Mg(OH)2 is the active ingredient in many laxatives. It can be made with the following reaction: Mg (s) + 2H2O (l) → Mg(OH)2 (s) + H2 (g)
If a laxative manufacturer has 2.8 x 10^5 grams of Mg, how many grams of water must be used to have all of the Mg react to form Mg(OH)2?

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.