Generally, when metals are treated with mineral acids, hydrogen gas is liberated but when metals (except Mn and Mg) are treated with ${\mathrm{HNO}}_3$, hydrogen is not liberated, why?

Compound $\mathrm{X}$ and aluminium are used to join railway tracks. (a)Identify the compound X. (b)Name the reaction. (c) Write down its reaction.

The following reaction takes place when aluminium powder is heated with ${\mathrm{MnO}}_2$   $3{\mathrm{MnO}}_2 \left(\mathrm{s}\right) + 4\mathrm{Al}\left(\mathrm{s}\right) \to 3\mathrm{Mn} \left(\mathrm{l}\right) + 2{\mathrm{Al}}_2{\mathrm{O}}_3 \left(\mathrm{l}\right) +\mathrm{Heat}$    

Is aluminium getting reduced?

The following reaction takes place when aluminium powder is heated with ${\mathrm{MnO}}_2$$3{\mathrm{MnO}}_2 \left(\mathrm{s}\right) + 4\mathrm{Al}\left(\mathrm{s}\right) \to 3\mathrm{Mn} \left(\mathrm{l}\right) + 2{\mathrm{Al}}_2{\mathrm{O}}_3 \left(\mathrm{l}\right) +\mathrm{Heat}$

  Is ${\mathrm{MnO}}_2$ getting oxidised?