Pure is the absence of the impure. People usually ask this question because they derive this understanding from the hadith of sperm. Sperm is inherently pure where as urine is not.
If the urine soiled bedsheet is put under sunshine and everything evaporates where you see no stain or smell then it can be considered pure. But if its drying in your room, your bedsheet will probably have a bad odour and mark of the urine spot. Then you have to wash it for it to become pure.
Shaykh Abu Eesa explains and answers further.