A sample is prepared by adding 10 grams of oxygen gas to 10 grams of nitrogen gas. No energy change is observed. Which of the following best describes this sample? 1 a mixture that is 50% oxygen by mass
a compound that is 50% oxygen by mass
a mixture in which 50% of the atoms are oxygen
a compound in which 50% of the atoms are oxygen

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.