its highly poisonous and can do some horrible things to you. dont do it again
As above its a sealed system, (and why it releases pressure if you try to take top off when hot.
liquids do not compress, but do expand when heated up so there must be some air in the expansion tank so the air compresses as the coolant expands.
This is why its called an expansion tank!
In my experience if you fill near to the max line when cold, it will be a fair bit above max when hot, which isnt a
problem.
Completely full when got or cold is a
problem!