Association of molecules in water is due to-

Concept:

• Hydrogen bond: The electrostatic force of attraction between a hydrogen atom that is covalently bonded to a highly electronegative atom and any other electronegative atom which is present in the same or different molecules.
• In the solid state, it is maximum.
• In the gaseous state, it is minimum.
• There are two types of hydrogen bonds found:
  • Intermolecular H-bonding: It occurs between different molecules of a compound and results in increasing solubility in water and a high boiling point.
    • e.g. HF, water [H2O] molecule
  • Intramolecular H-bonding: It occurs within different parts of the same molecule and results in decreasing solubility in water and a low boiling point.
    • e.g. o -nitrophenol
• The molecules having O—H, N—H or H—F bonds in them, show abnormal properties due to H-bond formation. For example;
  • Glycerol is viscous and has a very high boiling point due to the presence of intermolecular H-bonding.
  • H-bonding also plays an important role in the biological system and stability of proteins and nucleic acids(DNA RNA etc). 

Explanation:

• The extensive association of water molecules with one another in either the liquid or the solid state is due to hydrogen bonds (dotted lines) between the electronegative oxygen atom of one water molecule and the electropositive hydrogen atoms of adjacent molecules
• In water, H is attached to the more electronegative O atom. H atom of one water molecule forms a hydrogen bond with the O atom of another water molecule. 
• The hydrogen atom of the second water molecule forms a hydrogen bond with the O atom of the third water molecule. This results in the association with water molecules.

Hence, the Association of molecules in water is due to hydrogen bonding.