Atoms in covalent bonds do combine so as to be stable. As covalent bond consist non metals e.g O2 in this example each atom has vacance of 2 orbitals/ electrons so shairing electrons result their stability
Double replacement is happening
<span>The balloons volume as it
rises increases because at higher altitude external pressure is lower, so the
air molecules inside the balloons tends to expand more to attain mechanical
equilibrium, where the external pressure and internal pressure are equal. Unlike
when the balloon is on the ground, external pressure is great so the gas
molecule cannot push the walls of the balloon so it is like underinflated.</span>
