Page 132 - Physics

Physics for Secondary Schools
Mercury forms a convex meniscus
because of the stronger cohesion between
its molecules compared to the adhesion
between glass and mercury molecules.
Water forms concave meniscus because
adhesive forces between water and glass
molecules are stronger than cohesion
between water molecules.
Acti vity 6.2
Aim: To observe the adhesion
and cohesion.
Materials: Water, glycerine, glass
slides, cylinders and
droppers
Procedure
1. Pour about 50 ml of glycerine and
water into separate cylinders and
carefully observe the meniscus of
each liquid.
2. Using a dropper, place a drop of
glycerine on glass slide and note
down your observation.
3. Use another dropper to place a drop
of water on another glass slide and
note your observation.
Questions
(a) Compare the menisci of water with
glycerine.
(b) What did you observe when drops
of glycerine and water were placed
on separate glass slides?
(c) Explain your observations.
The meniscus of water curves upwards
forming a concave shape while the
meniscus of glycerine curves downwards
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Physics Form 1 Final.indd 126
forming a convex shape. When each
of these liquids is dropped on a glass
slide, the water spreads further unlike
glycerine. This is because glycerine has a
high cohesive force among its particles. A
drop of glycerine on a glass slide remains
almost spherical because the cohesive
force is stronger than adhesive force.
Applications of adhesion and cohesion
The following are applications of
adhesion and cohesion:
1. If we want to stick two different
objects together, we use adhesive
effects of the tape or glue.
2. Adhesion helps to remove harmful
materials such as bacteria from
drinking water.
3. Adhesive forces are the source of
friction between surfaces.
4. Cohesion supports water transport in
plants and animals.
5. Animals and plants also use cohesion
of tissue to repair damage.
6. Cohesion is responsible for viscosity.
7. Ink sticks on paper because adhesive
forces between ink and paper are
greater than the cohesive forces in the
molecules of ink. This explains why
your writing remains on the paper.
Task 6.2
Think of your daily life, paying
attention to situations where cohesion
and adhesion are involved. Discuss
how these concepts are crucial to your
daily activities.
Student’s Book Form One
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