Final answer:
To demonstrate the reactivity of Group 2A alkaline earth metals, one can observe their reaction with water, where they form hydroxides and hydrogen gas, specially magnesium, calcium, strontium, and barium.
Step-by-step explanation:
One way to demonstrate reactivity among the alkaline earth metals, Group 2A, is to observe what happens when they are placed in water. The alkaline earth metals, specifically magnesium, calcium, strontium, and barium, react with water to form hydroxides and hydrogen gas. For example, the reaction of calcium with water can be represented as:
Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)
The formation of hydrogen gas during these reactions is evidence that these metals, especially the heavier ones, act as better reducing agents than hydrogen and are quite reactive. Their reactivity increases down the group, so barium, being heavier, will show a more vigorous reaction with water at room temperature compared to the lighter alkaline earth metals like magnesium or calcium.