Q.Which is the correct sequence of solubility of carbonates of alkaline earth metals ?

"Blue John" is calcium fluoride (CaF2) in its ornamental blue-violet banded mineral form.

Calcium fluoride occurs naturally as the mineral fluorite (fluorspar). A rare, decoratively banded blue-and-purple variety of this mineral, historically mined in Derbyshire, England, and used for ornamental carvings, is traditionally known as "Blue John". Chemically it is still CaF2; none of the other …

Matching each substance to its characteristic industrial/medicinal use gives (A)-2, (B)-4, (C)-1, (D)-3 - option (a).

Each compound in List-I has a well-known specific use:

• Magnesium (A) burns with a dazzling white light and is used in flares, fireworks and photography flashbulbs -> Pyrotechnics (2).
• Magnesium sulphate (B), commonly called Epsom salt, is used medicinally as a laxative/purgative -> Purgative (4). …