Overview
This lecture explains why water is essential for life, focusing on its unique molecular structure and the resulting physical and chemical properties.
Structure and Bonding of Water
- A water molecule consists of two hydrogen atoms and one oxygen atom.
- Water molecules can form hydrogen bonds with several other water molecules at once.
- The polarity and hydrogen bonding of water give it unique physical properties.
Universal Solvent
- Water is called the universal solvent because it can dissolve many substances.
- Its polarity allows water molecules to surround and dissociate ionic compounds (e.g., sodium chloride).
- Dissolving ions enables frequent chemical reactions essential for cellular processes.
Cohesion and Adhesion
- Cohesion is the tendency of water molecules to stick together due to hydrogen bonding.
- Adhesion is water’s ability to cling to other polar surfaces.
- Cohesion and adhesion allow efficient transport of nutrients and wastes in living organisms.
Surface Tension
- Water has high surface tension because surface molecules are more strongly attracted to each other than to the air.
- High surface tension explains phenomena like water beading and insects walking on water.
High Heat Capacity
- Water’s hydrogen bonds let it absorb large amounts of heat without changing state.
- This stabilizes temperatures in organisms and large bodies of water.
- Slow temperature changes help living things maintain homeostasis.
Density and Ice Formation
- Water expands upon freezing because of hydrogen bond arrangement, making ice less dense than liquid water.
- Ice floats, insulating the water below and protecting aquatic life in winter.
Key Terms & Definitions
- Polarity — Uneven distribution of electrical charge within a molecule, leading to positive and negative ends.
- Hydrogen bond — Weak attraction between the hydrogen atom of one molecule and an electronegative atom of another.
- Cohesion — The attraction between molecules of the same substance.
- Adhesion — The attraction between molecules of different substances.
- Surface tension — The tightness across the surface of water caused by cohesive forces.
- Universal solvent — A substance (water) that dissolves more substances than any other liquid.
Action Items / Next Steps
- Review how water’s properties support life processes.
- Prepare examples of how these properties affect organisms for discussion.