Suppose that one wishes to use reverse osmosis to reduce the salt content of brackish water containing 0.20 m total salt concentration to a value of 2×10^−2 m , thus rendering it usable for human consumption.
what is the minimum pressure that needs to be applied in the permeators to achieve this goal, assuming that the operation occurs at 280 k ?

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.

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.