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- from : Edwin Edser. Heat for Advanced Students.
Macmillan and Co., Limited. London (1911) pp. 188-190.
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- Copyright © 1998 Richard A. Paselk
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- Experimental
Determination of Boiling Points.--From
what has previously been said, it appears that the temperature
of a liquid may, in certain circumstances, rise considerably
above its normal boiling point. Consequently it is only in determining
the boiling points of solutions that the thermometer is placed
in the liquid, and some arrangement must then be made to promote
free ebullition.
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- The usual method of determining the boiling
point of a pure liquid is to place the bulb of the thermometer
in the vapour given off during ebullition. The space containing
the thermometer should be jacketed in the manner described in
connection with the determination of the boiling point of a thermometer
(p. 11)
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- The actual temperature of the vapour above
a boiling solution is generally slightly lower than the temperature
of the solution. Thus above a salt solution, the temperature
of which is 110° C., the steam may reach a temperature, say,
of 105° C. A thermometer placed in this steam will, however,
indicate a temperature of 100° C. The reason of this is,
that the steam condenses to pure water on the bulb and stem of
the thermometer, and as this water has a large surface, any further
heat communicated will cause part of the water to evaporate without
a rise of temperature occurring.
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- On the other hand, if the temperature of
the thermometer falls below 100° C., steam will be condensed
on it, and the temperature will be raised by the latent heat
rendered up. In order to provide a larger surface for evaporation,
the bulb of the thermometer is often surrounded loosely with
cotton wool.
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- Determination of the Boiling Point of
a Solution.--As pointed out above,
a thermometer when placed in the steam given off from a boiling
aqueous solution of a salt, will indicate the boiling point of
the water, and not that of the solution. A similar law applies
to solutions in general. In order to determine the boiling point
of a solution, the thermometer bulb must be completely immersed.
Special precautions must be taken to prevent the occurrence of
boiling by bumping, and the consequent rise of temperature of
the solution above its boiling point.
- Beckmann's Boiling Point Apparatus.--Fig. 85 represents diagrammatically the essential
points of Beckmann's apparatus for determining the boiling points
of solutions. A test tube A, provided with a side inlet tube
U, is used to contain the solution. A piece of platinum wire
P, is fused through the bottom of the test tube, and a number
of glass beads, G, are also contained by it. The beads and platinum
wire serve to promote free ebullition. The bulb of a sensitive
thermometer T, similar to that described. on p.
15, dips into the solution. A spiral glass tube, K1,
serves to condense the vapour given off; the condensed liquid
runs back into the solution, so that the strength of the latter
is maintained constant. The test tube containing the solution
is surrounded by a glass vessel C, provided with double walls,
forming a vapour jacket. A liquid, possessing a boiling point
slightly higher than that of the solution in A, is placed in
C; some pieces of porous earthenware, D, serve to promote free
ebullition, whilst the condensing spiral K2, prevents
loss of the liquid. The whole of the above arrangement is mounted
on a stand made from asbestos mill-board. Two pointed Bunsen
flames play on the wire gauze E, and the hot gases impinge directly
on the lower surface of the vapour jacket C. Direct communication
of heat to the vessel, A, is prevented by the double cylinder,
F1, F2, of asbestos mill-board, and by
a roll, H,of asbestos paper. Thus the solution in A receives
heat only from the liquid and vapour in the vessel C.
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- The solution may be introduces into the vessel
by way of the side inlet tube B, the condenser K1
being removed.
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- © R. Paselk
- Last modified 22 July 2000
