Why does wet paper tear so easily?

As the southwest monsoon sweeps over India, people brace for storms, floods, and the occasional sight of a damaged bookstore or library. What makes the last a particularly sad sight is that paper tears easily when it’s wet.

Paper is made primarily from wood pulp, which contains a dense network of cellulose fibers. Cellulose consists of the compound C₆H₁₀O₅ attached to each other in long chains. Many of these chains clump together to form fibres, and the fibres are attached to each other using hydrogen bonds, i.e. a bond involving a hydrogen atom on one end.

While hydrogen bonds are weaker than covalent, ionic or metallic bonds, millions of them act to keep paper together, along with the fibres also being entangled.

When paper is wet, water molecules penetrate between the cellulose fibres. Because water can also form hydrogen bonds, it competes with and disrupts some of the hydrogen bonds that normally hold neighbouring fibres together.

Water also causes the cellulose fibres to absorb moisture and swell, loosening the network and reducing friction. As a result, the fibres can slide past one another more easily, so less force is needed to tear wet paper.