If the inside surface of a glass tube were coated with wax, which is a nopolar hydrocarbon, how would the general shape of the water meniscus change?
The correct answer and explanation is:
Correct Answer: The water meniscus would change from concave to convex.
When water is placed in a clean glass tube, it forms a concave meniscus. This occurs because water molecules are polar and exhibit strong adhesive forces with the polar surface of the glass, which is composed primarily of silica and contains hydroxyl groups. These adhesive forces between water and the glass cause the water to climb up the sides of the tube slightly, creating a concave shape. At the same time, the cohesive forces among water molecules also play a role, trying to keep the water molecules together.
However, when the inside of the glass tube is coated with wax, the situation changes significantly. Wax is a nonpolar hydrocarbon, meaning it has no attraction to polar water molecules. In this case, the adhesive forces between the water and the wax are much weaker than the cohesive forces within the water. Since water molecules prefer to stay bonded to each other rather than adhere to the wax surface, the water will not climb up the sides of the tube. Instead, it will pull away from the walls, causing the meniscus to become convex.
This shift from a concave to a convex meniscus illustrates how surface interactions affect the behavior of liquids in narrow spaces. The key idea is that the shape of the meniscus depends on the balance between adhesive and cohesive forces. When adhesion is stronger than cohesion, as with water in a clean glass tube, a concave meniscus forms. When cohesion is stronger than adhesion, as with water in a wax-coated tube, a convex meniscus forms. This principle is important in fields such as capillary action, fluid mechanics, and surface chemistry.