Hydrogen gas ignited by a spark. All car batteries produce it as a byproduct of the chemical reaction that makes the electricity. That's why it is recommended when jumping a battery that you make the last connection, of the negative clamp, to the frame rather than directly to the battery.
Yes
Normally connecting the cables in reverse order can cause it to explode. Overcharging can also cause the battery to rupture but it is highly unlikely it would explode.
If you explode you die and the same applies to a car battery.
Yes, a car battery can explode if jumped improperly. Car batteries contain sulfuric acid that can produce gasses. If a spark ignites the gases the battery can explode like a bomb.
Yes, if you short the battery out it can explode and burn you.
You would short out the battery. If it was a car battery capable of 300 amps and the wire was big enough, the battery could explode.
Yes, it can ruin a battery and it can explode.
If you explode you die and the same applies to a car battery.
If your car battery smells like "rotten eggs" then your battery is producing excessive hydrogen and is probably not safe. Please check you cars' charging system and make certain that the battery is not being overcharged. The battery can possibly explode.
You would make a car battery comparison by comparing which car battery seems better. You shouldn't keep the old car battery near because you can get it mixed up with the newer ones.
No, because if it is frozen it will not take a charge.
It will leak the electrolyte and is more likely to explode