Molecular Biology - Water

Water molecules ($H_2O$) are formed by polar covalent bonds between one oxygen atom and two hydrogen atoms, resulting in a dipole.

The oxygen atom is more electronegative, attracting electrons more strongly and acquiring a partial negative charge ($\delta^-$), while hydrogen atoms acquire a partial positive charge ($\delta^+$).

Hydrogen bonding is the intermolecular force that forms when the $\delta^+$ hydrogen of one water molecule is attracted to the $\delta^-$ oxygen of another.

Cohesion is the tendency of water molecules to stick to each other due to hydrogen bonding, which creates surface tension.

Adhesion is the attraction of water molecules to other polar or charged surfaces, essential for capillary action in the xylem of plants.

Thermal properties include a high specific heat capacity ($4.186 \, J \cdot g^{-1} \cdot ^\circ C^{-1}$), meaning it takes significant energy to change water's temperature.

High latent heat of vaporization ($2257 \, J \cdot g^{-1}$) allows water to act as an effective coolant through evaporation (e.g., sweating).

Solvent properties: Water is the 'universal solvent' for polar and ionic substances (e.g., $Na^+$, $Cl^-$, glucose) because its dipoles surround and isolate the solute particles.

Methane ($CH_4$) vs. Water ($H_2O$): Methane is non-polar and does not form hydrogen bonds, leading to a much lower boiling point ($-161^\circ C$) compared to water ($100^\circ C$).

Hydrophilic substances are chemically attracted to water (polar), while hydrophobic substances (non-polar like lipids) are repelled by it.