When cats smell something and then open their mouth slightly and wrinkle their nose and lips they are exhibiting the characteristic of "flehming".  Cats do this as an additional scent function. During "flehming" the cat draws air into its mouth so that the "jacobson's organ" at the roof of its  mouth can send signals to the brain regarding the scent of the object

"Flehming" can happen on almost any scent but happens primarily when a cat smells urine. Many male cats will show this behavior when smelling a female cat in heat.