Experiment 37
Testing liquids for polarity
Aim
To test nine liquids in order to find out if their molecules are polar.
Theory
In this experiment, the effect of a charged rod on a stream of a liquid from a
burette. A deflection of the stream indicates that the liquid consists of polar
molecules. On the contrary, non-polar liquid show no deflection.
This is because when the molecule is polar, one end of the molecule will be
attracted by the oppositely charged rod. This attraction will cause the molecules as
well as the running liquid to deflect towards the charged rod.
Structural formulae of some of the organic liquid used in this experiment are shown
as follows:
Procedure
1 The polythene rod was rubbed with fur. A negative charge was given to the
rod.
2 A beaker was positioned beneath the jet of one of the burette. The cork was
removed and a stream of the liquid was allowed to run from the burette with
the tap fully open.
3 The charged rod was brought close to the stream of liquid and any deflection
that occurred was noted. The extent of deflection was also noted on an
arbitrary scale from 0 to 3. (0 = no deflection; 3 = greatest deflection.) The
results were recorded.
4 The liquid was poured from the beaker back into the labeled burette. The
above procedure was repeated for the other organic liquids.
Questions
1 Water is a polar molecule and thus showing a deflection under the influence of
charged rod. Polar molecules have permanent dipole moments, when a
charged rod is put near the stream of water, the water molecules are attracted
towards the charged rod and the stream is deflected.
2 The polar liquid will be deflected towards the charged rod. The deflection is
independent of the charges carried by the rod. This is because when a
positively charged rod is used, the negative ends of the dipole in the polar
molecule are attracted towards the rod and vice versa. Therefore, the jet of
polar liquid will always be attracted towards the charge rod.
Discussion
1 Safety precautions
a) Toxic:
Trichloromethane and tetrachloromethane
b) Flammable: Cyclohexane, Cyclohexene, Hexane, Hexan-1-ol,
Methyl ethanoate and Propanone
Precautions:
a) Perform the experiment in a fume cupboard
b) Keep the stoppers on the bottle as much as possible
c) Wear gloves and safety spectacles
2
Sources of errors
a) The charged rod
Error #1: The distance between the charged rod and the jet of liquid may not
be constant for different trials, causing variation in the electrostatic field.
Improvement #1: The distance between the charged rod and the jet of liquid
should be precisely measured so that it is the same for all the trials. It is better
to clamp the rod into position to prevent non-uniform electrostatic field due to
shaking hand.
Error #2: The density of the electric charge may decrease due to ionization of
air molecule from time to time.
Improvement #2: Rub the rod with fur repeatedly before a new trial. The times
of rubbing should be constant and be great enough compare to the size of rod
so that the charge is “saturated” with charges.
Improvement #3: The rod can be made sharp-end and point it towards the jet
of liquid so to increase the charge density as well as the sensitivity of the
experiment.
b) The burette and the containers
Error: The burette and the container may have electrostatic charge induced in
them (e.g. by rubbing). Thus it may cause the liquid to be charged and deflect
towards the charged rod even when it is not polar.
Improvement: “Earth” the burette and containers with copper wire at once
before starting each trial.
Conclusion
______________________________________________________________
_________________________________________consist of polar molecules;
while ___________________________________________________ do not.