Pure anhydrous $MgCl_2$ can be prepared from the hydrated salt by

$A$ pair of substances which gives the same products on reaction with water is

Match the compounds in List-$I$ with their chemical formulas in List-$II$ and select the correct answer using the codes given below:
List-$I$ (Compounds) | List-$II$ (Formulas)
$(A)$ Plaster of Paris | $(i) \, CaSO_4 \cdot 2H_2O$
$(B)$ Epsomite | $(ii) \, CaSO_4 \cdot \frac{1}{2} H_2O$
$(C)$ Kieserite | $(iii) \, MgSO_4 \cdot 7H_2O$
$(D)$ Gypsum | $(iv) \, MgSO_4 \cdot H_2O$

Fill in the blanks:
$(1)$ Calcium and beryllium have flame colors of ...... and ....... respectively.
$(2)$ Quantitative analysis of calcium is carried out by ........
$(3)$ Beryllium powder on combustion burns brightly in air and gives ...... and .......
$(4)$ Beryllium hydroxide on dissolving in alkali gives .......

What are methanides?

Which of the following statement$(s)$ is/are incorrect?