Fe(s) + cuso4(aq) ⇒ cu(s) + feso4(aq)

which reactant is a substance that is dissolved in solution?

The answer to your question is CuSO₄

Explanation:

To answer your question just remember the following information

- A chemical reaction is divided into sections

 reactants and products

 reactants on the left side of the reaction

products on the right side of the reaction

- All the symbols have a meaning

 (s) means that that compound is in solid phase

 (aq) means that that compound is dissolved in solution.

Then, the answer is CuSO₄

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.