form when cooled ; dissolved salt
may be recovered by evaporating the water ; even steam
may be collected, condensed, and frozen. In these cases it
is plain that the changes are only changes in form. A
change in the form of matter without change in its composi-
tion is called a physical change. When, however, melted
candle grease becomes gaseous, it will not return to its origi-
nal form ; burnt sugar will never be sweet and white again ;
water acted upon by sodium is neither water, steam, nor ice.
In these cases the new substance has a different composition
from the old one. A change in the composition of matter is
called a chemical change. Heat, especially in the presence
of moisture, often brings about chemical changes.
102. Elements and compounds. — Some substances are
simple ; that is, they consist of but one thing. Examples :
iron, oxygen, carbon. A simple substance is an element.
Other substances are composed of two or more elements.
Examples : water, carbon dioxide. Into what elements may
water be sej^arated ? Carbon dioxide ? A substance com-
posed of two or more elements combined is a compound. In
a mixture each substance keeps its own properties; in a
compound these give place to new properties belonging to
the compound. Every chemical change involves either the
forming or the decomposition (breaking up) of a compound,
usually both.
103. Four elements found in food ; some of their proper-
ties.— Foods consist of compounds formed chiefly from
oxygen, carbon, hydrogen, and nitrogen, with small quan-
tities of other elements. Oxygen is an invisible, odorless gas.
It is a very active element, always ready to unite with other
elements to form new compounds. The combining of oxygen
with another element is called oxidation. The rusting of
metals and the decay of organic matter are slow forms of
oxidation. Oxygen forms about one-fifth of the volume
PREPARATOHY LESSONS 51
Page 26
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100. We have seen that familiar substances may undergo
great changes. The burning candle changes from an opaque
white solid to a translucent liquid, and then to a mixture of
invisible gases. Salt, upon being mixed with water, becomes
a clear liquid not distinguishable from water itself. The
solid carbon of wood and the gas oxygen unite to form car-
bon dioxide, a gas quite different from oxygen ; and when
the action is over, a handful of gray ashes is all the solid
substance left. Heat readily changes ice to water, and
water to steam.
We are so used to these happenings that they excite in us
no wonder ; yet, for hundreds of years men have been study-
ing these and similar changes without finding out all there
is to be known about them. So important is the part they
play in our everyday work, especially in cooking, that a
50 THEORY AND PRACTICE OF COOKERY
knowledge of the simpler facts about them is a great help
to housekeepers.
101. Physical and chemical changes. — Liquid candle
grease returns to the solid form when cooled ; dissolved salt
may be recovered by evaporating the water ; even steam
may be collected, condensed, and frozen. In these cases it
is plain that the changes are only changes in form. A
change in the form of matter without change in its composi-
tion is called a physical change. When, however, melted
candle grease becomes gaseous, it will not return to its origi-
nal form ; burnt sugar will never be sweet and white again ;
water acted upon by sodium is neither water, steam, nor ice.
In these cases the new substance has a different composition
from the old one. A change in the composition of matter is
called a chemical change. Heat, especially in the presence
of moisture, often brings about chemical changes.
102. Elements and compounds. — Some substances are
simple ; that is, they consist of but one thing. Examples :
iron, oxygen, carbon. A simple substance is an element.
Other substances are composed of two or more elements.
Examples : water, carbon dioxide. Into what elements may
water be sej^arated ? Carbon dioxide ? A substance com-
posed of two or more elements combined is a compound. In
a mixture each substance keeps its own properties; in a
compound these give place to new properties belonging to
the compound. Every chemical change involves either the
forming or the decomposition (breaking up) of a compound,
usually both.
103. Four elements found in food ; some of their proper-
ties.— Foods consist of compounds formed chiefly from
oxygen, carbon, hydrogen, and nitrogen, with small quan-
tities of other elements. Oxygen is an invisible, odorless gas.
It is a very active element, always ready to unite with other
elements to form new compounds. The combining of oxygen
with another element is called oxidation. The rusting of
metals and the decay of organic matter are slow forms of
oxidation. Oxygen forms about one-fifth of the volume
PREPARATOHY LESSONS 51