Under what conditions can hydrogen behave as a metal?

When sugar is brought into contact with concentrated $H_2SO_4$,it turns black due to .......

On heating a mixture of $NH_4Cl$ and $KNO_2$,we get:

What do you understand by the term non-stoichiometric hydrides? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.

Consider the following statements about the hydrides: $(I)$ Sodium hydride with water liberates oxygen gas $(II)$ Methane,silane are examples of electron precise hydrides $(III)$ Ammonia and water molecules are examples of electron deficient hydrides $(IV)$ Hydrides of beryllium and magnesium are polymeric in structure. The correct statements are:

Which of the following is a hydride?