9. Magnesium reacts with hydrochloric acid according to th following equation:
Mg(s) + 2HCl(aq) MgCl (aq) + H2(g)
What mass of hydrogen will be obtained if 100 cm of
2.00 mol dm HCl are added to 4.86 g of magnesium?
A. 0.2 g
C. 0.8 g
B. 0.4 g
D. 2.0 g

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.

can you list the answers so i can answer