hydrogen bonding

Polarity of water ($H_2O$)

Oxygen ($\delta^-$) attracts electrons more strongly than hydrogen ($\delta^+$), creating a bent, polar molecule.

     (δ-) O
         / \
   (δ+) H   H (δ+)

Hydrogen bonding in water

Attraction between the partial negative oxygen ($\delta^-$) of one water molecule and the partial positive hydrogen ($\delta^+$) of another.

   (δ-) O — H (δ+)
        :
   (δ-) O — H (δ+)

Cohesion vs. Adhesion

Cohesion: Water sticking to water. Adhesion: Water sticking to other polar surfaces.

Cohesion:  [Water] <---> [Water]
Adhesion:  [Water] <---> [Surface]

Low density of ice

At $0\,^\circ\text{C}$, hydrogen bonds freeze into a rigid, open lattice, making ice less dense than liquid water.

Liquid:  O..O..O  (Packed)
Ice:     O --- O  (Open lattice)
         |     |
         O --- O

Hydration shell

Water surrounds dissolved ions with partial negative oxygen ($\delta^-$) facing cations ($Na^+$) and partial positive hydrogen ($\delta^+$) facing anions ($Cl^-$).

Hydration shell

Water surrounds dissolved ions with partial negative oxygen ($\delta^-$) facing cations ($Na^+$) and partial positive hydrogen ($\delta^+$) facing anions ($Cl^-$).