Question

Why does a lump of cotton wool shrink in water?

Answer 1

Hint: Try to think about why cotton wool occupies more volume compared to the weight which is relatively less. Think about molecules trapped in between a lump of cotton wool and what happens to them after cotton wool is immersed in water.

Complete step by step answer: Cotton wool is a fluffy wool-like substance which is obtained from raw cotton after removing seeds and other impurities. Cotton is an organic polymer obtained from cellulose. Cotton wool lump contains air molecules trapped by polymers in between the fibers. These trapped air molecules occupy large volumes but weigh very less. When this lump of cotton wool is immersed in the water we can see that it shrinks down this is due to the replacement of air molecules with water molecules. Water molecules though do not occupy the volume the same as that of air molecules so cotton wool shrinks down.
Polymer molecules in cotton do not react with an air volume of air occupied unless external pressure is applied. Cotton is a cellulose polymer which contains many hydroxy and other polar functional groups. As a result, Cotton is hydrophilic. Air mostly contains Nitrogen which is inert because the bond energy of the Nitrogen molecule is high. And oxygen is also non-polar due to the polar covalent bond in the oxygen molecule so oxygen also does not interact with a polymer.
Water is a polar solvent and polar molecules interact with water molecules. When water molecules come in contact with cotton air molecules trapped will escape out. As a result cotton wool shrinks down when immersed in water.

Note: We may think that water molecules occupy the same volume as that of air molecules and cotton wool does not shrink but due to dipole-dipole interactions water cotton is closely packed. So cotton will shrink.