When a metal X is treated with cold water

Explanation:
Given, reaction can be written as :
 X + cold H2O→Y(XOH) + Z(g)
Since, X reacts with cold water to yield hydroxide, so it must be either Na or K.It is specified that molecular mass of formed hydroxide is 40,so it can't be KOH(MM=56), it is rather NaOH, having molecular mass 40(23+16+1) as described.
Hence, X must be Na and Y must be NaOH.
The reaction can be written as :
          2Na + 2H2O→2NaOH + H2
Therefore, Z is H2, which indeed catches fire easily.

Final Answer:
Hence, X, Y and Z are identified to be Na, NaOH and H2(g) respectively and the reaction involved is :
   2Na + 2H2O→2NaOH + H2