u tell them that the ionic bonds form co-valent bonding and hence the valencies of the two elements change which forms together to create a exothermic reaction resulting finally in a liquidfied form
i 'd say it's because sodium is more active than hydrogen in the acitvity series , thus sodium would replace the hydrogen, producing sodium hydroxide
2Na(s)+2H2O(l)=2NaOH(aq)+H2(g)
that's the best i can do, but i could be wrong