Question

What are the properties of water that results in cohesion or adhesion?

Answer 1

Polarity: due to the electronegativity difference between the bonded atoms in molecule create dipole. If the electronegative difference between two atoms in between the 0.4-2.0 then the bond is polar. If the electronegative difference between two atoms are equal then that bond is nonpolar. Due to the electronegative difference between the atoms of a molecule (eg: water) the covalent bond between the atoms show partial ionic behavior and generate the partial positive and partial negative charges in the molecule. In the case of water the bond between the oxygen an two hydrogen’s are covalent but due to the electronegativity difference between the oxygen (electronegativity is 3.44) and the hydrogen (electronegativity is 2.2) the covalent bond show partial ionic behavior and it leads to generation of partial negative charge on oxygen and partial positive charges on hydrogen atoms and create a dipole in single water molecule. This charges (+ and -) in the polar molecules make them attract the opposite charges (- and +) of other polar molecules.

Cohesion is an attraction of molecules towards the molecules of the same kind. The water molecules have strong cohesive forces and have the ability to form hydrogen bonds with one another. The positive and negative charges of the hydrogen and oxygen atoms (dipole of water) that allow the water molecules makes them attracted to each other.

Consider a single water molecule, the two hydrogen atoms align themselves along one side of the oxygen atom, the negative charge one oxygen atom attracts the positively charged hydrogen atoms of other water molecule and forms the hydrogen bond. This bipolar behavior of water gives cohesive nature.

Adhesion is the tendency of one kind molecule to attract molecules of a different kind. Adhesion allows the water molecules to climb upwards through capillary tubes and this capillary action depends upon the attraction between water molecules and the walls of the glass tube. When water contact with any molecules which are more polar than the water, they make hydrogen bonds with that molecule.

During the capillary action, the water molecules are more strongly attract the glass than other water molecules because glass molecules are even more polar than water molecules. This bipolar nature of the water molecules responsible for adhesion property.