IB Environmental systems and societies 4.1B Characteristics of Water Notes

Water: How Hydrogen Bonding Turns One Small Molecule Into the Engine of Life

1. Water sustains ecosystems, drives biological processes, and regulates climate because of a set of unusual physical and chemical properties.
2. Almost every one of these properties traces back to a single cause: polarity and the hydrogen bonds it creates.

Polarity and Hydrogen Bonding: The Root Cause of Every Other Property

1. A water molecule is two hydrogen atoms covalently bonded to one oxygen atom in a bent, V-shaped structure.
2. Oxygen is more electronegative, so it pulls the shared electrons closer and gains a partial negative charge (δ–) while the hydrogens become partially positive (δ+).
   1. This uneven charge distribution makes water a polar molecule.
3. Polarity lets adjacent molecules form hydrogen bonds, weak attractions between the δ+ hydrogen of one molecule and the δ– oxygen of another.
   1. Individually weak but collectively strong, these bonds give water its stability and cohesive strength.

Without hydrogen bonding water would be a gas at room temperature, and life as we know it could not exist.

Cohesion: Why Water Columns Hold Together and Insects Walk on Water

1. Cohesion is the attraction of water molecules to each other through hydrogen bonding.
2. It makes water molecules stick together, forming droplets and creating surface tension strong enough for water striders to stand on.
3. Cohesion holds an unbroken water column in plant xylem, letting transpiration pull water from roots to leaves.

Adhesion: Why Water Climbs Soil Pores and Plant Xylem

1. Adhesion is the attraction of water to other polar or charged surfaces.
2. It lets water climb narrow spaces against gravity through capillary action, moving water through soil pores and up xylem walls.
   1. Adhesion to cellulose in xylem walls works with cohesion to drive transpiration.