Page 136 - Physics

Physics for Secondary Schools

tension attempts to form the minimum Hence hot soup is tastier than cold
surface area and a spherical shape has the soup.
smallest surface area for a given volume. 4. Soap bubbles that consist of a layer
The surface tension of any liquid is of water between two layers of soap
affected by the following factors: fi lm can have a large surface area

1. Nature of the liquid: Different types because the soap acts to decrease the
of liquids have different surface surface tension of water. This is how
tensions. For example, mercury has surfactants are used to enhance the
a higher surface tension than water. cleaning ability of water. Surfactants
2. Impurities: Impurities in a liquid are also used to make emulsions of two
lower its surface tension. liquids oil and water, which normally
3. Temperature: The surface tension do not mix.
of a liquid decreases with an increase 5. In the extraction of impurities during
in temperature. laboratory processes.

Applications of surface tension Task 6.3

Surface tension has several practical Float two matchsticks on the surface
applications: of the water. Touch the water surface
1. The cleaning action of soap (detergents between the matchsticks using a hot
in general) is due to its ability to needle. Record and discuss your
lower the surface tension of water. observations.
For example, when washing clothes,
soap decreases the adhesion forces Capillarity
between the particles of cloth and Capillarity is another consequence of
those of dirt. As a result, the dirt is cohesive and adhesive forces.
easily removed since both water and
soap particles penetrate the pores of
the fabric. It is defi ned as a phenomenon in
2. Mosquitoes normally lay their eggs which liquid rises or falls in a narrow
space such as a thin tube or in the
on water. The eggs hang on the water
surface. When a small amount of oil voids of a porous material without
is poured into the water, it reduces any assistance from external forces.
the surface tension. This breaks the
elastic fi lm and the eggs are drowned The effect of capillarity can be seen in
and killed. drawing liquid between hairs of a painting
3. Hot soup has a lower surface tension brush, in a thin tube, in porous materials
than cold soup. As a result, hot soup such as sponges and some non-porous
spreads over a large area of the tongue. materials such as sand.

130
Student’s Book Form One

Physics Form 1 Final.indd 130 16/10/2024 20:57