Water is a simple polar molecule having a bent shape; it has one oxygen atom bonded to two hydrogen atoms. The bonds in water molecules are polar covalent because oxygen has higher electronegativity than hydrogen
· The electrons of the covalent bond are pulled more by the oxygen atom towards itself than the hydrogen atom.
· This results in the development of a partial negative charge (δ-) on the oxygen atom and partial positive charge (δ+) on the hydrogen atom.
· The bent shape of a water molecule is due to a lone pair of electrons present on the oxygen atom. These lone pairs can form bonds with hydrogen atoms of other water molecules. The bonds formed between different water molecules are called hydrogen bonds.
· The H-O-H bond angle is 104.45o, which is smaller than the ideal 109.5o of a sp3 hybridised atomic orbital because lone pairs on the oxygen atom cause repulsions and decrease the bond angle.
Forces due to polarity in water
Hydrogen bonding in water molecule gives rise to cohesive and adhesive forces.
Cohesion:
· Cohesion is the natural phenomenon in water molecules that causes water molecules to attract each other. It is the type of attraction that makes molecules of same type stick together.
· Cohesion arises due to the shape and structure of a molecule. When electrons surrounding the molecules come close to each other, then the molecules become irregular. This creates an electrical attraction that maintains the structure, such as in a water drop.
Adhesion:
· Adhesion is the attraction of a water molecule to another polar molecule. Molecules like soil and clay, surfaces like glass or certain clothing fibres, and animal hairs show adhesion with water.
· Adhesion can be of the following types:
o Mechanical adhesion: In this, voids or the pores of surface are filled with adhesive material, which holds the two surfaces together by interlocking.
o Chemical adhesion: In this, adhesion occurs when the surface atoms of two separate surfaces form ionic, covalent, or hydrogen bonds.
o Dispersive adhesion/Physiosortion: In this, two materials are held together by Van der Waals forces. This type of adhesion occurs in polar molecules.
o Electrostatic adhesion: In this, few conducting materials may pass electrons to create an electrical charge difference at the joint, as seen in a capacitor. This creates attractive force between two materials.
o Diffusive adhesion: In this, some materials may merge at the joint by diffusion. Diffusive adhesion occurs when the molecules of both the materials are soluble in each other.
Significance of Cohesive & Adhesive properties of water
Surface tension:
· Surface tension is explained by cohesive properties of water.
· Low levels of external force (surface tension) are resisted due to hydrogen bonding between water molecules.
· The surface of a liquid contracts to the smallest possible surface area due to cohesive forces between molecules.
· The water is made dense by the high surface tension of water, which allows certain smaller organisms to move along its surface.
Capillary Action:
· The capillary action is explained by adhesive properties of water.
· When the water is exposed to charged surface such as thin glass tube (capillary tube), adhesion is observed when water travels up the tube which is placed in water.
The water on sides appears higher than in middle.
· The water on sides appears higher than in middle because the force of attraction between the water molecules and the charged glass walls of capillary is more than the force of attraction between the water molecules.
· Capillary action is necessary to allow water to be transported from the roots to the stem and leaves of plant via transpiration pull.
