Water molecules stick to each other due to a type of intermolecular force called hydrogen bonding. Hydrogen bonding is a special type of dipole-dipole interaction that occurs between the hydrogen atom of one water molecule and the oxygen atom of another water molecule.
In a water molecule (H2O), the oxygen atom is more electronegative than the hydrogen atoms. As a result, the oxygen atom attracts the shared electrons more strongly, creating a partial negative charge (δ-) near the oxygen atom and partial positive charges (δ+) near the hydrogen atoms. This separation of charges within the molecule is referred to as polarity.
When water molecules come close to each other, the δ- oxygen atom of one molecule is attracted to the δ+ hydrogen atoms of neighboring molecules, leading to the formation of hydrogen bonds. These hydrogen bonds are relatively strong compared to other intermolecular forces and contribute to the cohesive properties of water.
The presence of hydrogen bonds between water molecules gives water some unique characteristics, such as high boiling and melting points, high surface tension, and the ability to dissolve a wide range of substances. These properties are crucial for many biological processes and are responsible for water's role as a universal solvent and its importance for life.