Determination of

4 Boiling Point
The boiling point ofa liquid may be defined as the temperature at which the vapour pressure of the
liquid is equal to the atmospheric pressure exerted upon the liquid surface. )
The boiling point of the liquid depends upon the pressure exerted upon the liquid surface. Since
atmospheric pressure is different at different places, therefore aliquid has different boiling points at
ifferent places. For the sake of comparison we use normal boiling points. The normal boiling point
of aliquid may be defined as the temperature at which vapour pressure of theliquid is equal to one
standard atmospheric pressure (760 mm).
(The boiling point of a liquid increases if non-volaile impurities are present in it. )
. Tru metho wwed to cheeb Hhe
EXPERIMENT 4.1
|:AIM: To determine the boiling pointofthe liquid. RHS
3- REQUIREMENTS RHS
100 ml corning glass beaker, a small thin walled test tube, thermometer, a capillary tube,
a tripod stand, wire gauze, stirrer, iron stand with clamp, liquid paraffin or conc. sulphuric
acid and the given liquid.
4- PROCEDURE RHS
1. Take a small test tube and fill it two-third with the given liquid whose boiling point is to be
determined. Fix this tube to the thermometer with a rubber band. The rubber band should be
fixed near the mouth of the tube so that it remains outside the liquid paraffin bath. Adjust the
tube so that the bottom of the tube is somewhere at the middle of the thermometer bulb.
2. Clamp the thermometer carrying test tube in an iron stand through acork. Lower the thermometer
along with the tube into a liquid paraffin bath. Adjust the thermometer so that its bulb is well
under the acid and open end of the tube with the rubber band is sufficiently outside the acid
bath.
3. Take a capillary tube 5-6 cm in length and seal it at about one cm from one end by heating it in
flame and giving a slight twist. Place this capillary in the test tube so that sealed part of it stands
in the liquid.
4. Start heating the liquid paraffin bath slowly and stir the bath gently. Keep an eye on the liquid
and the test tube and also on the thread of the mercury in the thermometer. At first, a bubble or
two willbe seen escaping at the end of the capillary dipping in the liquid, but soon a rapid and
continuous stream of air bubbles escapes from it. This is the stage when the vapour pressure of the
 whenteriperature
these the capillary
use heatins
Chermistry
Practical
Comprehcnstue point. of and
temperature compound.
evolution tube
of the uniform
rnean boiling level.
the in capillary
The Capillary rising
note the
organic
samecontinuous ensure
the stops. tube
Noteand note the seenthis
at liquid Reject liquid. to
just
flame again is stirred
pressure.
tube bulband liquid
tlhe
capillary
atmosplheric
Remove
LHS point.
and
boilingheating

the
thermometer
brisk in the
dipped
sealed. the paraffin
within
=
t°C.
liquid. Bubbles Liquid which tube, properly 2
the the well the
out. the capillary
test Mean
boiling
point=
of the tube
-Capillaryband
Rubber
paraffin
Liquid of
Determination
the at be and
thecoming
endof repeat point thenot should slowly
and
exceedsthepoint Liquid the in is
tubeboiling tube tube
Stirrer and
starts
fromboiling of tubevery
10°C ignitionend capillary
capillary
just bubbles 4.1. heated
capillary
bubbles the lower
capillary the Fig.
by tC(ii)
() t,C
fall as
the the be
of of
gives temperature temperature
of
the sealed the must
sealed evolution
stream temperatures RHS
PRECAUTIONS
end from that of OBSERVATIONSHS
indicates
lower startsthe one.pointbath
thecontinuOus placing Boiling
point
in the the the
the bubbles
new
sealedparaffin
liquid when Allow Record it sealed
two Keep on tube, The The
If
20 5. 1. 2. 3. a 4. 5.
 Determination of BoilingPoint
RESULTS SHS 21

Theboiling point of the

given liquid -t°C
Table 4. 1. Boilh: oints of Some Oranit
Compomds
Compound Boiling point (°C) Compound Boiling point ("C)
Benzylalcohol 205 Ethylbenzoate 212
Glycerol 290 Methyl salicylate 223
Ethylene glycol 197 Nitrobenzene 211
Phenol 182 Aniline 184
o-Cresol 190 o-Toluidine 200
Benzaldehyde 179 Chlorobenzene 132

Acetophenone 202 Bromobenzene 157

Phenyl acetate 196 Benzoyl chloride 197

Note: Paraffin can be safely heated upto 220°C while conc. H,SO, can be heated upto 280»C. For finding the
melting pointsof solids, having lower melting points, liquid paraffin may be used while for solids having
melting points greater than 200°C conc. H,SO, may be used.

VIVAQUESTIONS WITH ANSWERS