Answer 1

Sodium and potassium are two metals that react violently with cold water.

Three observations when such a metal is dropped in water are:

1. Reaction is highly exothermic i.e. a huge amount of heat is produced.
2. Hydrogen gas is evolved in the reaction.
3. A small explosion takes place because the heat produced in the reaction burns the hydrogen gas. Hydrogen can be identified by the 'pop' sound it makes while burning.