Structure of Water

Water is polar because it has a slight charge difference across different poles of molecule (dipole)

This allows water to form weak associations (H bonds) with other polar or ionic molecules

Hydrogen bonding and the angular shape of a water molecule gives it unique properties essential to existence of living things

Unique Properties of Water

1. Water is an excellent solvent

Solvent - substance that is able to dissolve other substances

Water is a polar molecule

Break ionic bonds

Hydrophilic “water loving” - substances that freely associate and readily dissolve in water (all polar molecules and ions)

Hydrophobic “water hating” - substances that do not freely associate and readily dissolve in water (fats and oils)

e.g. Water soluble substances (NaCl, O2, glucose, ionized aa) can travel freely in blood plasma, whereas water insoluble substances (lipids) cannot (form complexes with proteins - lipoproteins)

2. Water is both adhesive and cohesive

Adhesive - tendency of dissimilar particles/surfaces to cling to one another

Cohesive - tendency of similar or identical particles/surfaces to cling to one another

3. Water has a high specific heat capacity

Specific Heat Capacity - amount of energy needed to change the temperature of 1 kg of a substance by 1°C

Water is slow to change temperature

H-bonds broken before change state (absorb heat)

4. Water has a high heat conductivity

• Hydrogen Bonding: Strong hydrogen bonds form between water molecules due to attraction between the partially negative and positive ends, making them relatively robust.

• Heat Absorption: When heat is applied, energy is absorbed by breaking these hydrogen bonds, rather than increasing the kinetic energy of individual molecules.

• Temperature Resistance: Water's resistance to temperature changes results from the need for a substantial amount of heat energy to break its strong hydrogen bonds.

• Efficient Heat Transfer: The increased kinetic energy of water molecules, when heated, is efficiently transferred through hydrogen bonding, making water an excellent conductor of heat.

5. Water has a high boiling/freezing point

• Boiling Point: The high boiling point of water results from the need to break numerous hydrogen bonds before transitioning from a liquid to a gas.

• Freezing Point: Water's freezing point is elevated due to the formation of a stable and structured network of hydrogen bonds in the liquid state before transitioning to the solid state.

6. Water is more dense as a liquid then as a solid

• Unique Molecular Arrangement: Water molecules form a hexagonal lattice structure when transitioning from a liquid to a solid state.

• Hydrogen Bonding's Role: Hydrogen bonds cause molecules to arrange themselves with slightly more space in a solid (ice) compared to a liquid (water).

• Increased Molecular Spacing: In the solid state, water molecules arrange with a specific structure that leads to increased spacing between them, making ice less dense than liquid water.

• Density Inversion: Most substances become denser in their solid form, but water exhibits an anomaly due to the unique properties of hydrogen bonding and molecular arrangement.

Summary