Like all displacement reactions, these are redox processes. That means electrons are exchanged. Which substance is oxidized?
If you compare the vigor of the reactions between Li, Na, K, Rb, Cs and water you should be able to discern a trend. How can this trend be explained in terms of one of the periodic properties we have discussed?
The alkaline earth metals are also above hydrogen in the activity series. But the reactions with water are much less vigorous. Sometimes they cannot even be observed at room temperature. For the reasons we have already discussed, by the time we reach calcium the reactivity of these metals has increased enough so that the reaction is noticeable at room temperature.
Because the alkaline earth metals have two loosely held outer electrons, the displacement of hydrogen results in the formation of a +2 ion:
M + 2 H2O ® H2 + M(OH)2