Why is the first jar of hydrogen gas collected during preparation discarded

Hydrogen is collected by downward displacement of water as it is less denser than water. Hence it comes out at the surface of water . It is insoluble in water so it does not dissolve in water

Hydrogen gas is almost insoluble in water,it can be collected by displacement of water using an inverted apparatus. Hydrogen is highly explosive when mixed with air. Hydrogen is explosive when mixed with air so burns with explosion near the mouth of gas jar but burns out inside the jar because the burning requires the presence of oxygen which in not present inside the jar

increased stability in the completion of fluorine's outer shell.

it is stable as it is in group 0 which have full outer shells of electrons