Comparisons with Red Drop should be made. At this time, students add a drop of plain water to paper. Student Questions 1. A force is defined as something which exerts push or a pull on an object . One type of force that exerts a pull is a cohesive force.
Cohesion is the attractive force between similar particles.
For liquids, especially water, there is a strong cohesive force between molecules of the liquid. At the surface of a liquid the molecules experience an uneven attraction. This uneven attraction is caused by the liquid molecules inability to form bonds with the surrounding air. Molecule A at the surface is drawn downward by the forces acting on it by the other molecules Molecule B, within the liquid, is pulled equally in all directions by the forces of all the other molecules. It is surface tension which explains how a liquid forms a drop in a spherical shape, since a sphere has the smallest surface area for a given volume.