Q.What happens when

1. Magnesium burnt in air Magnesium burns with a dazzling white light, combining with both the oxygen and nitrogen of air:

   $2Mg + O_2 \rightarrow 2MgO$

   $3Mg + N_2 \rightarrow Mg_3N_2$

   so the ash formed is a mixture of magnesium oxide and magnesium nitride.
2. Quicklime heated with silica Quicklime (CaO) reacts with silica (SiO$_2$) at high temperature to form calcium silicate (a type of slag/glass-forming reaction):

   $CaO + SiO_2 \xrightarrow{\Delta} CaSiO_3$

3. Chlorine reacting with slaked lime Chlorine gas passed over dry slaked lime, Ca(OH)$_2$, gives bleaching powder:

   $Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$

4. Calcium nitrate heated …

Treat each part as a standalone standard reaction: recall magnesium's reaction with both O$_2$ and N$_2$ in air, CaO's acid–base reaction with acidic SiO$_2$ to form a silicate, Ca(OH)$_2$'s reaction with Cl$_2$ to give bleaching powder, …

• Writing only 2Mg + O$_2$ → 2MgO for magnesium in air and omitting the nitride (Mg$_3$N$_2$) formed alongside it.
• Mislabelling the product of Ca(OH)$_2$ + Cl$_2$ — it is bleaching powder, CaOCl$_2$ (calcium oxychloride/hypochlorite chloride), not simply CaCl$_2$. …