Overview
This lecture covers the molecular structure of water, the concept of polarity and hydrogen bonding, and how these properties give rise to water's unique behaviors essential for life.
Structure and Polarity of Water
- Water (H2O) consists of two hydrogen atoms and one oxygen atom joined by polar covalent bonds.
- Polar covalent bonds result in uneven electron sharing, making water a polar molecule.
- The oxygen side of the molecule has a slight negative charge, and the hydrogen side has a slight positive charge.
- Polarity causes water molecules to behave like tiny magnets, attracting opposite charges.
Hydrogen Bonds and Their Effects
- Hydrogen bonds form between the positive hydrogen of one water molecule and the negative oxygen of another.
- Hydrogen bonds are intermolecular forces, not true chemical bonds, but make water molecules stick together.
- These bonds are responsible for most of water’s unique properties.
High Specific Heat and Heat of Vaporization
- Water has a high specific heat capacity, requiring much energy to raise its temperature.
- High heat of vaporization means water needs a lot of energy to become a gas (evaporate).
- These properties allow efficient temperature regulation in living organisms through processes like sweating and evaporative cooling.
Cohesion, Adhesion, and Surface Tension
- Cohesion: Water molecules stick to each other due to hydrogen bonds.
- Adhesion: Water molecules also stick to other polar surfaces.
- Surface tension: Cohesion creates a strong surface layer, allowing objects (like insects) to float or water to form droplets.
- Capillary action in plants is possible due to cohesion and adhesion, enabling water to move upwards.
Recap and Biological Significance
- Hydrogen bonds make water hard to break apart, giving it a high specific heat and heat of vaporization.
- Water’s stickiness (cohesion and adhesion) allows for surface tension and the rise of water in plants.
- These properties make water vital for temperature regulation and life processes.
Key Terms & Definitions
- Polar molecule — A molecule with uneven distribution of charges, having positive and negative ends.
- Polar covalent bond — A bond where electrons are unevenly shared, creating partial charges.
- Hydrogen bond — A weak attraction between the hydrogen of one molecule and the electronegative atom (like oxygen) of another.
- Cohesion — Attraction between molecules of the same kind (water to water).
- Adhesion — Attraction between water molecules and different (usually polar) substances.
- Specific heat capacity — The amount of energy needed to raise the temperature of a substance.
- Heat of vaporization — The energy needed to turn a liquid into a gas.
- Surface tension — The force that creates a "skin" on water’s surface due to cohesive forces.
Action Items / Next Steps
- Review topic 1.2: Elements of Life.
- Prepare questions on water’s properties for class discussion.