Is it obligatory to purify impurities that have dried and whose substance has disappeared, or is their dryness sufficient?

The original state is purity, and a spot is not judged to be impure except with certainty. It is purified by pouring water on the impure spot until the impurity itself is removed. If the impurity has no physical form, then water should be poured until it covers the spot. Some scholars are of the opinion that the earth and what is permanently attached to it can be purified by the sun and wind. It is not obligatory to purify impure ground except for prayer upon it. One must beware of obsessive doubts (waswas).