Chemistry Reference
In-Depth Information
portional to the original length and the change in temperature. The
coef-
ficient of thermal expansion
of a material is the fractional change in length
(or in volume) that a body of a material undergoes for a unit change in
temperature. Most materials expand when heated and contract when
cooled, and Table 101b lists the values of the coefficients of linear expan-
sion of some materials. Some materials, however, have abnormal coeffi-
cients of thermal expansion at particular temperature ranges, contacting
when heated and expanding when cooled at particular temperature
ranges. Water, for example, expands when heated and contracts when
cooled. When it cools down to about 4°C, it begins to expand, however,
this expansion peaks (reaches a maximum) as the temperature approaches
the freezing point (see Textbox 72). Although the expansion is relatively
small, it is sufficient to exert pressure on surrounding the matter; freezing
water trapped in the pores or crevices of stone or pottery, for example,
often causes the breakup and disintegration of these materials into small
pieces.
TABLE 101b Thermal Properties of Some
Archaeological Materials
Coefficient of Linear Expansion (at 25°C)
Material
Expansion coefficient
a
N × 10
−6
Metals and Alloys
Lead
29
Brass
20
Silver
19
Copper
17
Gold
14
Iron
12
Steel
10-17
Nonmetals
Marble
4
Brick
5- 7
Glass
5-11
Mortar cement
7-14
Wood
6-11
a
Expressed as the change in length per degree centigrade.
