Why does water in a glass tube with grease or oil residue display a flat meniscus, whereas water in a clean glass tube displays a concave meniscus?

Verified step by step guidance

1. Understand the concept of meniscus: A meniscus is the curve seen at the top of a liquid in response to its container. It can be concave or convex depending on the liquid and the container material.
2. Identify the forces involved: The shape of the meniscus is determined by the balance between cohesive forces (attraction between like molecules) and adhesive forces (attraction between unlike molecules).
3. Consider water in a clean glass tube: Water has strong adhesive forces with glass, which are greater than its cohesive forces. This causes the water to climb up the sides of the glass, creating a concave meniscus.
4. Consider the effect of grease or oil: Grease or oil residue on the glass reduces the adhesive forces between the water and the glass. This diminishes the water's ability to climb the sides of the tube.
5. Explain the flat meniscus: With reduced adhesive forces due to the grease or oil, the cohesive forces within the water become more dominant, leading to a flatter meniscus compared to a clean glass tube.