To determine the surface tension of given liquid using Stalagmometer.

To determine the surface tension of given liquid
(ethanol) using Stalagmometer
Dr. N. G. Telkapalliwar
Associate Professor
Department of Chemistry
Dr. Ambedkar College, Nagpur
1

Presentation Overview
1) Aim
2) Apparatus and Chemicals
3) Theory
4) Procedure
5) Observations
6) Calculation
7) Result
Aim: To determine the surface tension of given liquid (ethanol) using Stalagmometer.
2

Theory
Surface Tension is the one of the property of liquid.
This property of liquids arises from the intermolecular forces
of attraction.
A molecule in the interior of a liquid is attracted equally in
all directions by the molecules around it and net force is zero.
But, a liquid molecule present on the surface is attracted by
a net inward force called as surface tension.
The liquid surface is, therefore, under tension and tends to
contract to the smallest possible area in order to have the
minimum number of molecules at the surface.
It is for this reason that in air, drops of a liquid assume
spherical shapes because for a given volume a sphere has the
minimum surface area.
The surface tension (γ) is defined as the force in dynes
acting along the surface of a liquid at right angle to any line
1 cm in length.
Surface tension is caused by
the net inward pull on the
surface molecules.
The inward forces on the surface
molecules minimize the surface
area and form a drop.
Units of Surface Tension
CGS unit of Surface is dyne cm–1.
SI unit of Surface is is N m–1. 3

Stalagmometer
Surface tension gives a definite shape to drop of liquid. More is the surface tension,
larger is the drop size. This principle is used in Stalagmometer.
Stalagmometer is an apparatus with bulb and capillary. Liquid is filled in the bulb
and allowed to flow through capillary in the form of drops.
If the surface tension is high, larger drop is produced and number of drops are less
for a given volume. Thus more is the number of drops, less is the surface tension.
4

Apparatus Required
Beaker Stalagmometer Stop Watch
The relative surface tension is calculated from the expression:
Where,
1 and 2 are the viscosity coefficients of water and given liquid.
n1 and n2 are the drops of water and given liquid
d1 and d2 are the densities of water and given liquid.
Knowing the value of the surface tension of water (1 ) at the temperature of the experiment,
the surface tension (2 ) of the given liquid can be found.
Formula
Density bottle
5

Procedure
Part-I : Determination of densities of water and given liquid (ethanol)
1. Clean the density bottle with chromic acid solution and then with distilled
water. Rinse with water and ether and then dry.
2. Weigh an empty density bottle (W1).
3. Fill the density bottle with distilled water completely, insert the stopper and
weigh again (W2).
4. Take out the distilled water and rinse the density bottle 2-3 times with given
experimental liquid (ethanol).
5. Fill the given experimental liquid (ethanol) into the density bottle and record
the weight again (W3).
6

Procedure
Part-II : Determination of surface tension
1) Clean the stalagmometer with chromic acid solution and then with distilled
water. Rinse with water and ether and then dry.
2) Attach the stalagmometer to the burette stand in exactly vertical position.
3) Hold a beaker containing distilled water and dip the lower end of
stalagmometer in it. Suck the distilled water just above the mark ‘A’.
4) Allow water to flow freely. Adjust the
flow rate using pinch screw clip so that it
should deliver 15 to 20 drops per minutes
(3-4 drops in 15 sec).
5) Count the number of drops falling
between marks ‘A’ and ‘B’.
6) Repeat the recording of number of drops
three times for distilled water as well as
for the given experimental liquid
(ethanol).
7

Observations and Calculations
Part-I : Determination of densities of water and given liquid (ethanol)
1) Room temperature = …………… OC
2) Weight of empty density bottle= W1= …………. g.
3) Weight of density bottle with water= W2= …………. g.
4) Weight of density bottle with given liquid (ethanol) = W3= …………. g.
5) Volume of density bottle =V= ………. ml
8

Observations and Calculations
Part-II : Determination of surface tension of given liquid (ethanol)
Sr.
No.
Number of drops Relative
Surface Tension
Surface Tension
For
Water
Mean
(n1)
For
Ethanol
Mean
(n1)
1 --
--
--
--
2 -- -- -- --
3 -- --
Result: 1) Relative surface tension of given liquid (ethanol) relative to water was found to be ……
2) Surface tension of given liquid (ethanol) was found to be ………… dyne cm-1
Surface tension of water at room temperature = 1 = ……… dyne cm-1
9

References
• Essentials of Physical Chemistry, S. Chand Publication by Arun Bahl, B.S. Bahl, G. D. Tuli.
• University Chemistry, Volume I, Alliance & Co., by R. M. Jugade.
• A Text Book of Chemistry, B.sc. SEM- I, Himalaya Publishing House.
The contents, materials, images, etc. used in this video/presentation are
taken from open sources (Internet and Books) for educational purposes
only and not meant for any commercial distribution.
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To determine the surface tension of given liquid using Stalagmometer.

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