why water cant control the fire in sodium?

dear ankur 
both lithium and sodium will react very fast with water , and will produce poisonous gas in that process . 
and it can harm human being , . 
and also , some chemical and physical properties of sodium having relevancy to fire . 
it react to liberate hydrogen more slowly than with water , 

so, argon are used to liberate the fire caused by sodium . 
HOPE IT CLEARS 
REGARDS