When a metal `X` is treated with cold water, it gives a basic salt `Y` With molecular formula `XOH` (molecular mass `= 40`) and liberates a gas `Z` which easily catches fire. Identify `X , Y` and `Z` and also write the reaction involved. 

Explanation

Molecular formula of Y = XOH.
Let the atomic weight of metal X is N.
Then, molecular mass of XOH = N + 16 + 1 = 40(given)
N = 40 - 17 = 23
Thus, metal X is sodium (Na) because 23 is the atomic weight of sodium metal.
Sodium reacts with water as
`2Na + 2H_(2)O rarr 2NaOH + H_(2)` _ Heat energy
Sodium (Na) gives hydrogen gas (Z) on reaction with cold water. So, Y is sodium hydroxide (NaOH) and Z is `H_(2)` (hydrogen gas).