Correct Option
(A) Liquid sodium is utilized as a coolant in nuclear reactors, particularly in fast breeder reactors. Its properties, such as high thermal conductivity, low neutron moderation capability, and a high boiling point, make it an efficient heat transfer medium. This allows it to effectively remove heat from the reactor core while maintaining the fast neutron spectrum necessary for sustaining the chain reaction in such reactors.
Incorrect Options
(B) Calcium carbonate was historically used as an abrasive in some toothpastes. However, modern toothpastes predominantly use abrasives such as hydrated silica, calcium hydrogen phosphate, or aluminium hydroxide. These alternatives offer improved cleaning efficiency and better compatibility with fluoride compounds commonly found in toothpaste formulations.
(C) Bordeaux mixture is a well-known fungicide and bactericide used in agriculture. It is prepared by mixing copper sulphate (CuSO₄) and calcium hydroxide (lime), not sodium sulphate. The combination creates a protective layer on plants to prevent fungal and bacterial diseases.
(D) Dental amalgams, used for dental fillings, are typically alloys of mercury with silver, tin, and copper. While zinc can be present as a minor component in some amalgam formulations to improve properties, "zinc amalgams" as a primary or standalone dental filling material are not standard. Zinc's specific properties and reactivity limit its extensive use as a main constituent in dental fillings compared to traditional amalgam components.