Which of the following scenarios is an example of cohesion?

Step 1: Analyze the question The question asks which scenario shows cohesion. The key is to know what cohesion means and to tell it apart from adhesion.

Step 2: Explain the concept

• Cohesion is the attraction between molecules of the same substance. Water molecules are attracted to each other through hydrogen bonds.
• Adhesion is the attraction between molecules of different substances, for example water and glass, or water and a cell wall.

Step 3: Test each option

• A. Water sticks to a mirror. This is water attracted to glass, which is a different substance, so it is adhesion.
• B. Water sticks to the walls of blood vessels. This is water attracted to the vessel wall, so it is adhesion.
• C. Water stays on a pine needle because it is attracted to the needle's surface. This is water attracted to the needle, so it is adhesion.
• D. Water molecules stick to one another and form a bead. Here water is attracted to water, so it is cohesion. Surface tension makes the water pull into a rounded bead.

Step 4: Answer The correct option is D.

Exam tip: Remember "Co = Same, Ad = Different."

• Cohesion: like molecules stick together (water to water).
• Adhesion: molecules stick to something else (water to glass, a wall or a leaf).

This pair often comes up in JAMB when explaining how water rises in the xylem. Cohesion holds the water column together, and adhesion makes it cling to the xylem walls.