Overview
This lecture covers the unique chemical and physical properties of water that make it essential for life on Earth, including its structure, bonding, behavior as a solvent, density changes, and heat capacity.
Water’s Molecular Structure
- Water (Hâ‚‚O) consists of two hydrogen atoms covalently bonded to one oxygen atom in a V-shape.
- The oxygen atom is more electronegative, creating a polar molecule with partial negative (oxygen) and positive (hydrogen) charges.
Hydrogen Bonding
- Polarity causes water molecules to form hydrogen bonds with each other.
- Hydrogen bonds are weak attractions between the positive side of one water molecule and the negative side of another.
- Hydrogen bonding is responsible for many of water’s unique properties.
Cohesion, Adhesion, and Capillary Action
- Cohesion: Water molecules stick to each other, leading to high surface tension.
- Adhesion: Water molecules stick to other substances, such as glass.
- Capillary action occurs when adhesion and cohesion allow water to climb against gravity (e.g., in plant stems or straws).
Water as a Solvent
- Water’s polarity makes it the universal solvent, dissolving more substances than any other liquid.
- Polar and ionic substances are hydrophilic, meaning they dissolve in water.
- Non-polar, hydrophobic substances do not dissolve in water because water’s cohesive forces exclude them.
Density and Floating Ice
- Solid water (ice) is less dense than liquid water due to a crystalline structure formed by hydrogen bonds.
- Ice floats on liquid water, which is crucial for aquatic life and climate stability.
Heat Capacity and Temperature Regulation
- Water has a high heat capacity, meaning it can absorb or release large amounts of heat with little temperature change.
- Oceans act as heat sinks, moderating Earth’s climate.
- Evaporation of water from skin cools the body by carrying away heat.
Key Terms & Definitions
- Covalent Bond — a chemical bond where atoms share electron pairs.
- Polar Molecule — a molecule with areas of positive and negative charge due to unequal sharing of electrons.
- Hydrogen Bond — a weak attraction between a hydrogen atom of one molecule and an electronegative atom of another.
- Cohesion — attraction between molecules of the same substance.
- Adhesion — attraction between molecules of different substances.
- Capillary Action — movement of liquid within narrow spaces due to cohesion and adhesion.
- Hydrophilic — attracted to water; dissolves easily in water.
- Hydrophobic — repelled by water; does not dissolve in water.
- Heat Capacity — amount of heat needed to change a substance’s temperature.
Action Items / Next Steps
- Review lecture sections on hydrogen bonding, surface tension, and the role of water in life.
- Prepare for test questions on water’s unique properties and definitions.