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.

Solvent Properties: Why Water Carries Nutrients and Pollutants Everywhere

1. Because it is polar, water is the universal solvent, dissolving ionic compounds like NaCl and polar molecules like glucose.
2. In hydration, δ+ hydrogens surround negative ions and δ– oxygens surround positive ions, pulling the compound apart and dispersing it.
3. This is why water transports nutrients through soils and organisms, and why it also spreads dissolved pollutants through ecosystems.

Transparency: Why Light Penetrates and Photosynthesis Happens at Depth

1. Water is transparent, so sunlight penetrates and drives photosynthesis in algae, phytoplankton, and submerged plants.
2. How deep light reaches sets the biological zones of a water body.
   1. Euphotic (sunlit) zone: to about 200 m, where most photosynthesis occurs.
   2. Dysphotic (twilight) zone: 200 to 1,000 m, dim light and no net photosynthesis.
   3. Aphotic (midnight) zone: below 1,000 m, no light, so photosynthesis is impossible.

High Specific Heat Capacity: Why Oceans Buffer the Climate

1. Water has an exceptionally high specific heat capacity, about 4.2 J g⁻¹ °C⁻¹.
   1. Energy must first break hydrogen bonds before the temperature can rise, so water heats and cools slowly.
2. This thermal inertia lets oceans and large lakes absorb and release heat slowly, moderating coastal temperatures and buffering global climate.
3. Within organisms, the water in blood and tissues resists temperature swings and helps stabilise body temperature.

Density Anomaly: Why Ice Floats and Lakes Don't Freeze Solid

1. Water is densest at 4°C, not at its freezing point, which is unusual for a liquid.
2. Below 4°C the molecules lock into an open hydrogen-bonded lattice that spaces them farther apart than in liquid water.
   1. Because this lattice makes ice less dense than liquid water, ice floats.
3. Floating ice forms an insulating lid that keeps the water beneath it liquid, so lakes and ponds do not freeze solid.
   1. This lets fish and other organisms survive through winter beneath the ice.

Gas Solubility: Why Cold Water Holds More Oxygen and CO₂

1. Water dissolves atmospheric gases, including oxygen (O₂) and carbon dioxide (CO₂).
2. Gas solubility is inversely related to temperature: colder water dissolves more gas, warmer water dissolves less.
   1. Cold, oxygen-rich polar and deep waters support high productivity and abundant marine life.
   2. Cold oceans also dissolve more CO₂, making them a major carbon sink.
3. As oceans warm, they hold less dissolved oxygen, expanding hypoxic "dead zones" and taking up less CO₂.

High Latent Heat of Vaporization: Why Evaporation Cools and Moves Heat

1. Breaking the hydrogen bonds to turn liquid water into vapour takes a large amount of energy.
2. Evaporation therefore removes heat efficiently, cooling organisms through sweating and cooling leaves through transpiration.
   1. On a global scale, evaporation and condensation move huge amounts of heat through the water cycle, redistributing energy around the planet.