Overview
This lecture explains the concept of surface tension in water, its molecular basis, and factors that affect it, with real-life examples.
What is Surface Tension?
- Surface tension is a property of liquids that enables them to resist external force.
- Water's surface tension comes from hydrogen bonding, which causes water molecules to stick together.
- Surface tension allows insects (like water striders) to walk on water.
- In microgravity (space), surface tension causes water to form spherical droplets.
Molecular Explanation of Surface Tension
- Water molecules in the center of the liquid can form hydrogen bonds with more neighboring molecules.
- Surface molecules have fewer bonding opportunities, so their existing bonds are stronger.
- Stronger surface bonds pull surface molecules closer together, creating surface tension.
Hydrogen Bonding in Water
- Water molecules are polar, with oxygen (negative) and hydrogen (positive) ends attracting each other.
- Hydrogen bonds are weak individually but significant in large numbers.
- Water's molecular structure results in spheres due to even distribution of hydrogen bonds.
Factors Affecting Surface Tension
- Adding soap weakens hydrogen bonds and reduces surface tension.
- Changes in temperature or adding other solutes/solvents also affect surface tension.
Key Terms & Definitions
- Surface tension — The tendency of a liquid’s surface to resist external force due to molecular cohesion.
- Hydrogen bond — A weak attraction between the positive hydrogen of one molecule and the negative oxygen of another.
- Polarity — Distribution of electrical charge leading to positive and negative ends on molecules.
Action Items / Next Steps
- Review examples of surface tension in daily life.
- Experiment: Add soap to water and observe changes in surface tension.