Overview
This lecture explains why liquids form menisci in containers and introduces the concepts of adhesion, cohesion, and capillary action.
Meniscus Formation
- The surface of water in a glass container is not flat; it curves upward near the glass, forming a concave meniscus.
- A concave meniscus occurs when the liquid is more attracted to the container than to itself (e.g., water in glass).
- A convex meniscus forms when the liquid is more attracted to itself than to the container (e.g., mercury in glass).
Molecular Interactions: Water and Glass
- Water molecules are polar, with partial positive charges at the hydrogens and partial negative charges at the oxygen.
- Glass (silicon dioxide) is also polar, with silicon atoms having partial positive charges and oxygen atoms partial negative charges.
- The greater electronegativity difference between silicon and oxygen makes glass more polar than water.
Adhesion and Cohesion
- Adhesion is the attraction of water molecules to the glass surface, causing them to "stick" and climb up the container walls.
- Cohesion is the attraction of water molecules to each other, mainly via hydrogen bonding.
Capillary Action
- Capillary action is the movement of water up a narrow tube due to adhesion between water and the glass and cohesion among water molecules.
- Capillary action requires a polar (hydrophilic) container, such as glass, not plastic.
- This phenomenon allows water to rise against gravity in thin tubes and also occurs in natural and everyday contexts (e.g., capillaries in the body, paper towels absorbing spills).
Key Terms & Definitions
- Meniscus — the curve at the liquid's surface in a container.
- Concave meniscus — surface curves upward toward container walls; typical for water in glass.
- Convex meniscus — surface bulges downward; typical for mercury in glass.
- Adhesion — attraction between different substances (e.g., water and glass).
- Cohesion — attraction between identical molecules (e.g., water to water).
- Capillary action — movement of liquid within narrow spaces due to adhesion and cohesion.
Action Items / Next Steps
- Observe the meniscus in various containers (glass vs. plastic, water vs. mercury if possible).
- Try the capillary action experiment using a thin glass tube and water.
- Read about additional real-world applications of capillary action.