Which physical property of the alkali metal group decreases down the group

Melting and boiling point

Explanation:

For all group 1 elements, the melting and boiling point of the various members of the group decreases down the group. Hence, as the elements become heavier, they tend to have lower melting and boiling points.

Thus in moving from lithium to francium, there is a steady decrease in the melting and boiling points of elements down the group.

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.