LEP
3.4.03
Boiling point elevation
R
PHYWE series of publications • Laboratory Experiments • Physics • PHYWE SYSTEME GMBH • 37070 Göttingen, Germany 23403 1
Related topics
Raoult’s law, Henry’s law, ebullioscopic constants, chemical
potential, Gibbs-Helmholtz equation, concentration ratio,
degree of dissociation.
Principle and task
The boiling point of a solution is always higher than that of the
pure solvent. The dependence of the temperature difference
(elevated boiling point) on the concentration of the solute can
be determined using a suitable apparatus.
Equipment
Appar. for elev. of boiling point 36820.00 1
Heat. mantle f. round bott.fl. 250 ml 47550.93 1
Clamp for heating mantle 47557.01 1
Power regulator 32247.93 1
Laboratory balance, data outp. 620 g 45023.93 1
Weighing dishes, 8538537 mm, 100 45019.01 1
Temperature meter, digital, 4-2 13617.93 1
Temperature probe, immers.type 11759.01 1
Protective sleeves f. temp. probe, 2 11762.05 1
Retort stand, h 750 mm 37694.00 1
Right angle clamp 37697.00 3
Universal clamp 37715.00 3
Flask, round, 1-neck, 250 ml, GL25/13 35812.15 1
Glass beaker, tall, 250 ml 36004.00 1
Jointing f. GL25, 8 mm hole, 10 pcs 41242.03 1
Silicone tubing i.d. 7 mm 39296.00 1
Mortar w. pestle, 150 ml, porcelain 32604.00 3
Pinchcock, width 15 mm 43631.15 1
Microspoon, special steel 33393.00 1
Wash bottle, plastic, 500 ml 33931.00 1
Pellet press for calorimeter 04403.04 1
Funnel, glass, top dia. 80 mm 34459.00 1
Pasteur pipettes, 250 pcs 36590.00 1
Rubber caps, 10 pcs 39275.03 1
Beads, 200 g 36937.20 1
Sodium chloride 500 g 30155.50 1
Urea 250 g 30086.25 1
Hydroquinone 250 g 30089.25 1
Glycerol 250 ml 30084.25 1
Water, distilled 5 l 31246.81 1
Problems
1. Measure the increase in boiling point of water as a function
of the concentration of table salt, urea and hydroquinone.
2. Investigate the relationship between the increase in boiling
point and the number of particles.
3. Determine the molar mass of the solute from the relation-
ship between the increase in boiling point and the concen-
tration.
Set-up and procedure
Perform the experimental set-up as described in Fig. 1.
Push the protective sleeve for the temperature probe, to which
two drops of glycerine are added to improve heat conduc-
tance, into the GL screw top of the inner vessel of the boiling
point apparatus. In this way the apparatus can be operated
with the temperature immersion probe PT 100 and the digital
temperature meter 4-2. Having prepared the dry, clean inner
vessel in this way, weigh it and record its mass. During the
measurement, steam enters the inner vessel through the later-
al aperture. When assembling, pay particular attention to
ensure that this opening is not covered and is located below
the silicone rubber seal of the connecting cap. Slip two short
pieces of silicone tubing onto the two gas outlets of the outer
vessel and place the free ends in the 250 ml glass beaker. The
escaping water vapour can condense here. Attach a hose clip
to the lower of the two tubes coming from the outer vessel,
and which is initially left open. The round flask is filled with 150
to 200 ml of water, into which a number of beads are placed.
Press the substances to be tested into pellets in order to pre-
vent any particles of the substance from sticking to the walls
of the vessel while being added. Five portions of each sub-
stance (NaCI, urea, hydroquinone) are weighed out: each
should weigh approximately 700 mg. It is advisable to first pul-
verise the substances with a mortar and pestle. First insert the
small steel cylinder into the pellet press, and place the press
in a vertical position. Now pour the weighed substance into
the pellet press through a funnel. Next fit the large plunger;
form a pellet in a vice by applying medium pressure. Deburr
the pellets and then weigh them (weighing accuracy 1 mg).
Fig. 1: Experimental set-up: Boiling point elevation.
