Clarify why a scientist can never prove or disapprove a hypothesis but can only test it

Because it is only a prediction of what could happen as a result of a variable being manipulated. It is not a set in stone theory so it cannot be proved or disapproved. Tests can only be carried out to confirm cause and effect in the hypothesis + for comparison.

Kind of a broad question, but I will say the density of the air is the difference between the air movement in high and low pressure areas.