Cohesion and Adhesion in Fluids

Cohesion refers to the attraction between molecules of the same substance, while adhesion refers to the attraction between molecules of different substances. In the case of a fluid, such as water, the cohesion between water molecules is typically stronger than the adhesion between water and glass molecules.

Explanation:

1. Cohesion between molecules of fluid:
- Cohesion between molecules of a fluid, such as water, is the result of intermolecular forces, such as hydrogen bonding.
- These forces cause water molecules to stick together, creating a cohesive force that holds the fluid together.
- Due to the strong cohesive forces, the molecules in the fluid tend to attract each other and form a continuous liquid surface.

2. Adhesion between fluid and glass:
- Adhesion between fluid and glass occurs due to intermolecular forces between the fluid molecules and the glass molecules.
- However, in the case of water and glass, the adhesion between the two is generally weaker compared to the cohesion between water molecules.

3. Dipped glass tube and free level of fluid:
- When a glass tube is dipped into a fluid, such as water, the fluid will rise in the tube due to capillary action.
- Capillary action is the result of the combination of cohesion and adhesion forces.
- If the cohesion between the fluid molecules is greater than the adhesion between the fluid and glass molecules, the fluid will be pulled upward in the tube, creating a concave meniscus.
- The level of the fluid in the tube will be lower than the surface of the liquid outside the tube.
- This is because the cohesive forces between the fluid molecules cause the fluid in the tube to resist being pulled upward by the adhesive forces between the fluid and the glass.

Conclusion:

In summary, when the cohesion between molecules of a fluid is greater than the adhesion between the fluid and the glass, the free level of the fluid in a dipped glass tube will be lower than the surface of the liquid. This is due to the strong cohesive forces within the fluid, which cause the fluid to resist being pulled upward by the adhesive forces between the fluid and the glass.