You can try putting a burning splint in the test tube with the gas and it will go out. This is just a a lack of O2.

The bigger the star(in mass) the shorter its life span. More gravity, hydrogens in core are closer, quicker fusion.

Iron, or more specifically Fe-56. Making elements combine up to iron releases energy(fusion) and would use energy after iron(so it doesn't happen). On the other hand, all elements heavier than iron, if they could be persuaded to, would release energy when split in fission.

Just a guess, but hydrogens are small compared with fluorine and the fluorines will repeal each other much much more.

Covalent. The bond will be polar because chlorine is more electronegative.

Si4 is mysterious and only just discovered by your self. Si is normally a giant covalent structure(like diamond) with a very high boiling point(~2500C). SiF4 is a simple covalent structure(like methane) so is much easier to boil because only weak intermolecular forces are keeping the molecules together.