N2(g) + 3H2(g) ⇌ 2NH3(g)

How will the reaction be affected if the pressure on the system is decreased?

Reaction equilibrium will shift to the left (higher molar volume side of process).

Explanation:

For gas phase reactions, changes in pressure-volume conditions cause a stress on the process equilibrium shifting away from the applied pressure-volume change.

LeChatlier's Principle states for a gas phase reaction at equilibrium ...

if the pressure is increased (volume decreased) the reaction will shift toward the lower molar volume side of the process.

if the pressure is decreased (volume increased) the reaction will shift toward the higher molar volume side of the process.

if the sum of molar volumes of reactants equals the sum of molar volumes of products, changes in pressure-volume conditions will NOT affect the equilibrium stability.  Example => H₂(g) + Cl₂(g) ⇄ 2HCl(g) will not be affected by P-V changes as ∑Vm(reactants) = ∑Vm(products).

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.