A bottle containing a perfume is by mistake left open. After a few days, its volume was found to have reduced to half. Expain giving reason.

This is the phenomenon of diffusion of liquid into surrounding atmosphere. Generally the perfume is in form of liquid. When bottle is kept open, the molecules of perfume from the bottle start diffusing in surrounding air. Volume of perfume is reduced as number of molecules inside the bottle is reduced and diffuse into air.  Since the motion of molecules in liquid is small , it takes few days for the molecules to leave the bottle.