2HgO -> 1 O2 + 2Hg
remember first there's 2 oxygens so then you check the other side and make it 2 too. then you see that now there's 2Hg and then you put a 2 on the other side to make it 2Hg too. balancing is just making sure the number of elements on one side equal the other.
So if you put a holo ball of paper in the water it sinks and as it sinks it gets smaller cuse pressure so it cuseus things to shrink cuse of pressure
Since the atmospheric pressure at Mt Everest is constant, the process will take place at constant pressure and such process at constant pressure is called Isobaric process. Hope this helps!