Asolid is placed in a solution. which observation most likely indicates that a chemical reaction has occurred?
1) the size of the solid has rapidly decreased.
2) the color of the solution has not changed.
3) the solid has bubbles forming on its surface.
4) the temperature of the solution stays the same.

The solid has bubbles forming on the surface

n+h→nh3

we must remember that nitrogen and hydrogen are both diatomic molecules in their standard gas form. this adjusts the equation to

n2+h2→nh3

now we need to adjust coefficients in order to balance the atoms on each side of the equation. currently we have 2 atoms of nitrogen and 2 atoms of hydrogen on the reactant side and 1 atom of nitrogen and 3 atoms of hydrogen on the product side.

we can balance the hydrogens by placing a coefficient of 2 in front of ammonia and a coefficient of 3 in front of the hydrogen.

n2+3h2→2nh3

this gives us 6 hydrogen on each side and coincidentally the nitrogens now equal 2 on each side.