At a molecular level, how does water soften things?
Biscuits are basically lumps of starch glued together by sugar. When the hot tea or coffee enters the pores in the biscuit, the sugar melts and the structure becomes unstable. This gives a biscuit which is purely starch but rather softer than what you started with. Paper is made up of cellulose fibres containing OH groups. The OH groups form hydrogen bonds with other OH groups on adjacent cellulose fibres. The resulting polymer holds the fibres together in place, making the paper stiff. Water molecules are polar, and therefore are attracted to the OH groups. When the paper is wet it loses strength because the water molecules get between the cellulose fibres, weakening their attraction to each other.
