Explanation:
c and d. I'm not sure which one is better tho
C. Physical changes do not change the identity of a substance, only the physical appearance such as shape or form. If you break a pencil in half, it is still composed of the same material and therefore keeps its identity as the substance it was before the physical change. Phase changes, such as ice melting, is also a physical change because it changes form. But ice is still made of H20 just like water and steam are comprised of H20.
Answer: The molecular formula of the compound is 
Explanation:
Converting all these percentages into mass.
We take the total mass of the compound to be 100 grams, so, the percentages given for each element becomes its mass.So, the mass of each element is equal to the percentage given.
Mass of N = 30.45 g
Mass of O = 69.55 g
Step 1 : convert given masses into moles.
Moles of N=
Moles of O = 
Step 2 : For the mole ratio, divide each value of moles by the smallest number of moles calculated.
For N = 
For O =
The ratio of N: O = 1: 2
Hence the empirical formula is
.
Empirical mass of
is = 14(1)+16 (2)=46
The equation used to calculate the valency is:

Step 3: To calculate the molecular formula=
Hence, the molecular formula of the compound is 