I. boiling water and producing water vapor ii. freezing liquid water into ice cubes iii. mixing salt into a beaker of water until the salt dissolves iv. combining hydrogen gas and oxygen gas in a way to make liquid water when a chemical change occurs something new is formed. the reactants have different properties than the products. the product cannot easily be separated into its original parts. of the four examples listed above, which describe a chemical change? a) i. b) iv c) iii. d) ii.

B) IV, because all the others are physical changes and cannot be reversed.

the answer is c. nacl

a. the law of proportion states the combination of 2 atoms in a specific ratio to produce multiple compounds such as the given statement of question ‘1st compound contains 88.8 % cu ’ indicates 88.8 g of cu is present along with 11.2 g oxygen in 100 g of compound whereas the statement ‘2st compound contains 79.9 % cu ’ indicates 79.9 g of cu is present along with 20.1 g oxygen in 100 g of compound

1st compound contains 88.8 g cu

for cu

m = 63 g /mol

atomic % = 88.8 /63 = 1.42

for o

m = 16 g /mol

atomic % = 11.2 /16 = 0.7

b. the emperical formula of 1st compound is

cu1.4o0.7

cu2o