Surface Tension ( Definition + Measurement )

presented to : Dr | Hamdy El-Kady
Physical Chemistry Course 2016-2017
prepared By : Muhammad Mamdouh Abdulsalam
Faculty Of Petroleum Engineering, Suez University

1. SEEN

2. This is a for liquids and arises from intermolecular forces of
attraction . you can notice that a molecule in the interior of the liquid is
attracted equally from all directions by the molecules around it, while
a molecule at the surface of the liquid is attracted only sideways and
toward the interior. (figure 1) shows the two types of attractions as
discussed. The forces on the sides being counterbalanced the surface
molecule is pulled only inward the liquid . so the molecules have
tendency to go into the bulk of the liquid. Now the surface of the liquid
ion and in order to have the minimum number of

at the surface of the liquid so it contracts to have the smallest

le area.

smallest possible area.

AN Ds ei

AQ This property of liquids helps some ins |
to walk on the surface of the water as shown
in (figure 3).this also could explain the capillary action which will be
ussed in more details.

1 TR PEE
Fe x

UN

3. The legs of water strides can't get wet (as
if it is covered with wax). Which means no attraction between the
molecules of water and the molecules of the leg.so when the leg pushes
down the water, the surface tension of the water only tries to recover its
flatness from its deformation due to the leg. This behavior of the water
pushes the water strider upward so it can stand on the surface as long
as its mass is small so that the water can support. The surface of the
water is acting like an elastic film.

which is (dyne per centimeter) dyne.cm in the CGS system and
(Newton per meter) N.m” in the SI system.

inetic energy of the molecules (KE). [KE « T] and thereby decreasing

| the intermolecular forces . so the inward forces acting on the molecules |
of the first layer of the liquid decrease. in other words, by increasing
temperature the surface tension decrea

2

BAR, je

4. Determination
of surface
tension

.
NATI
nt kin abd sl “rom Demet

e é A

5. „the drop of wate
Surface tension force,
ward.

CLARAS = 4
na te ocaso sum gente, Qy
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