Q. Why does paper turn yellow and brittle with age?
Ans. Paper is made from wood, which is in turn made of carbohydrates like cellulose and lignin. Lignin acts as a glue to bind the cellulose fibres together, thereby making the wood stiff. But as the time passes, lignin gets exposed to the air and sunlight and gets broken down to form some phenolic acids which are yellow in colour. These acids also weaken the bonds that bind various cellulose fibres causing the paper to become very brittle.