Answer:
Generally, the first ionisation energy increases along a period. But there are some exceptions one which is not an exception
I would say water; water is extremely polar, and this is why it can break one of the strongest bonds, ionic bonds. NaCl, as you probably know, is a salt, and dissolves in water. However, the ionic bond holding the Na+ and the Cl- is extremely strong; the boiling point of NaCl is at 1413 degrees celcius (water is at 100 degrees celcius). This means that it requires A LOT of energy to break the bond, but water is able to dissolve and break the bond very easily. It is very polar, so I would answer your question with water. And the bond connecting the H and the O is a covalent bond.
Pushing down a piston in an airtight cylinder would tend to increase the pressure in the system since the particle collision and collisions with the walls are now more frequent. Also, pushing further, the gas would undergo a phase change from gas to a liquid.
