Overview
This lecture explores the unique properties of water, explaining why it is essential for life and reviewing its molecular structure, chemical behavior, and vital physical properties.
Importance of Water for Life
- Water exists naturally as a solid, liquid, and gas on Earth.
- All known life depends on water, making its presence on other planets significant for the search for extraterrestrial life.
- Discovery of water, especially liquid, on Mars raises hopes of finding life elsewhere.
Structure of Water Molecule
- Water (Hâ‚‚O) consists of two hydrogen atoms covalently bonded to one oxygen atom.
- The molecule is V-shaped and polar: oxygen has a slight negative charge, hydrogens have a slight positive charge.
- Polarity leads to hydrogen bonding between water molecules.
Hydrogen Bonds and Their Effects
- Hydrogen bonds are weak attractions between the positive hydrogen of one molecule and the negative oxygen of another.
- Hydrogen bonds create strong cohesion (attraction between like molecules) in water, resulting in high surface tension.
- Cohesion allows water droplets to bead up and enables some insects to walk on water.
Adhesion and Capillary Action
- Adhesion is the attraction between different substances, such as water and glass.
- Adhesion and cohesion together result in capillary action, allowing water to climb up narrow tubes against gravity.
Water as a Solvent
- Due to its polarity, water is an excellent solvent (universal solvent) and dissolves more substances than any other liquid.
- Substances that dissolve in water are hydrophilic (water-attracting and polar).
- Non-polar, hydrophobic substances do not dissolve in water because they cannot break water's cohesive forces.
Historical Perspective: Henry Cavendish
- Henry Cavendish identified hydrogen gas and determined that water forms from the reaction of hydrogen with oxygen.
- He conducted groundbreaking experiments in gas density and atmospheric composition.
Density and States of Water
- Solid water (ice) is less dense than liquid water because hydrogen bonds form a crystalline structure that spaces molecules apart.
- Ice floating is critical for aquatic life and the earth’s climate stability.
Heat Capacity and Temperature Regulation
- Water has a high heat capacity, meaning it absorbs and retains heat well.
- Oceans regulate Earth’s temperature due to water’s heat capacity.
- Water’s high heat of vaporization enables cooling through sweating.
Key Terms & Definitions
- Polarity — Molecules with uneven charge distribution, making them attract other polar molecules.
- Covalent Bond — Sharing of electron pairs between atoms.
- Hydrogen Bond — Weak attraction between a hydrogen atom and an electronegative atom like oxygen.
- Cohesion — Attraction between like molecules.
- Adhesion — Attraction between different substances.
- Capillary Action — Movement of liquid along a surface due to cohesion and adhesion.
- Solvent — Substance that dissolves other substances.
- Hydrophilic — Attracted to and easily dissolved in water.
- Hydrophobic — Repelled by and not dissolved in water.
- Heat Capacity — The amount of heat energy required to change a substance's temperature.
Action Items / Next Steps
- Review the key properties of water, especially hydrogen bonds, cohesion, adhesion, and heat capacity.
- Watch the lecture again if any concepts are unclear.
- Prepare for potential quiz questions about water’s properties and their importance to life.