In which of the following situations does water undergo a change in physical properties?
a. the bathroom fills with steam when you take a hot shower.
b. the neighbor's pond freezes over in winter.
c. water is broken down to yield h2 and o2.
d.
a. and
b. only

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.