What is meant by the very substance of impurity (عين النجاسة), and is it a condition that whatever pure substance has mixed with it – such as oil mixed with blood, or food mixed with vomit – must also be removed for the removal of the very substance of impurity to be considered achieved?

The religion of Islam is lenient and easy, unlike the strictness that some of us practice. It suffices to remove the substance of impurity with water or similar means. If an impurity mixes with something from which it cannot be separated, such as certain liquids, then if the trace of the impurity is visible, it is contaminated and must be avoided. Otherwise, a small amount of impurity whose trace is not visible is excused.

To purify the hand from blood and the mouth from vomit: What is obligatory is to remove the substance of the impurity. So, if the blood is removed, the place becomes pure. There is no need to remove grease from the hand unless it is a solid mass (a thick layer) and contains traces of blood. As for vomit, scholars have differed regarding its impurity, but the majority of them consider it impure. Rinsing the mouth from it purifies the mouth, and there is no need to remove food remnants from between the teeth, as rinsing purifies them. One should be wary of obsessive doubts and excessive rigor in these matters.