Question

what happens when a piece of sodium is dropped in water

Answer 1

The temperature of the water is not as important as it being in a liquid or gaseous state(i.e., > 32∘∘F or 0∘∘C). Once sodium has sufficient contact with water, then a very exothermic reaction occurs as follows:

Na+H2O⟶NaOH+H2Na+H2O⟶NaOH+H2

With all the heat being released from the reaction, it is common for a spark to occur. If there is any oxygen around, then typically the hydrogen generated will explode with further reaction:

H2+O2⟶H2OH2+O2⟶H2O(note not balanced)

One thing to note is that depending on the amount of sodium used, the sodium may float on top of water and actually skip across the top travelling some distance. It can be rather dangerous and any nearby combustible or flammable material may ignite.