The hydroxide of which of the following metals reacts with both acid and alkali?

$A$ chemistry student wants to identify the metal ion in a salt. They make a paste of the salt with concentrated $HCl$ on a clean platinum wire loop. If they take a small amount of this paste and place it in a non-luminous flame,it produces a green flame. Which metal is present?

What is the difference in the number of water molecules between gypsum and plaster of Paris?

Thermal stability of $MCO_3$ is in order:

Out of $BeF_{2}, MgF_{2}, CaF_{2}, SrF_{2}$,which has maximum solubility?

Which of the following elements belongs to the $second$ group and $fifth$ period of the periodic table?