Which correctly summarizes the trend in electron affinity?
it tends to be very high for group 2
it tends to be more negative across a period
it tends to remain the same across periods
it tends to be more negative down a group

The answer is it tends to be more negative down a group. This is because as you go down the periodic table, the elements have more electron shells in their atoms. This makes the outermost shells less attracted to the nucleus due to their greater distances from the nucleus. Therefore, these shells are less likely to attract electrons (hence lower electron affinity) and are even more likely to lose electrons from their outer electron orbits.

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.

if you're asking what comes out, it's steam.