Surface tension occurs at the surface of liquids due to intermolecular forces. Molecules below the surface experience equal intermolecular forces pulling in all directions, but surface molecules experience forces only downward and laterally, resulting in a net downward force. This causes surface molecules to compress more tightly than interior molecules, forming a "skin" that gives liquids their characteristic rounded shape and allows insects like water striders to walk on water.

1. SURFACE
TENSION

2. SURFACE
TENSION

3. What’s going on
at the surface
of a liquid?

4. What’s going on
at the surface
of a liquid?
Let’s take
a look!

24. Particles that make up a liquid
are in constant random motion;
they are randomly arranged.

25. You might expect the particles at the surface,
at the micro level, to form a random surface,
as shown below.

26. You might expect the particles at the surface,
at the micro level, to form a random surface,
as shown below.

28. = intermolecular
attractions
But how do intermolecular forces
influence the surface?

29. Under the surface,
intermolecular attractions pull on
individual molecules in all directions
= intermolecular
attractions

30. = intermolecular
attractions

31. = intermolecular
attractions

32. At the surface,
pull on the molecules is laterally and downward;
there is negligible intermolecular attractions
above the molecules (from the medium above, such as air).
SO, the net force on surface molecules is downward.

33. The result of this downward force is that
surface particles are pulled down until
counter-balanced by the compression
resistance of the liquid:

36. Surface molecules are compressed
more tightly together,
forming a sort of skin on the surface,
with less distance between them
compared to the molecules below.

37. Surface molecules also form a
much smoother surface than
one would expect from randomly
moving molecules.

40. This explains the characteristic rounded
shape that liquids form when dropping
through the air: The molecules are all
being pulled toward the center.

41. This explains the characteristic rounded
shape that liquids form when dropping
through the air: The molecules are all
being pulled toward the center.

42. Water in particular
has a very high
surface tension.
What property does
water have that would
give it such a strong
surface tension?

43. This is the water strider insect. Have you seen one?

44. How is this possible?