Take the case of a
liquid suddenly solidifying. The heat latent in that liquid, and
necessary to keep it a liquid, is no longer necessary and comes out, and
the substance appears to become hot. Quicklime is a cold, white, solid
substance, but there is a compound of water and lime--slaked lime--which
is also a solid powdery substance, called by the chemist, hydrate of
lime. The water used to slake the quicklime is a liquid, and it may be
ice-cold water, but to form hydrate of lime it must assume a solid form,
and hence can and does dispense with its heat of liquefaction in the
change of state. You all know how hot lime becomes on slaking with
water. Of course we have heat of chemical combination here as well as
evolution of latent heat. As another example, we may take a solution of
acetate of soda, so strong that it is just on the point of
crystallising. If it crystallises it solidifies, and the liquid
consequently gives up its latent heat of liquefaction. We will make it
crystallise, first connecting the tube containing it to another one
containing a coloured liquid and closed by a cork carrying a narrow tube
dipping into the coloured liquid.
Pages:
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62