Question

A metal (M) burns with dazzling brilliance in air to give a white powder.The white powder reacts with water to form a white precipitate and a colourless gas with a characteristic smell.The metal (M) decomposes hot water but not cold water, liberating the inflammable hydrogen gas. The metal (M) is:
A. K
B. Ca
C. Mg
D. Rb

Answer 1

Correct Answer - C
Hydrogen is liberated when Mg reacts with hot water. Rb, K and Ca gives hydrogen gas even with cold water.
`Mg+O_2toMgO , Mg+N_2toMg_3N_2`
`Mg_3N_2+6H_2O to 3Mg(OH)_2 + 2NH_3 uarr`
`Mg+2H_2O("hot")toMg(OH)_2+H_2`