Which substances have the same empirical formula and why?

Formaldehyde and glucose has the same empirical formula which contains one carbon, two oxygen and one hydrogen atoms. It is due to empirical formula because empirical formula shows the simplest ratio of the compound or molecule. The molecular formula of glucose is C6H12O6 and formaldehyde is CH2O while empirical formula of glucose and formaldehyde is CH2O.

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.