In liquids, molecules move about freely yet tend to cling together

In liquids, molecules move about freely yet tend to cling together. This tendency to cling together, which is not noticeable in gases, is characteristic of liquids and especially of solids. It is the cause of viscosity and is readily detected in a variety of ways. For instance, not only do liquid molecules cling together to form drops and streams, but they cling to the molecules of solids as well, as is shown by the wet surface of an object that has been dipped in water. The attraction of like molecules for one another is called “cohesion,” while the attraction of unlike molecules is called “adhesion,” although the force is the same whether the molecules are alike or unlike. It is the former that causes drops of water to form and that holds iron, copper, and other solids so rigidly together.

The adhesion of glue to other objects is well known. Paint also "sticks" well. Sometimes the "joint" where two boards are glued together is stronger than the board itself. The force of attraction between molecules has been studied carefully. The attraction acts only through very short distances. The attraction even in liquids is considerable and may be measured. The cohesion of liquids is also indicated by the tendency of films to assume the smallest possible surface. Soap bubble films show this readily. A soap bubble takes its spherical shape because this form holds the confined air within the smallest possible surface. A drop of liquid is spherical for the same reason. The surface of water acts as if covered by a film that coheres more strongly than the water beneath it. This is shown by the fact that a steel needle or a thin strip of metal may be floated upon the surface of water. It is supported by the surface film. If the film breaks the needle sinks. This film also supports the little water bugs seen running over the surface of a quiet pond in summer. The surface film is stronger in some liquids than in others. This may be shown by taking water, colored so that it can be seen, placing a thin layer of it on a white surface and dropping alcohol upon it. Wherever the alcohol drops, the water is seen to pull away from it, leaving a bare space over which the alcohol has been spread. This indicates that the alcohol has the weaker film.

1. Which of the following is the function of the first paragraph?

A. To emphasize cohesion’s importance over adhesion.

B. To review the causes of viscosity.

C. To explain how cohesion holds both liquids and solids together.

D. To define cohesion and adhesion and explain how solids/liquids interact

E. To show how liquid films are an example of cohesion.