When lithium and sodium react with water, hydrogen gas is released. Due to these hydrogen gas bubbles, lithium and sodium floats on water.
eg. 2Na + 2H2O → 2Na+ + 2OH– + H2↑
The reactivity of group 1 metals increases with increasing atomic radius and lowering of ionization enthalpy down the group.
Thus, sodium having lower ionization enthalpy, is more reactive than lithium.
Hence, lithium reacts slowly while sodium reacts vigorously with water.
Since the reaction of sodium with water is highly exothermic, it catches fire when put in water.